Zero Carbon ® Webinar Questions and Answers

The following questions were asked by attendees of one of my Zero Carbon®, Zero Bills webinars or by email. They are reproduced here, together with my answers.

In my opinion, the Q&A is the most valuable part of the webinars and these interactions provide a gold mine of information to anyone wishing to cut their bills by cutting their carbon footprint. After some general questions, the Q&A is organized roughly by HITS (heat pumps, insulation, triple-glazed windows and solar panels), but the questions often address multiple issues at once or come at an issue from a different perspective like the role of carbon offsets, how to apply the fab four to new construction, condensation, or how the fab four recipe changes based on which climate zone you are in.

You can search this using the search feature on the website in the top left corner of this page.

 General questions, not specific to one element of HITS:

Q: Will you describe how to calculate my home’s carbon footprint?

A: Yes, the simplest way to do it is described here:

Q:  How did you manage to measure daily usage of heating oil?

A:  I carefully calibrated the float gauges on my 330 gallon oil tanks. I could measuer accurately down to about 2 gallons of heating oil use. On cold days I used far more than this (at least before the fab four) and so it was easy to measure. On warm days I took the average over 5-10 days.

Q: You’ve reduced your carbon footprint and saved money, but how is the comfort of your house? What does your wife think?

A: The comfort of our house had increased considerably with the fab four. The cold drafts off the windows in the living room made is unpleasant to be there at Christmas time. Now the living room is pleasant even in the coldest parts of winter. The insulation on the ceiling of the basement helped here too. In addition, the noise from the ductwork is reduced because the heat pumps run almost continuously but at lower speed than the old on/off blower fans of the old heating system. Also, we no longer have an occasional faint whiff of burnt heating oil in the house. And the induction stove is a joy to cook on. My wife and the whole family really like living in our house. We are very happily married (to each other).

Q: Can you share any spreadsheets that will allow for scenario role play to determine cost/savings/credit impacts into any decision process? and do these calculations take into account the CF impact of the manufacturing of all of the utilized components, inclusive of supply chain? very cool talk! keep it up!

A: I have spreadsheets for almost everything I do. I have combined them into an integrated energy/financial model that allows for exactly these kinds of scenario planning. However, today, the software is far too complicated to use for anyone other than me to use it reliably. One day I hope to be able to offer it as an app on my website, but that will take a lot of work, so don’t hold your breath.

Q: if I plan to remain in my home 10 more yrs, would it pay to do HITS?

A: Definitely for insulation and draft sealing, these often pay for themselves in the first winter. Adding window insert, particularly on drafty sash windows, will pay for themselves within 10 years. Solar panels will probably pay for themselves in 5-8 years. However, large academic studies show that house prices increase by between 4-7% by adding heat pumps and by similar amounts for adding solar panels. Both of these house-price increases would exceed the installation cost (after the subsidies) so adding heat pumps and solar panels is actually a good way to prepare your house for sale.

Q: In your IRR Are you making any assumptions for price of energy over time?

A: Yes. Discounted cash flow analysis requires projecting the cash flows (cash in and cash out) over the life of the investment (here solar panels or heat pumps). So you need to project the future prices of electricity to calculate the cash saved by having solar panels and the cash spent on powering a heat pump. I was very conservative in my inflation assumptions in the DCF analysis I did and assumed 0% inflation for electricity. I would rather be more conservative and get a nice surprise if things turn out better. In fact, electricity-price inflation in MA has continued at about 2-3% per year so the IRRs I give in the books are actually about 2-3% lower than what I have actually realized on these investments. In other words the investments have turned out to be better than I projected. 

Q: Did you work with a rater at all, or run any simulations on choices vs savings?

A: I did not work with a rater. I built my own software (it is a very large Excel spreadsheet that incorporates both energy modelling and financial modelling) precisely because existing simulation software (such as REM/RATE – see other Q&A on this) has a terrible reputation for assessing energy use in existing buildings. I used my software to model everything I did and have used the same approach with our rental property and all my consulting clients. My software is based on the fundamental laws of physics applied to buildings and then uses standard discounted cash flow projections to do the financial analysis.

Q: Re cradle-to-grave accounting: When you add in the CO2 produced during manufacture and (later) disposal of the technologies, how does that change the math?

A: See my answers below on the carbon footprint of manufacturing the fab four. On cradle-to-grave accounting for CO2, more and more people are starting to think about this. Let’s take each one of the fab four in turn:

  1. Heat pumps. This is my biggest concern because the refrigerant gas is a very potent greenhouse gas, it is about 1,000x as strong as CO2 in causing global warming. If the heat pump is put in a land fill, or recycled for its metal, then the refrigerant will escape into the atmosphere. I hope that eventually a new generation of low-global-warming-potential refrigerants will replace the current generation, which is called R410A. R410A is a hydrofluorocarbon (HFC) that was developed to replace chlorofluorocarbons (CFCs) because CFCs depleted the ozone layer. Other applications (like making the bubbles in spray-foam insulation) also use HFCs. HFCs for spray foam are beginning to be replaced with HFOs (hydrofluoro-olefins) because HFOs have only about the same global warming potential as CO2. Without a new generation of refrigerants, we are going to have to rely on installers to safely remove the refrigerant gas before the unit is disposed of. This is probably going to require legislation. In Paul Hawken’s book, “Drawdown, The Most Comprehensive Plan Ever Proposed to Reverse Global Warming”, refrigerant management (or mismanagement) is listed as the most addressable cause of global warming.
  2. Insulation. It is very easy to separate fiberglass or rockwool insulation from the wooden framing of a house when the house is demolished. It is very hard to separate spray foam from the wood because it is stuck to the wood like glue. This, plus the flammability of spray foam (which is almost never discussed) is why I prefer either fiberglass or rockwool for insulation. Since both fiberglass and rockwool and very porous to drafts, they need to be installed with an air-tight but breathable membrane. Fiberglass and rockwool can either be reused or recycled. 
  3. Triple-glazed windows. These pose no additional problems at the end of their life than any other window. The glass and wood can easily be separated and recycled or disposed of in a landfill where the glass will stay forever. 
  4. Solar panels. A solar panel is essentially glass plus some metal. Both are valuable and easy to recycle.

Q: What about all the carbon it took to create, deliver and install all the materials to get your house to zero?

A: Solar panels erase the carbon footprint produced by manufacturing them with the first 18 months of the carbon-free electricity they produce. The heat pumps will pay back the carbon footprint of their manufacture in about 4 months. The insulation repays its carbon footprint of manufacturing in just a few weeks and the triple-glazed windows in a few months.

Q:What if I live in the South? Does HITS work in a hot climate?

A: The fab four recipe for cutting your carbon footprint will work in the southern half of the country as well as in the northern half. 

From the point of view of the laws of physics, a house is just a box. It has a top, a bottom and four sides. Energy comes into the box from electricity and heating fuel. Energy goes out through the walls, windows, attic and floor/basement. This is the same whether we call the box a house, an apartment block, an office or a factory, and the laws of physics are the same in Florida and North Dakota.

In a cold climate you need to keep the heat in. In a hot climate you need to keep the heat out. The answer is insulation and triple-glazed low-E windows in both cases. So, these parts of HITS (the I and the T) are the same in cold climate or a hot climate. 

However, a heat pump (the H in HITS) in cooling mode is the same efficiency as an air-conditioner. It is only in heating mode that a heat pump has two and a half times the efficiency of an oil-fired or natural gas-fired furnace or boiler. So, the year-round gain in heating/cooling efficiency is bigger in a cold climate than a hot climate.

Solar panels (the S in HITS) work better in the southern half of the US because there is much more sun there than in the northern half. In fact, where we live in Massachusetts, we have a rather poor solar crop to harvest. The desert southwest has almost 40% more solar energy per year. This makes solar power about 40% cheaper per kilowatt-hour in the southwest compared to Massachusetts. See this map from the NREL:

In addition to how much sunshine you have in your area, the price you pay for utility electricity will have a big impact on the payback period for solar panels. High electricity prices make the payback period on solar panels faster, because you are saving more money per kilowatt-hour of electricity generated by your solar panels.

Other than Hawaii, which has very high electricity prices, the highest prices for electricity in the U.S. are in New England, California and Alaska where you will pay about 20c per kilowatt-hour. In a band of states running down the center of the country from North Dakota to Louisiana, electricity is about 10c per kilowatt-hour. In most of the rest of the country it is about 15c per kilowatt-hour.

So overall, with insulation and low-E triple-glazed windows cutting your carbon footprint and utility bills across the entire country and with the lower benefit of using heat pumps for heating in the southern half of the country combined with the higher solar production in the southern half of the country, HITS will benefit you no matter where you live. The recipe just needs to be fine-tuned to your exact location and local financial subsidies.

Q: What would be the difference between Passive House and Green Zero Carbon houses?

1. What would be the difference between Passive House and Green Zero Carbon houses? 

A: PassivHaus focuses on passive solar gain as a big part of its energy saving. I do not. Passive solar gain from big south-facing windows leads to massive overheating on sunny days in winter, which is very bad for energy efficiency. Windows (good ones are R4-5) are less insulating than a wall (a bad one is R12) so any time you have a window rather than a wall, you have heat leaking out in winter and heat leaking in in summer. If you have big windows you have big holes in your thermal envelope. PH also is a philosophy of perfectionism, particularly around draft sealing. PH requires a level of air infiltration which is extremely low, very expensive to reach and likely to cause mold unless the house is equipped with a ventilator. I am totally pragmatic and have no ideology. I look FIRST at what saves money and makes a good return on investment. PH does not look at cost effectiveness at all. In my opinion, this is why the PH movement has really struggled to become established in over 30 years of trying. At the end of the day PH is expensive to implement and very time consuming because of their detailed audit requirements. Frankly, it is also a lot of hassle to do everything to the Germanic standards of perfection and record everything in their software package (the PassivHaus Planning Package), which is so complicated it takes days of training to get certified to use it. Also, PH makes no accounting for where the house is located (you may have noticed that Canada has colder winters than Florida but the PH standard is the same in all locations) or how big the house is. Both location and size are key drivers of energy use and energy efficiency. PH has no standard for a renovation, it only applies to new construction, which means it is irrelevant to the 99% of all houses that are already built each year. Finally, the PH emphasis on air-tight construction has led to excess condensation and mold in PH construction. My fab four recipe was developed for retrofits, but when applied to new construction it is cheaper and easier to implement than on a renovation. See my post here: https://greenzerocarbonhome.com/2020/06/can-you-use-hits-to-build-a-new-house-with-a-zero-carbon-footprint/
Also see this article on better ways to measure energy efficiency:https://greenzerocarbonhome.com/energy-and-finance-terms-explained/net-zero-passivhaus-leed-certification-zerh-and-hers/

I recently (July 2021) had a blower-door test done at my house and the result was 4.6 ACH50. ACH50 is a common standard for air infiltration and stands for Air Changes per Hour at 50 Pascals. Pascals are, like pounds per square inch, a measure of air pressure. 50 Pascals is about the pressure caused by a 20 mph wind. 4.5 ACH50 is equivalent to 1,035 CFM50 (cubic feet per minute at 50 Pascals). This means that the natural air exchange on my house (i.e., at 0 Pascals) is about 0.23 ACH (sometimes called ACH0). This means that the entire air volume of my house is replaced every four hours due to drafts around doors, windows, walls and chimneys. The natural air infiltration rate in my house is 238 CFM0.

This proves what I have long suspected, which is that it is absolutely not necessary to seal your house to the level of air tightness required by the Passive House (PassivHaus) Institute in order to cut your carbon emissions to zero. 

The Passive House standard is often held up as the ideal standard for low-energy consumption houses. But I have never seen any financial analysis accompanying this conclusion. This data proves that you can cut both your carbon emissions and bills to zero (and I am making a 15% return on investment too) without the expense of creating a very tight building envelope. 

Very few builders can build to a the Passive House standard of 0.6ACH50 and doing so often requires many hours of skilled labor plus the addition of an ERV (energy recovery ventilator) which, alone, can add $5,000 to the cost of the house. I know one such expert who recently did the air sealing on a Passive House project. He gets paid about 3x what a typical laborer on a construction site gets paid. Labor hours add up real fast at those rates! Hence, the Passive House standard for air infiltration can only be achieved at considerable expense – an investment that will never earn a return.

Much like geothermal, solar hot-water panels and thickening your walls with insulation, a super-tight building envelope makes energy sense but does not make financial sense.


Q2. Is there any certification to build Green Zero Carbon homes like Passive homes?

A2:  If you use my Zero Carbon Home consulting service then I will certify the house.

Q: about Passive House design?

A: I have a lot of concerns about Passive House designs including: the cost, mold, the use of big south facing windows for passive gain, the lack of a standard for renovations and the lack of adjusting the standards for either where the house is located or how big it is. Please see this for more details:

and this:

Q: So it is always the first step to create a high performance envelop then windows, H & C, then do solar…the battery option from eversource seems its counteractive.

A: This is not the order in which I did it, but it is the financially optimal order. First do air sealing and insulation. Second do the windows. This cuts your house’s energy use as much as possible. Then heat as efficiently as possible with a heat pump. Then build a solar array big enough to power it all (and don’t forget to add on 3kW for every electric vehicle you plan to charge.)

Q: Washer/dryers.

I’m researching the impact of line drying on total household carbon emissions for an essay I’m writing. It looks like washer/dryers account for close to 10% of household carbon emissions and that the dryer is close to 6% of that total. I also have a statistic that there are 90 million dryers, electric 75% and gas 25%, in use nationwide, which looks like an opening for those interested in household climate mitigation.

Have you given dryers thought in your own household or as part of this broader project? Do you have any statistics or where would you look to find them? I do not see the topic touched on in your blogs. 

A: I have not studied washer/dryers in any detail. Because I aim my work at everyone I have to keep things simple and for many people even thinking about the “fab four” is a lot. There are many things that work that I do not even mention, see the “20 tips” section for instance.
In my experience it is very hard to get people to change their habits. If someone likes long showers, wants to drive their car to work and eat meat it is going to be very hard to change them. This is precisely why I meet people where the are and show them (not tell them) what we did that worked. It is entirely possible to get to net zero, make money and maintain (or even improve) your lifestyle.  E.g. our house is quieter and has fewer cold spots after the Fab Four.
So, although I am certain drying clothes on racks cuts carbon emissions it is also a lot of work compared to using a clothes dryer. I think you have a greater chance at cutting carbon emissions by powering that clothes dryer with solar panels rather than hanging the clothes to dry. In our house we do both. We have 3 or 4 clothes-drying racks and still use the dryer 2-3 hours a week to get the wrinkles out and get emergency loads done – which seems to be most weeks. 
I hope that helps!


Q3. As far as cost comparison which would be less costly and more benefits value v/s money

A3: I do not yet have the side-by-side data to prove this, but since I start with cost-effectiveness I think it is likely that a Green Zero Carbon house is more cost effective than a PassivHaus. Here is one example of how Passive House leads to excessive costs:

I recently (July 2021) had a blower-door test done at my house and the result was 4.6 ACH50. ACH50 is a common standard for air infiltration and stands for Air Changes per Hour at 50 Pascals. Pascals are, like pounds per square inch, a measure of air pressure. 50 Pascals is about the pressure caused by a 20 mph wind. 4.5 ACH50 is equivalent to 1,035 CFM50 (cubic feet per minute at 50 Pascals). This means that the natural air exchange on my house (i.e., at 0 Pascals) is about 0.23 ACH (sometimes called ACH0). This means that the entire air volume of my house is replaced every four hours due to drafts around doors, windows, walls and chimneys. The natural air infiltration rate in my house is 238 CFM0.

This proves what I have long suspected, which is that it is absolutely not necessary to seal your house to the level of air tightness required by the Passive House (PassivHaus) Institute in order to cut your carbon emissions to zero. 

The Passive House standard is often held up as the ideal standard for low-energy consumption houses. But I have never seen any financial analysis accompanying this conclusion. This data proves that you can cut both your carbon emissions and bills to zero (and I am making a 15% return on investment too) without the expense of creating a very tight building envelope. 

Very few builders can build to a the Passive House standard of 0.6ACH50 and doing so often requires many hours of skilled labor plus the addition of an ERV (energy recovery ventilator) which, alone, can add $5,000 to the cost of the house. I know one contractor who recently did the air sealing on a Passive House project. He gets paid about 3x what a typical laborer on a construction site gets paid. Labor hours add up real fast at those rates! Hence, the Passive House standard for air infiltration can only be achieved at considerable expense – an investment that will never earn a return.

Much like geothermal, solar hot-water panels and thickening your walls with insulation, a super-tight building envelope makes energy sense but does not make financial sense.

Based on preliminary modeling work I have done using my integrated energy/financial modeling software, a Zero Carbon house is very close to being the lowest lifetime cost of ownership house. Lifetime cost of ownership means the combined cost of the mortgage (including any extra cost needed to add things like solar panels and heat pumps), plus the heating and electric bills, plus repair and maintenance plus insurance costs.

Q 4. Any other thing that you would like to highlight and add from your experience.

A4: Overall, I think my approach is practical and sensible. I am also completely independent and am not paid by or employed by any manufacturer or installer in the industry, so I don’t have the conflicts of interest that permeate the industry.

Structurally insulated panels and heat recovery ventilator (HRVs)

Q1. There was a question during the most recent webinar and I missed your response.  The question was what do you think of SIP’s (structurally integrated panels) – what are your thoughts about SIP’s?
Q2. BTW, I was just watching a Fine Home Building webinar entitled “Principles of Residential Ventilation,” most of it over my head, but the core message was that ERV’s (energy recovery ventilators) are the bee’s knees.  Do you have any thoughts on ERV’s (sort of outside your wheelhouse, I know)?  The sub-messages are moisture management & indoor air quality). 

A 1. I think SIPs are great, but they are only one of many ways to get insulation and structure. Walls have to perform a lot of functions in addition to holding the roof up. They must block rain, block wind, block cold/heat and block moisture that is in the air as vapor or humidity. The latter is because humidity/vapor becomes liquid water (condensation or dew) when the temperature drops. You do not want liquid water in a wall – it will lead to mold, rot and likely asthma for the family. This is not theoretical. A house in our town was condemned by the board of health after the owner was taken to the ER with asthma that was caused by mold ultimately caused by vapor condensing inside the walls that did not evaporate. See my other answers on the topic of condensation. Mold and rot, caused by condensation, are probably the most insidious problems in housing today.


Blocking rain and wind are fairly easy, this is what siding is supposed to do. However, shiplap (overlapped) siding and shingles do a great job of blocking rain and a fairly poor job of blocking wind. This is why, today, most builders install an air-tight membrane under the siding. Products like Tyvek house wrap allow humidity to pass through from the inside but block both water and wind. For Tyvek to be effective as a wind barrier the Tyvek sheets need to sealed by having their seams taped from the outside. This allows a wall to dry to the outside when it gets condensation inside it. This is exactly the same idea as in a GoreTex jacket – the fabric prevents rain from getting in but still allows humidity created by sweating to escape to the outside. Condensation in walls (and inside jackets when exercising) is inevitable but mold is not inevitable – you just need to allow that condensation to evaporate. This is the job of a vapor-permeable membrane like Tyvek. Or GoreTex. Some very high-end custom builders will now install vapor tight membranes on both the inside and the outside of the wall, which keep the interior of the wall completely free of condensation because vapor cannot even enter the wall and hence cannot condense. These membranes still allow vapor to travel from the inside to the outside (just like Tyvek or GoreTex) so, if the inside of the wall gets wet for some reason other than condensation (like a water leak) it can still dry out. This is best practice in wall construction today and is so far beyond code that most builders will not be familiar with it.

And I have said nothing so far about insulation. Best practice is to have some insulation outside the air blocking layer (e.g., outside the Tyvek layer) to prevent (or reduce) thermal bridging which is where heat leaks out through less-well insulated parts of the wall like the studs – wood is a poor insulator compared to the air trapped in insulation like foam or fiberglass. Then have even more insulation in the cavities of a wood-framed wall. This would normally be the 4” cavity created by the 2”x4” wood studs (the upright planks of wood that hold up the roof). This cavity is normally filled with insulation like spray foam, fiberglass or dense-packed cellulose. When you combine this outer layer of insulation with the cavity insulation, plus the membranes to block wind and rain, plus the siding (which blocks most of the wind and rain), plus the painted drywall on the inside you have the perfect wall. 


This “perfect wall” is more difficult to build than making a wall from SIPs, but SIPs at least get you insulation and structure. Some SIPs come with the outer surface painted with a waterproof and windproof paint with the joints sealed with tape on site. This makes a very good, but probably not quite the best, wall at a reasonable cost. At the end of the day, SIP or no SIP, your wall must must block rain, block wind, block cold/heat and block moisture. Oh, and hold the roof up. How you get there is less important than that you get there.

A2: I hope this answers your first question, but it is also a lead into to the answer to your second question. When you have a very tight building envelope, (that is, less than about one air-change-per-hour at 50 Pascals, know as 1ACH50. A pascal is a unit of pressure similar to pounds per square inch. Fifty Pascals is about the pressure of a 20 mile-an-hour wind) the air leakage into the house will not be enough to evaporate all the condensation that inevitably happens in walls. You are effectively living inside a Ziplock bag. This is sometimes what happens in PassiveHouses because PH certification requires less than 0.6 ACH50 per hour. Hence, with tight building envelopes, you need great walls that can dry effectively – see above. To make a great house that is low cost to run, low carbon footprint and healthy to live in, all these separate parts (walls, windows, ventilation, insulation etc) all need to work together. You can’t fix one without fixing the others. 

So to provide the fresh air you need when you have a tight building envelope, you need ventilation. Heat recovery ventilators (HRVs) bring air into the house through a pipe rather than through gaps and cracks in the walls. The air that comes through this pipe is warmed up with the air that is leaving the house. This gives you fresh air without losing all the heat. If you have a very tight building envelope a ventilator is not really an option, you need to have it. An HRV is better than just an air pipe. I have never installed an HRV because retrofits are almost impossible to get to 1 ACH50 and my consulting work so far has been entirely on retrofits. So neither I, nor my clients, have ever needed an HRV. So, I think that HRVs make good energy sense and you really need one if you are going to have a very tight building envelope. Whether they are cost-effective is another matter and I do not know the answer to that. I would definitely ask the installer how much energy and money they will save you and then calculate the payback period. Payback periods on heat pumps, insulation, triple-glazed windows and solar panels range from  about 1 to 9 years. When I did some back of the envelope calculations on the amount of money I might save with a HRV I found that I could generate the energy I need at less money with extra solar panels than I could save by installing an HRV.

Can you use HITS to build a new house with a zero-carbon footprint?

Q: Does HITS apply to new construction?

A: The HITS recipe makes it fairly easy to make money by cutting your carbon footprint dramatically on existing houses. It is far easier to do the same on a new house. This is because it costs very little more to install 6” cavity walls and fill them with insulation than it does to install 4” cavity walls. If the sheathing (plywood) outside layer is made from structurally-insulated panels (which are boards made of an insulating layer like a 2” ISO board glued to a plywood structural layer which is painted in the factory to have a water-proof and vapor-proof layer on the outside which then has the seams between the panels taped and sealed on site) you can prevent water penetration, prevent vapor penetration, prevent drafts, gain rigidity and increase insulation in a single installation. This takes far less labor time to install than it does to install each component separately. Adding triple-glazed low-E windows costs only a few % more than double-glazed windows. 

Hence, it costs very little extra to build new a house with an excellent thermal envelope that will dramatically cut the carbon footprint and heating bills than it does to build a standard house. 

Since the heating and cooling loads in a well-insulated house are far lower than in a code-built house, the house probably needs smaller heat pumps to heat and cool the house, which saves money compared to a standard house. 

If the house is designed to have one side facing south with no shade, then the roof can generate all the electricity needed to cut the carbon footprint and energy bills to zero.

Estimates of the additional cost to build a zero-carbon house above that of a standard (“code-built”) vary from 0% to about 5% without the solar panels and 5-10% with the solar panels.

The moderator on our call built his own house with 10” thick walls. He heats it entirely with a single air-sourced heat pump, even in the depths of winter in St. Louis. He powers the entire house with solar panels on his roof. He pays no utility bills. 

While I have not built one of my own, I think the ROI on newly built zero carbon, zero bills houses can be excellent.

Q: I’m building a new home.  Where would you invest to get the best impact?

A: Thick well-insulated walls, a reasonably air-tight envelope (say 3.0 ACH50), low-E triple-glazed windows, a ducted heat pump system and solar panels on an unobstructed south-facing roof. 

Q:  How do you address a client who is concerned about shoddy contractor workmanship (possible roof leaks, breaking something else while installing, etc.) (note this question was asked by one of the volunteer coaches that I am currently training.)

A: I hope that we can provide a list of approved contractors because I agree this is a huge problem. Some people have even said they would hire me just to get my list of contractors! We aren’t going to make saints out of sinners, but I think we can weed out the awful ones. I also recommend getting warranties from the installers. Many solar installers do this, as do the bigger HVAC companies. Window manufacturers and some window installers will give warranties and I think it is unlikely that you would get a meaningful warranty from an insulation installer, but the MassSave program does provide at least a small level of quality control.

Q:  Client informs you that he plans to downsize in 2-3 years.  How does all this work and spending make financial sense?

It can still make sense, in two different ways.
First, there are many things that are so cheap and so effective they will pay for themselves in under a year. See the cheat sheet for examples. Also see the small book (called COVID19, special stuck at home edition) I wrote on exactly this which which you can download from this website for free using the code COVID19

Second, doing even the big things like solar panels and heat pumps can more than pay for themselves by increasing the house price. Very well-documented academic studies on tens of thousands of houses across the US show that houses with lower utility bills sell for more than comparable houses with higher bills. The rule is $20 of house price increase for every $1 in annual utility bill savings. Separately, Zillow data shows that houses with heat pumps sell for between 4-7% more than those without heat pumps and houses with solar panels (regardless of the size of the array) sell for 4% more than comparable houses without solar panels. These house price increases will often be more than the cost of installing the systems (after the tax breaks and subsidies). This means that installing the fab four is actually a very good strategy to increase the price of your house before you sell it. 

5.  Your method makes sense for a home that is a good solar prospect and has forced air fed by a furnace, but how about if a client’s home has neither?

A: Heat pumps can work just as well for forced-hot water radiator systems as forced-hot air ducted systems, see answers on this in the Heat Pumps section.  Insulation, draft sealing and better windows will work for everyone regardless of how they heat their house. Having a good solar location is a huge plus but even roofs in 50% shade (like my garage) can still produce electricity at 10-12c/kWh which is half that from Eversource or National Grid. If even this is not possible for the client, then the next best option is to get community-sourced solar which is available with 100% solar electricity generated in MA for a 12.5% discount to utility electricity making it about 20c/kWh. This is much more expensive than roof-top solar at 5-8c/kWh but still low enough to make a ducted heat pump a cheaper way to heat than burning heating oil or propane, though it will still be more expensive that burning natural gas. 

What is the source of the $20 house-price increase for every $1 in utility bill savings?

Q: Can you link to the DOE study on home value? 

You can download it here: https://www.thefreelibrary.com/Evidence+or+rational+market+valuations+for+home+energy+efficiency.-a021276611

It was published in The Appraisal Journal in October 1998 and authored by Rick Nevin and Gregory Watson. Their conclusions are based on Census Bureau data on 49,000 houses across all states and with all types of heating fuel. The studies were done over several years in the 1990s. Remarkably similar results were obtained by different researchers using different data sets in both the 1970s and 1980s. 

Q: for this DOE study of $20 gained for $1 savings – is that $1 per annum or $1 per month? 

A: It is $20 for every $1 in annual bill savings.

Q: Where do you find state/federal subsidy programs?

A: www.dsireusa.org

Q: Is there a website you recommend that is current with all of the various 2021 Massachusetts / Federal green energy incentives? (I know the 28% solar Fed credit was just extended for 2 years!). Related question, can you recommend a local tax advisor who could answer questions regarding green energy tax incentives/tax credits?  

A1: Yes: www.dsireusa.org

A2: I do not know anyone locally.

Q: I just replaced my natural gas furnace so I don’t want to do heat pumps. Does it still make sense to do the other 3. If so, in which order?

A: Yes it does. Insulation (and draft sealing), and triple-glazed windows (either replacing the whole window if the window is rotten or adding window inserts if the window is in good shape) will cut your energy bills not matter what the source of heat is. I suggest doing insulation and draft sealing first because they have the highest return on investment and often pay for themselves in a few months. Windows take longer to pay for themselves. The incremental cost of triple-glazed windows over the cost of double-glazed windows will pay for itself on our house in about 6 years. The cost of window inserts pays for itself in about 5 years – see chapter 3 in Zero Carbon Home for details. After that, I would do enough solar to offset your entire electric bill. If you are thinking of getting an electric vehicle or two then add on about 3,000kWh a year for each car. In most places this would be about an additional 3kW of solar panels or about 8 additional panels. This will get you about 12,000 miles per car at around 2c per mile. A gasoline car doing 30 mpg costs about 10c per mile. An average roof in MA can generate 3,000kWh per year from a 3kW array which will cost you about $8,000 before subsidies and about $3,000 after the federal and MA subsidies.

Q: In what ways do you cut your non-home carbon footprint, such as that from traveling, driving, products you purchase, etc., if at all?

A: We work hard to minimize the carbon footprint we create from all sources not just our home and swimming pool, both of which have zero carbon footprints. Any carbon footprint from air travel, selling paperback versions of Zero Carbon Home and selling the T-shirt  is offset with audited, verified-incremental, carbon offsets that we buy from Cool Effect. 

We are big recyclers. We buy only organic food in the first place. We throw out almost nothing. Any edible waste goes to our chickens. The chickens fertilize our garden making our fruit and vegetable gardens very productive. We have done a taste test of our tomatoes compared to the most expensive, local and organic tomatoes from Whole Foods Market and ours tasted far better. The same was true for our peaches, it was literally no comparison. The chickens give us eggs and meat. So, we eat very well. We are not even close to being self-sufficient and do not aspire to being so. But we do love the taste of asparagus in April, rhubarb in May, tomatoes and peas in June, cherries in July, peaches in August, just about everything in September, apples in October, pears even into November and fresh eggs year-round. Last October, I succeeded in transplanting peppers and tomatoes in pots to be grown indoors (growing under LED grow lights powered by my solar panels) and we were eating them up until Christmas. Anything the chickens won’t eat (onions and citrus for instance) gets composted as do all our used paper tissues. Almost everything else such as paper, glass, metal and plastic gets recycled and we trash only about a single 50-liter (13-gallon) kitchen waste bag each week.

I bought a Tesla this year, which I charge from my solar panels which means it costs 2c per mile compared to my old SUV which cost 15c per mile on gasoline. The Tesla, when charged by solar panels, has a zero-carbon footprint. This covers most of our local travel, but we still have two gasoline-powered cars. When they die they will be replaced with EV’s too. My wife recently drove the 400 miles to New York City and back in the Tesla. It cost $8. the Greyhound costs $26, and it emits pollution and CO2 every mile of the way.

When we buy things, we buy almost always local and sustainable. For examples:

  • For clothing we only buy organic, mostly cotton and almost all grown and sewn in the U.S.
  • For food we buy only organic and usually U.S.-grown only though we do make a few exceptions for some rather excellent Swiss cheese and Italian balsamic vinegar. I used to drink mostly French wine but now drink mostly Californian organic wines. We have visited farms that provide many of our favorite foods like tomatoes grown at Longwind Farm in Vermont, cheese made at Gray Barn Farm on Martha’s Vineyard, and blueberries grown at D’Ottavio’s farm in New Jersey.
  • For construction products (wood, paints, door hardware and light fixtures) and furniture almost everything we buy is made in the U.S. including many made in New England. We buy a lot of construction products because we are renovating, or have renovated, three properties. The wood that will become the flooring in the extension that we are currently building on our house will come from trees that fell down on our land. We had these trees sawn into “1 by” dimensional lumber, and they are currently drying out. Recently we received the first batch of wide pine boards that will become flooring in our extension.
  • For cars, our Tesla was made in the U.S., the first American-built car we have ever bought. Before this, we bought only BMW and Mercedes.
  • We buy almost no gasoline or heating oil and we buy zero electricity as everything is powered by U.S. sunshine. The heating oil we do buy is BioHeat30 which is 30% vegetable oil.
  • For air travel, when we do travel by air (which we have not done so far this year, but not by choice) we offset the journey with carbon offsets. Any remaining purchases of gasoline, heating oil and electricity are zeroed out with certified incremental carbon offsets each year as Christmas presents from me to the other family members.

Q: If you purchase carbon offsets, how do you decide where to buy the carbon offsets from? What do you look for when purchasing carbon offsets? What are your purchasing criteria?

A: I am quite skeptical of the claims of many types of carbon offsets especially those that depend on planting trees in the Amazon. Trees grow really well in the Amazon all on their own. Hence it is hard to say that planting trees is removing more carbon dioxide than nature would remove by herself. I buy my carbon offsets from Cool Effect precisely because they are audited to be incremental. Even then, I buy only carbon offsets from a project that captures methane that would otherwise leak into the atmosphere from exposed coal seams on the Ute Indian reservation in Utah. This is genuinely incremental, and it is supporting U.S. jobs and Native American tribes. 

Q: If you have purchased carbon offsets, what prompted you to purchase them at the specific moments you’ve made the purchase? How did you decide how many offsets to purchase? With what frequency do you purchase offsets? 

A: I do it annually to offset any secondary carbon footprint we have from both cars and air travel. I also offset the carbon footprint of any paperback books I sell (the vast majority are sold as e-books) and any T-shirts I sell in the Zero Carbon business. The T-shirts have the lowest carbon footprint possible because they are made from unbleached, un-dyed organic cotton grown and sewn in the USA. However, I still offset the small carbon footprint they still have.

Q: How accurate is your house energy model compared to tools like Rem/RATE?

The integrated energy/financial model I use I built myself. There are lots of software packages out there for calculating the energy performance of buildings but the reviews of them are terrible. They have a very poor track record of predicting the real-world energy performance of any particular house. See this review I wrote:

Zero-Energy Ready Home (ZERH) and Home Energy Rating System (HERS) 

The Department of Energy offers its Zero Energy Ready Home (ZERH) program but it is more aimed at certifying builders rather than buildings. Hence, just like the PassiveHaus and LEED programs it is focused on new construction, not how to go zero on your existing home. The ZERH program relies heavily on EnergyStar standards for appliances and windows and the HERS (Home Energy Rating System) for performance. HERS is focused on energy use relative to a benchmark house (i.e., how your home compares to a model house of the same floor area)  rather than minimizing energy or spending. A HERS rating is only available on new houses, not for existing ones. A review of the HERS rating system in Home Energy magazine found that,  in practice, “there was no clear relationship between the rating score of an individual home and actual energy cost.” Hmmm.

My model began with simple curiosity. I began with just correlating (drawing a line graph) the actual energy (heating fuel plus electricity) that was used every day in my home (I have two years worth of daily data) and the average outside temperature. The r-squared (statistical correlation) of these models (there were 5 of them – one built each time I added one of the fab four) was over 80%. This is a very high correlation for a model that left out known influencers of energy demand like solar heat gain and drafts. Nevertheless, despite these obvious weaknesses, the outside temperature was by far the biggest driver of energy use and hence energy bills. This was an “ahha” moment.


In a separate “ahha” moment I realized that the u-value for windows was not just an arbitrary scale (unlike say a HERS rating) but actually was the rate of energy flow across the window. Since I could approximate the R-values of all my walls, attic and basement, (and the u-value is 1/R value) I could build a mathematical model of how the energy flowed out of my house. Basic physics requires that over any period longer than a few hours, the energy flowing into a house must equal the energy flowing out. This allowed me to build a predictive model of how the energy flows into and out of a house. Since I know the energy flowing in (the combined energy in the electricity plus that burned as heating fuel) I could anchor the model to reality before we even got started. Hence, the inaccuracies in my model are going to be in allocating where the energy flows out (e.g., I might over overestimate the energy flowing out through the walls and underestimate the energy flowing out through the attic) but the overall amount of energy lost must be correct because energy cannot be created or destroyed, the energy lost by your house must be equal to the energy you put in. Put another way, if you cut off the electricity and turned off the heating, your house would eventually reach the outside temperature.


As far as I know, all the other software modeling packages out there start with modeling the thermal envelope of the house. They then model the heating inputs and then hope that they have got it right. One of the most respected models out there is Rem/RATE. I have repeatedly asked the owners of this software for data on how accurate it is in real-world situations. They have never answered my questions. The most that they were willing to say is that “it meets the standards”, but could not even tell me what the standards were. There is almost no published data on the performance of Rem/RATE software. The only data that I have been able to find is the following chart from 2009:

https://www.energyvanguard.com/blog/41919/How-Accurate-Is-REM-Rate-as-an-Energy-Modeling-Tool


Although the average prediction of the model (which is not plotted on the graph, the straight line is what at 100% r-squared would look like – clearly the model is no where near 100% r-squared, and they never publish this most basic statistic) it is obvious that there is enormous error in the predictions for one house vs another. In some cases the error is equal to the mean value! We use models to predict the heating or cooling load for one individual house, not the average of thousands of homes. Hence, I really doubt the value of Rem/RATE for helping homeowners cut their bills and carbon footprints. The HERS rating systems (which is reviewed negatively in the Home Energy magazine article that I quote above) is built on the Rem/RATE software.  I, and Home Energy magazine, are not the only one with these concerns, see this quote below from the same article:


“Of course, modeling older homes and heating, water heating, lights, and appliance loads is a different matter, and the divergence between modeled and actual energy consumption may be quite different. According to Blasnik, “I know from experience that many energy modeling tools—REM included—often do a poor job of modeling heating loads in older, leaky, poorly-insulated homes.”

 And yet, cutting energy use on “older, leaky, poorly-insulated homes” is exactly the problem we need to overcome! 


So I built my own model, and it not only predicted my actual annual heating bills to within 10% of the actual bills (the most current model is accurate to 5%) but it has proven itself in practice with all of my consulting clients. It has enabled me to make predictions of the real-world impact on both the energy use and the financial bills for actual changes to that home like adding insulation or adding triple-glazed windows. This is why I do not use any off-the-shelf energy modeling software. They simply have a poor track record of predicting real-world energy and financial performance on houses such as those most people live in. 


If you use one of these software packages please let me know how you get on. I welcome any feedback that I can use to improve the model.

Q: Did you invest/change the controls philosophy of the property? For example by adding more thermosts in the property to creat heating / cooling zones.

A: No, we did not. In our home have the same zones before and after the fab four. However, in our rental property where we installed the ductless mini-split units we have eight zones and there were only three before.

Q: We live in Boxborough, MA where a municipal electric company has a very low rate, abut 12 cents Kwh. Makes solar very expensive because the pay back is about 20 years, discouraging people from puttng solar on roofs. Any ideas about making it worthwhile, aside from being a renewable?

A: With cheap electricity already you should focus more on installing heat pumps, a heat pump hot-water tank and an electric vehicle rather than adding solar panels.

Q: Any thoughts on phase change materials for thermal storage?

A: Phase changes (like water turning to steam) can store a lot of energy. But with net metering for solar panels you can effectively store the energy for free. You generate it in summer and build up credits that you can pull down in the winter. 

Conflicts of interest

Q:Aren’t selling your book and consulting service a conflict of interest?

A: The vast majority of what I do I do for free. The webinars are all free. Over 1,500 people have attended them. I know of at least 10 people who have decided to cut their carbon footprint (usually by installing solar or heat pumps) as a direct result of these webinars. None of these people are clients of my consulting service. These are just the ones I actually know of. There are probably far more. I believe the webinars are very effective at cutting carbon footprints, which is precisely why I do them frequently. This quote was recently posted on a Next-door Dover website by someone who attended one of the webinars “David is a true inspiration and asset to our community. He inspired us to get solar panels and a new front door. Next up, a heat pump. It’s so amazing that you can do good for the environment AND your bank account at the same time.”


In addition to offering free webinars, I give away the book (it sells for $15.99) to attendees at the webinar. I write up and distribute, for free, the Q&A from the webinars because people find this very useful. The webinar is on YouTube and available for free. I have edited the video into 6 chunks to make it easier for people to view what they want rather than have to watch the whole thing. I answer a lot of questions from people by email for free. My entire website is maintained at my own expense, there is an enormous amount of helpful material on the website and it is all available for free. 


The consulting service also may not be what you think it is. I have 8 clients today. The majority are elderly, single and living alone. They want to cut their carbon footprint (and bills) but are afraid that they will make an expensive mistake (one client came to me after he realized he had made a mistake) or fear being ripped off by contractors. Having someone who is on their side (I treat all my clients as though I am their fiduciary who must do the right thing for the client even if it is against my personal interest, i.e., the same way a doctor or lawyer treats a client) who gives independent advice. I do not work for or get paid by any installer of equipment or services. As you probably know, contractors sometimes have a tendency to exaggerate the benefits of their products. I honestly believe that these clients will not cut their carbon footprints without having someone they can trust who is on their side. It takes me about 40 hours of work to help a client cut their carbon footprint. This works out to getting paid about $50-100 an hour. If I was in it to make money I would sit on the boards of public companies, which I used to do, because that pays $500 an hour. If I offered this level of work for free I would be swamped with people wanting free advice, which many people would not act upon precisely because it was free. One of my clients said he was sick of getting stuff for free and he wanted to pay someone who knew what they were doing. His words, not mine. The consulting service is a lot of work for me and, like all consulting work, it is sometimes frustrating or ineffective. I have two clients, who I have spent a lot of time with, who have done nothing at all. This is a waste of my time, but it is not a waste of their money because they have paid me nothing. I only get paid on results and only after the client has actually saved the money. It takes two years for me to get paid. So far, for two years of consulting work I have actually been paid a grand total of $100. Well, while I was writing this email my son brought me a check for $300 that just came out of our family’s 3-day mail quarantine period. So now I have been paid a total of $400. 


It is because people can be uncomfortable with a conflict of interest that I explicitly state the ways in which I get paid for things (like books and the consulting service) in the webinar. I believe this discloser is essential. It could hardly be called advertising. I have tried very hard to avoid any conflicts in my approach to cutting carbon footprints, and where there can be perceived to be a conflict, I disclose it upfront. I believe that sunlight is the best disinfectant. 


However, I understand how people view possible conflicts of interest as a non-starter. But I am not willing to give up on these clients because, if I did, I think they would just carry on paying excessive heating and electricity bills and emitting CO2 into the atmosphere. So if you don’t feel comfortable promoting this event I am totally fine with that. Really, I mean it. We are all in the same boat and rowing together to try to save the planet from global warming and there are many different approaches to doing so. Mine is only one. Also, if you have any suggestions for things I can do that would make you comfortable with promoting the webinar, then I am very open to suggestions. My goal is to cut CO2 emissions. That is all. I am sure we agree on that. 

Q: All your information is very useful. Under what circumstances wuld we need a consult with you?

A: My advice in the books, webinars and Q&A is intended to empower people to act on their own. I do not do all this an advertisement for my consulting service. I have found that consulting to clients is very time intensive and that I can cut more tons of CO2 with every hour I spend on a webinar than I can with every hour consulting to a client. There is only one of me. However, there are some people who are not comfortable making these types of decisions on their own and want someone to help guide them through it. I can help a small number of people this way. If you are convinced that you need to take action to cut your bills and carbon footprint but are not ready to act because you are concerned that you might make an expensive mistake or that you might get taken advantage of by unscrupulous contractors (they have been known to exist) then it might make sense to ask for my help. But if you are comfortable hiring contractors to get things done in your house then I suggest that you just follow the recipe in the book.

Rapid-fire Q&A from an engineer (on multiple topics):

Q1. In the latest E&T magazine published by the IET they highlight a major reliability problem with solar panels which included a special report published by NIST. It concerns the loss of efficiency after 3 years and fire problems. Have you had any problems with your solar panels?

A1: I have had one panel replaced under the warranty and one inverter replaced under the warranty. I was not happy with the response times but they did it and did not charge me anything. I have never heard of either of the problems you mention.

Q2. It has been reported that placing solar panels on the roof (the resulting additional weight) has bowed the roof structure. Have you seen this problem?

A2: This has not happened with any of the 3 solar arrays I have. Each installation had a letter from a structural engineer saying the roof could take the load, including snow.

Q3. Does adding solar panels to the roof meet you local building code requirements or was additional structure work required?

A3: Yes on one, no on two.

Q4 Will the addition of solar panels on the roof cause any problems with high wind condition (say 110 mph) experienced under hurricane conditions.

A4: We never get winds this strong in MA. I know you do in Florida. I would ask local solar installers.

Q5. I assume you get snow and ice forming on your roof in winter time. How do you handle these problems?

A5: I just let it melt off when the weather warms up. The electricity production is very low in winter so missing a few days due to snow cover loses me very little money. I would be getting paid less than minimum wage if I went on the roof to clear the panels.

Q6. During fall conditions tree leaves and other material collects over the solar panels. How often to you have to clean them?

A6: I have never cleaned them. Modern panels with microinverters do not have the problem of older generation panels that would almost shut down if a single leaf landed on them.

Q7. I assume you use asphalt shingles on your roof. What happens say every 15 years when they have to be replaced?

A7: I replaced one roof (which was about 20 year old) before putting the solar panels on. The other two roofs were about 5 years old. The solar panels extend the life of the roof because the sun is no longer beating down on the rubber or asphalt. It is the sun that deteriorates the asphalt tiles, not the wind and rain. Even so, the panels will last longer than the roof and it will cost me a bit more to pay someone to take off the panels before the roof is replaced. I don’t expect to happen for at least 25 years.

Q8. What is the highest voltage developed by the solar panels when charging (example 400 volts DC)? Is this a safety problem in the event the house catches fire? 


A8: The max voltage is about 400VDC. There is an emergency disconnect switch that the fire department can use (it is a requirement in MA) to prevent the electrocution risk of soaking the roof with water.

Q9. How has installing solar panels impacted your house insurance?
A9: It has not.

Q10. You made no mention about powering water heater in your house!
A10: We use a heat pump hot water heater. It is detailed in the book.

Q11. You made no mention of changing over to LED lighting in the house!
A11: We did this, I need to keep to the bigger issues in the webinar, but the details are in the book.

Q12. You made no mention of using carpets in your house!
A12: We have hard wood floors throughout the house. Adding fiberglass to the ceiling of the basement is a lot cheaper, and a lot more effective, than adding carpets.

Q13 Regarding the heat pumps, what is the lowest outside temperature in your area and do you need to use use supplemental electric heating below freezing?

A13: Below about 20F outside neither the Bosch ducted system at our house nor the Mitsibushi ductless system at our rental property can keep the house at 70F without supplemental heat.

Q14 Do the heat pumps use just air or a deep ground water piping system?
A14: All our heat pumps are air-sourced. Geothermal is 4x the cost and does not pay for itself around here.

Q15. With solar power on the roof do have a higher risk of lightening strikes? If you continue to have a power company connection, is it on a pole or buried in the ground?

A15: I am not aware of this risk. The panels are grounded. Both houses are on underground feeds so the panels are below the utility wires on the poles, this makes the poles the highest conductor around so they would likely get hit first. 

Q: 16. Women like to cook on gas stoves for a more even heat. Do you have a gas connection to the house?
A16: We do not. I was brought up cooking on gas. After we moved in to our current home, we cooked for 20 years on an old GE electric ring-burner stove. We replaced it with a induction stove and I would never go back to either gas or ring-burners. Induction has perfectly even heat across the pan, which means never getting a ring of burnt food on the bottom of the pan. I also like having no source of ignition in the house. I think our house is much safer without gas.


I look forward to answering any further questions on the call.

More rapid fire questions from someone who is an engineer, has a lots of experience in energy conservation in homes, and is one of the first to get certified as a Zero Carbon (R) coach

Q1: The price of elec makes it uneconomical to go heat pump w/o solar. 

A1. Yes but it is very economical with solar and most houses can generate electricity at 5-10c/kWh (even with some shade on the roof). With a COP of 2.5 (the average in NE for ducted and ductless combined, my Bosch ducted system is about 3.0) the breakeven electricity price at which solar is cheaper than heating with natural gas (not an option in Sherborn) is 12.5c/kWh so most people can save money with a heat pump and solar even if they are on natural gas. With heating oil the breakeven at COP 2.5 is 20c/kWh which almost any roof can do. If your roof is in total shade or is slate or you think solar is ugly then you can buy 100% solar electricity made in MA for about 20c/kWh. This means that using a heat pump can cut your carbon footprint to zero at same cost as heating with heating oil. This won’t save you any money but it will cut your carbon footprint to zero and you will not have solar panels on your roof.


Q2. Despite what was discussed last night if the client thinks solar is ugly…it’s ugly. 

A2. Yes.. See above.


Q3. Geothermal is not feasible forcing the client to continue maintaining the existing system. 

A3. Agreed, geothermal rarely makes financial sense unless you can get the wells drilled for cheap. I am currently investigating if this is feasible on an existing well on our rental property in Dover. I will keep you posted.


Q4. Many ducted AC systems are in the attic outside the envelope. 

A4. Yes but if the sloped sides of the roof are insulated you have brought it inside. Fiberglass is dirt cheap and you can install it yourself or get an installer which MassSave may pay for.


Q5. Window incentives are only available when replacing single pane. 

A5. Yes. This is why I only recommend adding triple-glazed windows when the existing windows fail. Then the incremental cost of the new triple glazed over the cost of new double glazed usually pays for itself in a few years. This is how we did it on our house and clients of mine have done it this way too and they are very happy with the results. Not just lower bills and carbon footprint but fewer cold spots in the house.


Q6. Based on my experience and our discussions a Mass/Save audit is iffy at best.

A6. I agree I have had several done and some were next to useless. That is a government program and suffers from those usual ills. I use it only as a gateway to get the 0% heat loan. This is one of the reasons I think coaches can be so powerful, there is a real shortage of high-quality independent advice.


Q7. It is not possible to power a boiler with a heat pump unless the house is 100% radiant. 

A7. I am genuinely not sure about this. I have not done it myself so I have no direct experience. Manufacturers claim it can be done, but I am wary of their claims. I am about to install an ASHP to power a radiant system. However, I do not see having an existing forced hot water system as a big obstacle. Heat pumps do not replace boilers and radiators. They replaced AC units. At least that is my approach. I have never recommended to a client to take out their existing heating system, forced hot air or forced hot water. If the house has ductwork for AC already then that can be reused for heating. I always recommend leaving the boiler and radiators in place anyway as a back up for when the heat pumps can’t keep the house warm on their own (which is about 20 days a year on our house and our rental house). I am going to be replacing ancient AC units with heat pumps at the rental property and am going to get quotes for using both the radiators and the ducts for distribution. So watch this space! If the house does not have ductwork for AC then it can sometimes be added at reasonable cost from the basement to the first floor and from the attic to the top floor. If none of that works then mini-splits can make sense. We added mini-splits to the other rental house and they work fine for both heating and cooling, though they were about twice the cost of the ducted system at our home.

Q: By insulating your home and adding triple glazed windows you are significantly reducing your carbon footprint to nil but a side effect of a tighter home is a degradation of indoor air quality. Have you done anything to upgrade your ventilation, filtration and humidity control systems to ensure a optimal level of IAQ (indoor air quality)?
A: We use standard paper-based air filters in the air-handler units and humidifiers/dehumidifiers with that are integrated with the air-handler units but other than that we have done nothing. We monitor our indoor-air quality using a meter from Foobot (it cost about $100) and it is consistently excellent. Not burning fossil fuel greatly improves the indoor-air quality as does cooking on an induction stove.  

Q: As an average, how many apartments can you stack up with the same (shared) PV roof area and still cover the energy needs?

A: I can’t really answer this without doing a lot of modelling of the actual apartment building. However, at a very rough guess, I would say two to three. This assumes that the external envelope is well insulated and there are good windows without drafts.

Q:  What about lack of air circulation in the house? Don’t you need some fresh air?

A:  Yes. Please see other answers on this topic.

Q: The financial impact of heat pumps and solar panels in New Hampshire.

I watched your Arlington webinar on 4/8 and have a few follow up questions.

1. Sustainability of Net Metering
I asked a question about the longevity of the net metering program and Lori framed it as somewhat of a political question. I was trying to ask a question more about energy markets and energy storage. If enough people get solar, the supply of solar electricity at the time of production may exceed the demand. As far as I know, there is no cost effective way to store this excess energy on a large scale. Without storage, the excess energy becomes worthless and as you said, the financials of the net metering program will fall apart. 

Here’s a link to one article: https://pv-magazine-usa.com/2016/12/08/report-finds-net-metering-is-not-sustainable-over-the-long-term/
I suppose my questions would be: do you know of any data that suggest a time line for the decrease in value of solar energy? I guess most importantly would be whether the net metering program will change before the pay back period of the system (say 10 years for talking purposes). Or maybe there’s data that say even if there are solar panels on every roof, the production will still not exceed the daytime energy demand, in which case the net metering program can be considered more or less safe. 

I think there are two questions here: the first is for how long is net metering (as currently practiced in MA) sustainable and the second is can excess solar power be cost-effectively stored at grid scale?


A: The economics of net metering are very unfavorable to the utility company. By law they are forced to buy solar electricity from homeowners at the full retail price (in MA about 23c/kWh) when they can buy it from a power station at about 8c/kWh. This is a subsidy from all ratepayers to those ratepayers who have solar panels. This can only work as long as the amount of solar power subject to net metering is small compared to the total amount of electricity being consumed. This is true today but is becoming less so as more solar power is installed on rooftops and more commercial-scale solar panel farms are being constructed to supply community-sourced solar power as an option to ratepayers. Eventually the utilities will seek to negotiate less favorable terms for net metering. This is a negotiation with the government and hence things will move slowly. I do not expect net-metering to go away, probably ever, but I do expect to see its generosity to the homeowner decrease over time. The net-metering credit rate in MA is already cut back to 60% of the retail value for arrays over 10kW in size. It also already excludes some taxes and fees and  it excludes the fixed charge of $7 per month. In other states, net metering is already far less generous than it is in MA. Of all the subsides for solar panels in MA, the federal tax credit, the state SMART subsidy and the net-metering subsidy, net metering is the biggest of all three. 


On the second question about can excess solar electricity be cost-effectively stored at grid scale the answer is yes. Both California and Australia have installed very large batteries to store excess electricity. In MA Eversource (our utility company) is doing something similar by paying homeowners to have access to their home-based back-up batteries during times of peak demand on the grid. The program is called Connected Solutions and it seems to be quite generous. I have recently placed orders for two new solar panel arrays each with batteries to replace broken propane back-up generators. When a battery is installed with fairly large solar panel arrays (over about 10kW) the combination now pays for itself in seven or eight years. 

Q2. Heat Pump Efficiency


I asked a question about estimating the required electricity to run heat pumps and your answer was to calculate the heat energy contained in the oil I burned this winter (in kWh) and divide by 2.5. This makes sense to me since a 2.5 COP corresponds to a heat pump HSPF of 8.5, which I believe is fairly standard. I’m a little confused by the section of your book where you talk about heat pumps having 400% efficiency, or COP=4. Is this a theoretical value that maybe does not account for the electricity required to run the fans? Or is this based on a very efficient heat pump with HSPF=13.6?


In the book I do refer to heat pumps being 400% or 4x as efficient as a furnace. This is true for a heat-pump hot-water heater and a heat pump for heating a swimming pool. When I wrote the book I thought it was also true for heat pumps for heating the air in a house. However, since writing the book, research has been published by well-regarded scientists that shows that the year-round average COP in New England is about 2.5. In the webinar I now use 2.5, not 4.0. When I measured the real, year-round COP on my two houses the one with the Bosch heat pumps has a COP of about 3.0 and the one with the Mitsubishi heat pumps has a COP of about 2.5. This is very similar to what was found by the academic researchers i.e., that ducted systems were more efficient than ductless systems and that Bosch was more efficient that Mitsubishi. These numbers include the electricity to run the fans as well as the compressor.

Along the same lines, you talk about how it only makes sense to heat using heat pumps if you have cheap solar electricity. However, if utility electricity in MA is 23c/kWh, doesn’t that mean you’re making heat with heat pumps for 23/2.5=9.2c/kWh, making it only slightly more expensive than heating oil (6-7c/kWh)? 

I agree with you that the cost of heating the house is 23c/2.5 = 9.2c/kWh of heat in the house. Heating with heating oil (which contains about 40kWh of heat per gallon) at $2.59 a gallon (what I am currently paying) with an 85% efficient furnace (better than my dinosaur furnace) means that heat in the house costs about 8c/kWh. So paying 9.2c/kWh of heat in the house by using heat pump on utility electricity is about 15% more expensive than heating with oil. If you can achieve a COP of 3.0 (as I do with my Bosch ducted heat pumps) then the cost of heating the house is almost exactly the same as heating with heating oil. For me it was actually a saving because my dinosaur furnace is only about 75% efficient. However, even if the running cost is the same, you have to buy a new heat pump whereas you already have an existing furnace. This is why I recommend you install heat pumps when your AC units fail not when your furnace fails. Replacing a broken AC unit with a heat pump is about 40% are expensive than replacing it with a new AC unit. But now you get heating and cooling. However, leave the old furnace in place so you have a back-up heating system should the heat pump not be able to heat the house in very cold weather and so you have a heating system during grid outages. Note that you will still need either an back-up generator or a battery so that you can run the pumps and fans on electricity during the grid outage. If you don’t do this you will have a hot furnace and a cold house. 

3. Solar Financial Feasibility
I am having some trouble getting the financials of solar to make sense. I am using the numbers you  provided in your book as a general measuring stick (ie the array should produce at under 10c/kWh, and you have seen quotes down around 5c/kWh). I am in NH, and it seems one of the huge differences between MA and NH is in how SRECs are handled. I spoke with an installer today who said the current value of an SREC is NH is $5. Over 25 years, this adds up to about $1200, which stands in stark contrast to the $29000 in your book. (I have read a few articles about how the program is broken in NH because utilities are allowed to collect unclaimed SRECs for free, which depresses their value). Without this benefit, it looks impossible to me to get the solar electricity cost down to 1/3 of the utility cost, which is around what you quoted in the book. 

The value of an SREC in MA is currently about $250/MWh so it is very different to NH!

Here are some numbers, which are roughly accurate. I just received a quote for a 11.1kW system for $27000 (after the federal income tax credit). Production over 25 years is around 238,000 kWh, which works out to be 11c/kWh. 

Current utility rates from NHEC are around 15c/kWh and the net meter rate is around 10c/kWh. Assuming the system produces enough power that there are always net meter credits to work with, the cost of power when the system is overproducing is just the system cost (11c/kWh) and the cost of power in the winter when the system is underproducing is the system cost plus the difference between the utility rate and the net meter rate (11+(15-10) = 16c/kWh). When using heat pumps, a large % of annual power consumption happens in the winter. As far as I understand it, this power actually costs 1c/kWh more than the standard utility rate. But maybe this really doesn’t matter since, as discussed above, if I’m using a heat pump with COP=2.5, then the cost of heat is 16/2.5=6.4c/kWh, which is basically the same as oil. One of the points you make in the book is that the cost of solar electricity is fixed. However, if you are using a lot of electricity through the net metering process, you still have to buy this electricity at the utility rate. 

NH has much lower utility rates than MA. This is in part driven by all the subsidy schemes like full-retail-price net metering, SMART and Connected Solutions which drive up the price of electricity for all the MA ratepayers who are not taking advantage of the subsidies. So you are getting a net-metering credit of 10c/kWh on the excess solar electricity you produce in summer but are paying 15c/kWh in winter when the heat pumps are used the most. This suggests a slightly different version of the Fab Four recipe for you. I would invest much more in lowering the energy use of your house such as great insulation, air-sealing and upgrading you windows. Then, if your AC units fail, replace them with heat pumps and add solar panels if it is cost-effective which it may not be. With such cheap electricity, you may be better off financially using a heat pump (after insulation and air sealing) and replacing your gasoline vehicles with EVs rather than getting solar panels.

My conclusion here is that without significant SREC value in NH, there is no way for me to push the cost of solar low enough to be a great investment when compared to other investment opportunities (granted, I haven’t done a detailed financial evaluation yet). On the plus side, I don’t think I will lose money here, and it obviously still makes tremendous sense in terms of cutting carbon. Would you agree with this conclusion? Or would you consider cheaper solar panels (ie not LG, Sunpower, or Panasonic) here to try to improve the economics (I have received a quote using REC panels that is closer to 8c/kWh)?

REC makes a great panel – I almost bought it until my installer decided to quit installing batteries, since I wanted a battery I had to get a new installer. The new installer gave me a better deal on Solaria panels at about 6c/kWh including a battery. At 8c/kWh and COP 3.0 you would be heating the house at 2.7c/kWh of heat in the house which is 1/3 of the cost of heating with heating oil and about half the cost of heating with natural gas. This is roughly the situation at my house. I heat with a heat pump, cook on induction, run a heat-pump hot-water tank and drive my Tesla, all charged with the cheap electricity from my solar panels. All with a zero carbon footprint.


HEAT PUMPS

Questions to ask a heat-pump installer:

  • Does the Manual J show my house will be at 70℉ when it is 5℉ outside?
  • If you have no ductwork ask, “Can you quote this 1) with ductless units and 2) with ductwork in the basement for the ground floor plus ductwork in the attic for the upper floor?” Also ask for a quote for radiant-floor heating, this was surprisingly cheap in our rental house (but this will not do AC for you). Make sure the attic work is well insulated or you will get ice dams.
  • I have not tried AC using cold water in radiators. Some manufacturers say it can be done, but I doubt it without getting pools of condensation on the carpets.
  • For forced-hot water radiators, make sure the Manual J calculation is done with a water temperature of 110℉ not 140℉
  • How much money will I save on heating if I am paying 23c/kWh for electricity (or at your utility rate)? How much if I am paying solar rates (5-8c/kWh)
  • How much for a heat-pump hot water tank? How much will it save me per year?
  • How long is the warranty, does it cover parts and labor?

Heat pumps for forced hot air systems (i.e., with ductwork):

Q: What company do you recommend for a cold climate heat pump for a house with ducts, who can also change us from one zone on two floors to two zones, one for each floor. I live in Woburn, MA.

A: Either I or my clients have used Rodenhiser and New England Ductless and had good experiences with both. 

Q: I attended your seminar a yr ago, my hub last week.  He received your email stating the % costs savings by converting to heat pump.
We currently live in Wellesley, in a 1938 Cape (low ceilings on 2nd floor), that was built as a summer cottage, with 1990 addition.  Gas furnace is 28 yrs old, central AC is 30 yrs old.  AC broke down this summer so we are replacing our whole HVAC system.
Because our house is not tight, we are going with a heat pump hybrid (single stage 16 SEER), which includes a new gas furnace for the below 30 degree days.
I know we will lower our carbon footprint a bit with the hybrid.  Solar is not an option.  We have large trees, within 100 ft of wetlands, 3 skylights, and face east/west.
We do need additional insulation, and hopefully that will come in a year or two.
Is it worth bothering with the additional expense of the heat pump, if we’ll actually see our heating & cooling bills go up?
I have been consulting with Abode, who has a contract with Wellesley Municipal Light Plant to oversee a rebate program for heat pumps, and they’re definitely encouraging heat pumps.  But with the numbers you supplied, I’m really questioning if it will be that much more energy efficient or if there will be any reduction in costs.
Thank you in advance for any knowledge you can share with me.

A:

Hi L, if you live in Wellesley then you are probably on MLP electricity at 14c/kWh. See below table for what the effect of moving to heat pumps will be: 

Heat pump economics.png

If your AC is broken, then now is the ideal time to replace them with heat pumps because just replacing the AC units with new AC units might cost you $15,000 and replacing them with heat pumps might cost you $23,000. These numbers are actual costs for two Bosch units for my sister-in-law’s 3,400 sf home in Needham. But since the heat pumps can be financed with the 0% interest 7-year Heat Loan you can make $5,000 worth of interest by investing the $23,000 in bonds paying 3% (e.g. a mass muni ETF like VMATX) for seven years. This brings down the incremental cost of the heat pumps to only $3,000, which makes the heat pumps very good value. 
Unless the gas furnace is broken I would not replace it because now it will become your back up heating system and hence will get little use. 
I wouldn’t  be so quick to dismiss the potential of solar on your roof. East or west facing roofs generate 90% of the electricity of a south facing roof. I too have 100’ tall pines only 50’ away from my garage and that array still generates electricity at 11c/kWh which is less than half that of Eversource, though only 3c cheaper than your MLP. 
I hope this helps but if you are still not sure you can hire my company Zero Carbon LLC to visit your house and do the detailed energy and financial modelling. I am not looking to do more consulting work, I am about to start a (more than) full time job in biotech, but I have just engaged my first Zero Carbon coach (who I have trained and certified) and he could help you if you needed more guidance. My billing rate is $200 an hour but he charges only $75. If you need the energy/financial modelling (which I think you do) then I charge a flat fee of $250 for that. 
Also, I would do the insulation and air sealing first because that will save you money on the heat pumps and solar panels. 
I hope this helps!

Q: I’ve attended your webinar, and it is great. Thanks for all you do to move individuals to lower carbon footprints.
I want so much to change from our oil boiler (32 years old, so we need to do something) to a heat pump system. But the conversion prices are high, and the lowest ones that maybe we could consider are based on using our current AC ducting. Companies won’t warranty the ducting, and they often tell us we would be significantly more comfortable with minisplits. But we have many rooms and would need 6 minisplits – it is prohibitively expensive.
I’m just about ready to give up on the whole heat pump idea.
Can you help me think about this?

A:

Hi G, I suggest you install the heat pumps using the existing AC ductwork and leave the old furnace (and radiators) in place. The new thermostats that will come with the ducted heat pump system integrate the heat pumps and furnace so the furnace will just come on when ever the heat pumps can’t keep the house warm. This is what we did on our house. We had forced hot air ductwork (not just for AC) but we still left the dinosaur 1970s furnace in place. We use it about 10 days a year during bitterly cold weather. With ductwork that is only for the AC you may not get quite as good circulation of the warm air and so your furnace may come on a bit more often than ours but overall it will dramatically cut your carbon footprint and at considerably lower cost than either putting in new ductwork or adding many mini splits heads. 
However, if you are on Eversource or National Grid electricity (at 23c/kWh) your heating bill will probably double. To cut both your bill and your carbon emissions you need solar panels on your roof. If you don’t have enough cash to do both heat pumps and solar panels then get the heat pumps using the 0% interest Heat Loan (you need a  free MassSave audit to qualify) and spend the cash on the solar panels. Or if even that is too much cash you can get low interest rate loans (Sunlight Financial and UMass5 have good rates) for solar panels.
I hope that helps!

Q: Wondering how much noise the units produce (I live with hard-of-hearing folks for whom background noise is an issue)

A: For the outdoor units, our Bosch units are far quieter than the old AC units they replaced, but they are not as quiet as the Mitsubishi units which you cannot hear from more than about 20’ away. Inside the house the Bosch units are far quieter than the old blower-fan units but they are not silent and generate a low level of background noise. The Mitsubishi ones have about the same noise level inside the house.  Neither bothers us at all. 

Q: Do heat pumps require a 200amp electrical service?

A: Not necessarily, but if you only have 100A service today and don’t have AC then you will probably need to upgrade the service from the street. Utility companies charge for this, but it is not too bad if the cable comes through the air. If you need to trench the cable it will cost a lot more. At our house we got two heat pumps in on an existing 200A panel.

Q:  how does 2.5 time’s the efficiency relate to an 80% efficiency rating for an oil fired boiler?

A: 2.5x is 250%, which is about 4 times the efficiency of an oil-fired boiler.

Q: You said that it dehumidified in the basement.  Also dehumidify in the second-floor bedroomd?

A: Only slightly. 

Q: we have a high efficiency condensing direct flow gas furnace, new about 5 years ago.  Does it make sense to get a heat pump condenser to reduce the carbon footprint?  So to create a dual fuel system?  The furnace can already handle an external solar heating system…

A: Adding a heat pump will reduce your carbon footprint but it will about double your heating bills if you are paying 23c/kWh for electricity which most people in MA are. See the blog post on my web site on the cost and carbon footprints of changing to heat pumps from different heating fuels and at different electricity prices. If you can expand your solar array to make the electricity that a heat pump would use, then adding a heat pump will lower your bills and carbon footprint. If you do add the heat pump, just leave your furnace in place.

Q: What is the SEER and tonnage of Green heat pump?

A: I have two Bosch BOVA 5-ton heat pumps at my home. The SEER rating, which measures cooling efficiency is 17.5 and the heating efficiency called HSPF (which, in MA, is more important that the SEER) is 9.5. In practice I have found that the year-round COP (coefficient of performance) which is the number that determines your heating bills, is around 3.0. This is at the high end of the results found in independent academic studies of heat pump efficiency in year-round conditions in New England which averaged 2.3. With COP, higher is better. You can convert HSPF to COP by dividing by 3.4.

Q: If you don’t have enough roof space for a solar array that can provide enough electricity to cover your new higher electric bills do heat pumps still make economic sense?

A: Not in MA using Eversource or National Grid electricity at 23c/kWh. If you are in one of the towns using municipal light and power plants (like Wellesley and Boxborough) which charge about 14c/kWh then a heat pump (especially a ducted one that get higher efficiencies) will cost about the same as natural gas to heat the house, will cost less than heating oil and far less than propane.

Q: Lower-than-expected efficiency of heat pumps?

A: People, including me, other people I know, and academic researchers, have reported lower-than-expected efficiencies on heat pumps. The efficiency of heat pumps is measured using a confusing number of terms including COP, or coefficient of performance, HSPF or heating season performance factor and SEER or seasonal energy efficiency ratio. Let’s explain each one of them:

COP is the heat energy delivery by the heat pump divided by the electrical energy used by the heat pump. If the heat pump delivers say 4kWh of heat by using 1kWh of electricity, then the COP is 4. People are often familiar with using kWh to measure electricity because that is how their utility company delivers and bills them for electricity. People are less familiar with kWh to measure heat but both heat and electricity are forms of energy and so can be measured using any unit of energy such as kWhs, BTUs, therms or joules. Using these different terms for measuring energy is similar to using centigrade and Fahrenheit to measure temperature or miles and kilometers to measure distance, they are just different units for measuring the same thing. People are more familiar with using BTUs to measure heat because that is how the energy in natural gas is sometimes measured (though it is also measured in therms, one therm is equal to 100,000 BTUs). One kWh of energy is equal to 3,412 BTUs of energy. As long as you use the same units, (either kWh or BTUs) for the electricity used by the heat pump and the heat output of then you will get the right COP. COPs for heat pumps are usually between 2.0 and 4.0. The higher the COP the more efficient and the more money it will save you.

HSPF is similar to COP, except it measures the heat output in BTUs and the electricity used in watt-hours. There are 1,000 watt-hours (Wh) per kilowatt-hour (kWh). Hence, in the above example with 4kWh of heat output for every 1kWh of electricity used, there are 4 x 3,412 BTUs = 13,648 BTUs of heat delivered for every 1,000 Wh of electricity used. So the HSPF is 13,648/1,000 = 13.6. Mathematicians will quickly realize that HSPF can be converted to COP by dividing the HSPF by 3.412. The higher the HSPF the more efficient and the more money it will save you.

SEER measures the efficiency of the heat pump in cooling or air-conditioning mode. Now the output is the amount of energy removed from the room (which cools it down) in BTUs and the input is the amount of electricity used by the heat pump measured (just like HSPF) in watt-hours. 

I believe that consumers would be far less confused by these terms if HSPF was simply called “heating efficiency” and SEER was simply called “cooling efficiency”. 

For SEER, HSPF and COP higher is better. Manufacturers are required to state the SEER and HSPF on the units in the energy label which is a bit like EPA’s the miles-per-gallon sticker you see on the windows on new cars.

However, the SEER and HSPF that are required to be stated on the equipment’s label are measured under continuous usage under ideal laboratory conditions. Under real-world conditions, HSPFs can be far lower than those stated on the equipment. It is as though the EPA measured the mpg on cars when they were all going downhill with a following wind. 

In the real world, heat pumps often work in conjunction with a fossil-fuel heating systems and have to heat the house in short bursts in the spring and fall whereas the tests are done over long time periods of continuous operation, in other words, the tests simulate winter or summer operating conditions but not the conditions in which the heat pumps operate in spring and fall. Both the integration with the fossil-fuel furnace and the operation in spring and fall reduce the efficiency of the heat pump. Hence, the real-world, year-round COPs, HSPFs and SEERs are often lower, sometimes much lower, than those advertised by the manufacturers.

The following comment is in reply to data shared by someone who had installed Mitsubishi Hyper Heat heat pumps and, like me, was not getting the performance he had expected.

Hi, XXX,  

This is great data and very consistent with both my own experience, that of academic researchers and the members of the Heat Smart Alliance. If you did a year-round average COP calculation (weighted by the energy used) then I would guess you would come out to a COP of between 2.5 and 3.0. When I do consulting work for other people, I use 2.5 in my calculations for ductless systems and 3.0 for ducted systems. Both numbers are far below the claimed COPs of manufactures which are in the 4-5 range. COPs, HSPFs and SEERs are all good ways to compare heat pumps from different manufacturers, but they are highly misleading if you use them to predict your energy-bill savings.

I agree with you that the short cycling reduces COP. Short cycling is not only a problem in the shoulder seasons but is also a problem if there is a back-up furnace that comes on with an outdoor temp of say 40°F. This mean that both the furnace and the heat pumps are short cycling which kills the efficiency of both. From the heat pump’s point of view, it does not care if the outdoor temperature is mild or if the back-up furnace is coming on to help, it leads to short cycling either way.

I believe that this effect is why people who have only a heat pump (and no back-up furnace) in cold locations (and have enough insulation to enable the heat pump to maintain 70°F year round) actually get better year-round COP than those with back up furnaces and / or milder climates. I have had such people on my webinars, and they cannot believe that other people (like you and I) aren’t overjoyed with our heat pumps. 

These short-cycling issues lead to lower COPs in practice than the manufacturers advertise. Academic research shows that the year-round actual COP of Mitsubishi Hyper Heat units in MA is about 2.5. If you are paying 23c/kWh (current Eversource or National Grid rates) then the 2.5 COP translates to a cost per kWh of heat in the house of 9.2 cents. Oil heat costs about 8c/kWh of heat in the house. This is paying $2.75 a gallon with an 85% efficient furnace. This is a 15% increase in your heating bill. If you are heating with natural gas, which costs about 5.2c per kWh of heat in the house (paying $1.45 per therm with a 95% efficient furnace), then your heating bill will almost double. This is why I disagree with the common conventional wisdom in MA that heat pumps are for everyone. The only way (in MA) to make heat pumps pay is to install solar at the same time (in which case, if the solar generates electricity at 8c/kWh of electricity – which is easy to do) then with a COP of 2.5 the cost of heat in the house is 3.2 cents or a 40% cut in your heating bill if you are heating with natural gas and a 60% cut if you are heating with heating oil. This cost saving can pay back the cost of the new heat pump in around 10 years for a return on investment of about 7% per year. It is only in these far more limited circumstances that I recommend heat pumps to people. My advice often comes as a nasty shock to the proponents of “electrify everything”. This is why I think it is essential to take a “whole house” approach to energy efficiency that looks at the financial returns on these investments. The momentum to cut carbon will be stopped dead in its tracks as soon as stories of doubled heating bills with heat pumps start to spread.

So, if you are suffering from high bills after installing a mini-split heat pump, here is my advice on how to improve the performance:

  1. Do not have upstairs heads and downstairs heads on the same outdoor unit. The upstairs heads will sometimes be trying to cool while the downstairs heads are trying to heat. This gets very confusing for a heat pump with very little brain.
  2. If the heat pumps have enough heat output to heat the house at a very low temperature, like 0°F, (this should have been determined in a Manual J calculation before the system was installed) then it will likely be more efficient to set the temperature at which the back-up furnace comes on to a very low temperature like 10°F rather than the common practice of setting it to 40°F which leads to short cycling (and inefficiency) in both the heat pump and the furnace. 
  3. If it gets too cold in the winter with the heat pump thermostat set at say 70°F, turn up the thermostat rather than turn on the furnace or turn on an electric fan heater. Keep turning it up until your feel comfortable. Only when this fails to keep the house warm, turn on the furnace.

And let me know if you have found any tips for getting better performance out of heat pumps!

Q: How do you measure the CO2 your house produces?

A: It is easy to calculate the carbon emissions of your house, just follow this link:

Q: I installed heat pumps around 2010. Between November and March, they are MUCH more expensive for heating than my natural gas furnace. Are 2021 heat pumps sufficiently improved that I will be able to use them all winter? Would I need to replace both the indoor unit and the outdoor compressor, or only the latter? What questions do I need to ask to be sure replacing my current units makes sense?

A: If you are using utility electricity at around 23c/kWh then heating with heat pumps will about increase your heating bill about 150% if you had previously been heating with natural gas. This fact never seems to get mentioned by either heat pump installers or heat pump advocates. Even if you are heating with heating oil, your bill will increase about 25% by switching to a heat pump on utility electricity.  The key to cutting your bills AND carbon emissions is to generate cheap solar electricity from your roof. Doing this will cut your heating bill (if you are on natural gas) by about 10% and if you are on heating oil it will cut it about 50%. In either case your heating will now have a zero-carbon footprint. You do not need a new heat pump. You need solar panels.

Q: Are you aware of any restrictions on using heat pumps on commercial buildings?

A: No. They make excellent sense in commercial buildings when powered by solar panels on the roof. This combination heats the space at half the cost of heating with natural gas. This would be by far the cheapest way to heat any low office, warehouse, school or manufacturing facility that often have large, unshaded flat roofs that are ideal for solar panels.

Q: how loud are the heat pumps?

A: The noise level inside our house has dropped considerably with the new heat pumps. Outside the house you cannot hear the Bosch units at 50’ away and the Mitsubishi units are even quieter.

Q: How do ASHPs work with existing forced hot water heating systems?

A: Yes. There is a whole section on this on my website under Q&A. You are in the right section!

Q:  Do heat pumps remove humidity in the air?

A: Yes, when in cooling mode. In New England the only time we need dehumidification is in the summer when the heat pumps are operating in cooling mode, but in Seattle where is it cool and humid you might still need a dehumidifier in winter, although some heat pumps (my Bosch ones for instance) will temporarily go into cooling mode even in winter time if they need to to reduce the humidity.

Q:  In a 100 year-old house, does every room require a ASHP?

A: If you have ductwork (either for a forced hot air heating system or an AC system) then you just need one air handler unit for each zone. The air handler is the part that goes inside the house. This is what we have at our home, two air handlers and two zones one upstairs and one downstairs. If you have ductless mini-splits (like we do at our rental property) you can, fairly inexpensively, add ductwork to the ground floor from the basement (if you have a basement) or add ductwork to the upper floor from the attic. If you have neither ductwork, nor a basement nor an attic (the situation at our rental property) then minisplits make sense. We have three outdoor units feeding eight indoor heads, roughly one per major room. One bathroom was too small for its own head and we put in a small electric-resistance heater (called a toe-kick heater because it is mounted under the bathroom cabinet).

Q: The 2 estimates I’ve had for heat pumps (a couple yrs. apart) for our 106-year old 2-family house were in the $36-40K.  We do have PV and also (as of 12 mos. ago) solar hot water.  Would LOVE to have heat pumps, but for an an older, non-open-plan home, we were told we would have to put a unit in almost every room, thus the high cost.  Any suggestions?

A: This is complicated situation. I can’t really make recommendations without knowing more about the situation. My primary questions would be: Do you have AC? Is the AC ducted? How old is the AC system? Also, in older houses it will pay greater dividends to focus first on great air sealing and great insulation, then on the windows. For windows replace ones that need to be replaced with triple glazed rather than double glazed and consider window inserts for the ones that are in good shape. These actions will all lower the cost of heating and cooling the house and is likely to mean that you need smaller heat pumps and fewer indoor units if you end up with mini-splits. 

Q: humidity??

A: Mold, rot and asthma are all caused by humidity, so you are right to ask about it. Please see many other answers on this topic on my website.

Q: Can one fit air-sourced heat pump(s) to a forced hot water heating system sourced from a gas furnace?

Y: Please see multiple other answers on this topic in the Q&A section of my website in the next section.

Q: Are these heat pumps connected to geothermal?  Or something else?  And do your savings account for how much electricity they use?

A: My heat pumps are air-soured heat pumps not geothermal heat pumps which are also known as ground-sourced heat pumps. Yes, my savings count the decrease in my use of heating oil and the increase in the electricity usage at utility rates, i.e., not at the much-reduced cost of using solar power from my roof. The cost savings from the solar panels are counted in the evaluation of the solar panels, so I am not double counting anything. 

Q: Do heat pumps work to heat, using existing radiator systems that use steam or hot water?

A: Yes, heat pumps work with water-based radiator systems, forced-hot air systems and underfloor radiant heating. I have never seen one that works with a steam heating system. Please see other answers on this topic.

Q: Does it take a heat pump longer to bring the temperature up to 75 degrees in winter than a conventional furnace?

A: If you are starting from cold (such as after coming home from a winter vacation) yes it will take longer. However, all modern heat pumps come with wifi thermostats which means you can control the temperature of your house from anywhere. I set the system to come on the day before I come home.

Q: Was your basement still warm enough to keep pipes from freezing?  And to be comfortable enough to use for various living tasks; e.g., laundry?

A: Our basement is now at about 60°F (down from about 68°F before the fab four). We don’t do laundry in our basement but it would be fine if we did.

Q: A heat pump runs only on electricity — do you recommend a generator for electrical outages?

A: Where we live in Dover a generator is standard issue because we get frequent power outages in winter. However, no generator will produce enough power to run your heat pumps. This is one good reason to keep your fossil-fuel furnace when you add a heat pump. The back-up generator will run the circulating fans so the combination will keep you warm. I have recently bought a battery to replace my defunct back-up generator. The subsidies are very generous on batteries in MA right now, so if you are considering a back-up generator now is a good time to consider using a battery instead. Please see other answers on batteries.

Q: are heat pumps 2.5 times more efficient than a natural gas furnace rated at 95% efficient?

A: Yes, please see chapter 2 in Zero Carbon® Home

Q:  What did you do to cover the peak cold days when you need backup heat with your heat pumps. How do you avoid having to have an expensive auxiliary heat backup system.

A: We kept our dinosaur oil furnace in place and still use it on about 20 days a year when it is colder than about 20F outside. There was no expense here because we already had the furnace.

Q: Could you repeat the temperature at which the heat pump becomes 1-1 efficient?

A: It varies by manufacturer but below 0°F the efficiency will be little higher than 100% or no better than an electric fan heater or electric radiator.

Q: If you are building a new house and you use a heat pump, do you still need to add a gas furnace for a New England home.

A: You would not need a furnace if you had very well-insulated walls (like R30), a well-insulated attic (like R50), and basement (R40) and also had very good windows (R5 or greater). You would also need a very tight building envelope, probably less than 3.0ACH50. This is almost impossible to achieve on an existing house, but it is possible on new construction.

Q: Our  home is heated using forced hot water in baseboard units and under some floors. Is a heat pump adaptable to this arrangement?

A: Yes, see other answers on this topic.

Q: Ground source or air source heat pumps?

A: All my heat pumps are air sourced. Ground sourced, or geothermal, heat pumps make energy sense but they are 4x the cost of equivalent air-sourced heat pumps and so do not make financial sense.

Q: Is variable speed worth the money?

A: Yes. Variable-speed pumps (and circulating fans on forced hot air systems) use about ¼ of the electricity of a traditional fixed-speed pump or fan.

Q: Does the heat coming down from the ceiling heat well? Dont you want the heat to rise from floor level?

A: Ideally yes. But the fans on our ceiling-mounted units on our ductless mini-split units do a good job of distributing the warm air around the room and we do not feel any cold spots. 

Q: Can the heat pumps connect with radiant floor heating systems?

A: Yes, if the radiant heating is by hot water. 

Q: I build NZE new houses using Mitsubishi Hyper heat pumps and they heat the houses to 70+ even when the temp is below zero!

A: This is very doable on new construction. The experience I was referring to is on retrofits to existing homes, where I have found that heat pumps (including Mitsubishi’s) cannot keep a house at 70F if the outdoor temperature is below about 20F.

Q: Can you explain how the heat pumps are fueled by oil or gas or electricity?

A: It takes electricity to run a heat pump. There is no fuel. A heat pump is like a water pump, it moves heat from one place to another just like a water pump moves water from one place to another. A water pump does not create water and a heat pump does not create heat. A water pump needs electricity to turn the motor and a heat pump needs electricity to turn its motor too. When I calculate the cost savings from using the heat pump it is the savings on the heating oil bill less the cost of the electricity needed to run the heat pump. The same is true for the carbon footprint.

Q: Will a heat pump work with a gas boiler and indirect water heater or do those have to be replaced?

A: Yes, they work well together. In fact, I do not recommend taking out your gas boiler. Leave it in place as a back up for the heat pump on very cold winter days. You can also leave the indirect water heater in place though I did not. I replaced the water heater with a heat-pump hot-water tank which is much cheaper to run – especially when powered by cheap solar panels. See the section, below, on heat-pump hot-water heaters.

Q: How does your furnace know when to kick in when it is too cold for the heat pump?

A: Our thermostats were replaced with ones that integrate the heat pumps with the oil-fired furnace. I do not even think about it, it just works. 

Q: What was the type of furnace that your replaced with heat pumps?

A: We did not replace our furnace. We still have the furnace and still use it on about 20 days a year when the outside temperature is below about 20F. Our furnace is a 45-year old A.O. Smith furnace. 

Q: Would you recommend the company that installed your heat pumps?

A: It was Rodenhiser. I think they did a good job. I also think they are a bit on the expensive side.

Q: Also, do you have any thoughts on whether cold climate heat pumps work well?  I also have oil heat, and am considering either replacing it entirely with a cold climate heat pump or getting a heat pump that can work with my oil furnace.

A: See Chapter 2 in the book. I recommend keeping the furnace and replacing your AC units with heat pumps. Heat pumps cost less to run if you power them with solar panels. See below for the likely difference to your heating bill and carbon footprint from adding heat pumps:

Adding a heat pump will have dramatic effect on a home’s heating bill and carbon footprint, but the effect varies considerably depending on what heating fuel is currently being used and what the source of electricity is for the home.

Effect of adding a heat pump:

 If currently heating with natural gasIf currently heating with heating oil
If using Eversource or National Grid electricity (at 23c/kWh and 0.87lbs CO2/kWh)150% increase in heating bill
10% cut in carbon footprint
25% increase in heating bill
45% cut in carbon footprint
If using Municipal Light and Power electricity (at 14c/kWh and 0.87lbs CO2/kWh)50% increase in heating bill
10% cut in carbon footprint
25% cut in heating bill
45% cut in carbon footprint
If using Rooftop solar panel electricity (at 8c/kWh and 0lbs CO2/kWh)10% cut in heating bill
100% cut in carbon footprint
50% cut in heating bill
100% cut in carbon footprint

This assumes that the heat pump achieves a year-round COP (coefficient of performance, or efficiency) of 2.3, which is the average found in a review of the work of several researchers on Mitsubishi, Daikin and Bosch heat pumps in New England.  Both ducted and ductless systems were included. In general, ducted systems had higher COPs than ductless systems and Mitsubishi had the lowest COPs, as low as 1.3 in one case. The manufacturers claim COPs of between 3 and 5 for these units. None of them perform to the manufacturers’ specifications under real-world year-round conditions in New England.

Q: I installed solar 2 years ago – if I decide to install heat pumps, would I need to add more solar panels?

A: It is not necessary,  but it is a lot cheaper to power your heat pumps with solar panels rather than power them with utility electricity.

Q: Is it wise to use a gas fireplace as backup heat since the heat pump will not be efficient when very cold and also provide heat if there is a power failure? 
A: Yes. 


Q: With net metering always changing, do you recommend battery storage yet to store excess instead of only receiving avoided cost payments?
A: Batteries are currently much more expensive than a diesel or propane back-up generator. However, that is very dependent on the local subsidies. In MA now, if you pair a battery with a fairly large array (about 20kW) and get all the subsidies, then the battery can pay for itself in just a few years.   

Q: Any idea on the operational life of a heat pump.  Will you have to replace them in 20 years?

A: Probably. 

Q: Would a mini split heat pump help to dehumidify a basement?

A: A mini-split heat pump head (i.e., the part that is inside the house or basement) WILL dehumidify the air inside the house but only when it is operating in AC mode i.e., during the summer months. Since basements are generally cooler than the rest of the house, it may not be operating as an AC unit very often and so it may not actually be dehumidifying the basement much at all. In the winter, when it is operating as heater it will not be dehumidifying the basement air. 

How effective this is for you is going to depend on what the source for the humidity is in your basement. If the humidity comes in summer (which is quite common because warm humid air enters the house and then condenses on surfaces in the basement because the basement is colder than the rest of the house), then the heat pump will dehumidify the basement air but only when it is in AC mode, which as I mentioned it probably is not doing very often even in summer. However, if the source of humidity in your basement is due to rising damp, i.e., from a high-water table or from rainwater leaking in from the soil then this tends to happen in winter when the ground water is highest or we get rain on top of thick snow. In winter the heat pump will be heating your basement. So now you will have warm humid air in the basement. Warm air rises. Humid air rises. Warm humid air really rises. When this warm humid air contacts a cold roof surface it will condense. This will lead to mold and rot. This is a common problem in cathedral ceilings. Because of this, I think relying on a mini-split heat pump to dehumidify your basement is not a good idea. I think you will be much better off installing a heat-pump hot water tank. These dehumidify the basement air whenever they are heating the hot water, which is year round. Heat-pump hot-water tanks are much cheaper to run than fossil-fuel hot-water heaters. The dehumidification comes as a side benefit, but in my opinion, it is a major benefit. If you don’t want a heat-pump hot-water tank, then just getting a dehumidifier will probably do the job. Dehumidifiers provide a small amount of heat in the basement too. 

Q: Any recommendation for ducts located in the attic space?

A: If the attic has good insulation above the ducts then you do not need to do anything. This is called having the ducts inside the thermal envelope of the house. However, if your attic gets cold in winter and hot in summer then you need to insulate the ducts or you will be losing a tremendous amount of energy in both summer and winter. 

Q: About the finances of operating gas heat versus air source heat pump?

A: Heating with a heat pump powered by solar panels generates heat in the house at about half the cost of heating with natural gas. My panels produce electricity at about 7c/kWh. The COP (efficiency) of our heat pumps is about 2.5, so we are producing heat in the house at about 2.8c/kWh. Heating with natural gas costs about 4-5c/kWh of heat in the house (assuming $1.42 per therm of natural gas and a 95% efficient furnace). Heating with heating oil costs about 8c/kWh of heat in the house (assuming $2.75/gallon of heating oil and an 85% efficient furnace).

Q: When we calculate energy costs of high efficiency natural gas furnaces vs heat pumps, the furnaces always win out over HP operational costs.  On-site Solar is not factored into this however.  Not everyone has the cash on hand for adding solar nor qualify for tax credits to offset these installation costs.  Q? How to manage lower (not necessarily Low) income families with this program?

A: If the electricity costs more than about 12.5c/kWh then the natural gas will be cheaper, see the answer above. In most of New England and the West Coast electricity is much more expensive than this (it is 23c/kWh for us). In these states you will need solar and a heat pump to both save money and cut your carbon footprint.  Part of the reason electricity is so expensive in these states is because the electricity bill includes many subsidies for energy efficiency, solar panels and heat pumps. These subsidies include generous net-metering tariffs, generous solar panel and battery subsidies and low-interest-rate or zero-interest-rate loans, which are often even more subsidized for low-income owners.  The best way to cut bills and carbon footprints in low-income communities is to use these subsidized loan programs. If the homeowner does not have a credit score high enough to get a loan, then a leasing arrangement for the panels is always possible. These leases can be very expensive for the homeowner but in the last year or so I have seen lease rates come down, making them at least reasonable options if the homeowner can’t get a loan. In a band of states in the middle of the country from the Dakotas to Louisiana electricity is about 10c/kWh and so there is little money to be saved with solar panels. For homeowners in these states, it will be cheaper to heat with a heat pump than to heat by burning natural gas. It will also be very cheap to run an electric vehicle. 

Q: Leave the furnace in place?

Yes, our heat pumps cannot keep our house at 70F if the outdoor temperature is below about 20F which happens on about 20-30 days each year. To be able to heat year-round using only a heat pump in New England requires superb insulation (such as R40 walls, which requires 10” of sprayfoam. Our walls, with 4” of fiberglass, are R10). This is almost impossible to do (i.e., it is very expensive) on an existing house. However, it is possible on new construction.

Q: Why not electric backup heat?

A: Electric back-up heating is feasible, and many contractors push it as the source of back-up heat when heating with heat pumps. But they are not paying the bills. Using utility electricity at 23c/kWh of electricity to run an electric resistance heater means heat would cost me 23c/kWh of heat in the house (electric-resistance heating elements are 100% efficient at converting electricity to heat) but my oil furnace costs about 8c/kWh of heat in the house. Natural gas costs about half this.

Q: What about the 300 gals of oil? But since there is still a furnace, therefore not zero carbon footprint.

A: The carbon footprint of the oil we burn is more than offset by the excess solar electricity that we export to the grid. So, our house has a year-round zero-carbon footprint even if on some days we still burn heating oil.

Q: Is there a way to heat a house without using any fossil fuels? 

A: Yes, you can heat a house using only a heat pump, but you need excellent insulation and excellent windows to do it. It is really only possible on new construction. I have never seen a renovation that was able to keep the house at 70F all winter without using some back up heating. You can use back-up electrical heating, it is just very expensive.

Q: I have an almost ten-year old AC system like you pictured, in a house that is steam heated with gas. Can I simply get the AC system replaced with a combination AC/heating air heat pump system?

A: Yes. Replacing an AC system is often the most economical way to install heat pumps. With the heat pumps in heating mode, you will be heating with forced hot air through the ducts not forced hot water to the baseboards. Many people prefer this, but some do not like it. I prefer forced hot air because the air distribution is better with forced hot air. This leaves fewer cold spots and removes any stale air in the house. You can also filter the air which can remove allergens if anyone in your house is sensitive.

On my sister-in-law’s house, replacing her failed AC units would have cost $15,000 and replacing them with heat pumps cost $23,000. However, the heat pumps qualify for the 0% interest rate Heat Loan program. Assuming she could invest at 3% interest for the 7 years of the loan she could invest $23,000 rather than spend that cash on the heat pumps. 7 years at 3% on $23,000 is almost $5,000. Deduct this from the $23,000 and she is effectively paying $18,000 for heat pumps when new AC units would have cost her $15,000. So, in this situation, the effective cost of heat pumps is $3,000. This is a real bargain.

But, there is a caveat. Since you are heating with gas today, (and I am assuming you are paying about 23c/kilowatt-hour of electricity which is the current price for both Eversource and National Grid) switching to heat pumps will increase your overall utility bills, i.e., the total of the gas bill and the electricity needed to run the heat pump. The total bills could double. The heat pumps will cut your carbon footprint by about 10%. The better way to add heat pumps is to add solar panels at the same time. Solar panels are very profitable investments these days. Most roofs can generate solar electricity at between 5-8c per kilowatt-hour. If you use this electricity to power your heat pump you will be generating heat in your house for less than the cost of heating it with natural gas. If you can generate solar electricity at 5c/kilowatt-hour your overall utility bills for heating your house will drop by almost 50%. And your carbon footprint will drop to almost zero. Heat pumps and solar play nicely together. And don’t forget to insulate and draft seal first, these will reduce the size (and cost) of the solar array you need.

 Q: Was a pressure test conducted on the building and ducts to determine their leakage rates?
A: No, but I sealed the holes with aluminum tape so I think the system is pretty good. 

Q: Before the heat pumps were installed, was a manual J & manual D conducted to properly size?
A: No. But remember this was a retrofit and we already knew that the old system had sufficient heating and cooling output to create a comfortable house. Since I was significantly reducing the heating and cooling load with the fab four, I was very confident we had enough heat and cool in the system. 
Q: My understanding if you do not size the HP system using manual J & Manual D and the building and duct leakage are not verified, then NO unit will meet loads.
A: Not true, see above.

Q:  does the same heat pump connect to the water heater, or do you get a separate heat pump for heat vs. for hot water? Thank you both!

A: I have two heat pumps for heating the house and one for heating the hot water for the taps and sinks.

Q: If I want to switch my house to heat pumps, will it be a lot more expensive if I don’t already have forced hot air heating? My house does have central AC but hot water and baseboard radiators for heating. Can those same ducts likely simply be used for heating as well?

A: Yes, the existind ductwork for the AC system can be used for the hot air for the heat pumps. So you will be switching from forced hot-water heating to forced hot-air. 

Q: : so presumably the heat pump still has a carbon footprint through electricity, just not as bad as fossil fuels?

A: Yes, the heat pump uses electricity and so creates a carbon footprint from the fossil fuel burned to create the electricity at power stations. However, I power mine from my solar panels, hence the heat pumps have a zero-carbon footprint.

Q:   Heat pumps are electric. How do electric heat pumps compare to Navien units for example which are fed by natural gas? In my experience the naviens fed by gas are significantly more efficient and result in better savings than any electric system?

A: I am not familiar with the Navien systems. Presumably they are high efficiency gas hot-water heaters, probably tankless. These are the most efficient hot-water systems that burn fossil fuel and can reach 95% efficiency. They can deliver heat at about 5c per kilowatt-hour of heat in the house. My solar panels generate electricity at about 8c/kWh of electricity. The efficiency of my heat-pump hot-water tank is about 400% because it draws heat from the air in the basement which is at about 60F. Hence, I am generating heat at about 2c/kWh of heat in the house, which is less than half the cost of heating with a even the most efficient tankless hot water heater powered by natural gas.


Q: Is the cost of heat pump proportionate to the size of your house?

A: Yes, but the relationship is not linear. Bigger houses are fundamentally more energy efficient than small houses (tiny-house movement proponents please take note) just because the heat leaves a house through its surface area but the amount of heat needed is proportional to the volume of air. The ratio of surface area to volume decreases as the house gets bigger. This is the same reason that thin people get colder faster than large people when swimming in the ocean. 

Q: Did you consider electric radiant heat as a supplemental heat source rather than the oil?

A: We did not, but you can add electric heat (either as fan heaters or as radiant heat) as the source of back-up heat for really cold days in winter. It is feasible, it is just expensive to heat with electricity directly rather than using a heat pump.

Q: Are there limitations on outside temperatures in winter to have the heat pumps still work?  would they work in minus 30 degrees celcius?

A: In my experience heat pumps on their own cannot keep a house at 70F if the outside temperature is below about 20F. You could go lower than this but only with excellent insulation and windows which are really only achievable in new construction, not on a renovation. Below about 10F the heat pump is creating little more heat than an electric resistance heater and so becomes very expensive to heat. Most heat pumps would stop operating somewhere between 5F and -15F so at -30C (approx. -30F) you would get no heat at all.

Q: My house has 2 gas furnaces. Outside temps dip to -30 degrees C.  Can a heat pump be installed for base load heating, with the gas furnaces cutting in for peak loads?

A: Yes this is a good way to use heat pumps – as supplement for a fossil fuel system not as its replacement. If it gets that cold in winter, it is probably fairly cold most of the year and heat pumps operate best between about 10F and 60F. If you have a very short season at these temperatures than heat pumps could cost too much to pay for themselves in any reasonable number of years. 

Q: So mini-splits are also heat pumps but work with systems w/o ductwork? Is there any other way they differ?

A: Correct. Academic studies have found that ducted heat pumps are about 20% more efficient that ductless installations. Personally, I prefer the air circulation from a ducted system and I prefer the look of traditional floor registers to having units on the walls or floors.

Q: Is the $26K you referenced for both heat pumps on your house?

A: Yes and it includes the installation.

Q: Can the tax credits be stretched over several years?

A: I am not a tax advisor but in my experience, yes any unused part can be carried over to future years. 

Geothermal Heat Pumps

  Q: Have you done any calculations (what % higher than air source) to show when geothermal is a better option from a financial sense?

A: I have not done precise modeling, but if you can generate electricity from solar panels at 5c/kWh (doable with a sloped south-facing roof), and you can get the wells drilled a lot cheaper than my quote, and you get the federal tax credit then I think geothermal starts to make financial sense. 

Q: Can I use well water instead of a ground source arrangement for a heat pump?

A: In theory yes, but I doubt that the volume of well water is high enough to provide all the heating needs of a house unless it is very well insulated. I am investigating this possibility at our rental property.

Q: Has he done a more-recent Geothermal cost-savings analysis for a client? Does it still not making financial sense?

A: I have not directly evaluated geothermal since 2016, but the big cost is drilling the wells, not buying the heat pumps itself. The cost of drilling wells has not come down in the last few years, so I doubt that the economics have changed much. In MA, geothermal now qualifies for a MassSave rebate of up to $15,000, but even with that I think that the vast majority of homeowners will be better off financially with air-sourced heat pumps. ASHPs are especially cost effective if they are bought to replace failed AC units. 

Heat pumps for forced hot-water systems (i.e., with hot water circulating around radiators)

Q: Are there heat pumps for hot-water radiator systems?

A: The question I asked during the webinar was not very clear.  We have a water based heating system with radiators and a boiler fueled by gas.  We also have an air-sourced A/C system with small tubes running throughout the house (a UNICO, high velocity system) without the hanging of mini-split systems in each room.  Is there any way of using an air sourced heating system to heat the hot water in our radiator based heating system (either through the existing boiler or directly to the hot water circulation system)?

Hi Alan, yes is the short answer, you can connect an air-sourced heat pump to a water radiator system. These are sometimes called hydronic systems. But the long answer is a bit more nuanced. These type of air-sourced heat-pump to hot-water systems are made by Jaga, Daiken and Spacepak and other manufacturers. I have never used one so I have no particular recommendations. The hot water in a boiler-driven radiator system is typically at 140F. Air-sourced heat pumps have a hard time getting the output side (whether it is air or water) above about 110F. So your radiators will no longer have that piping-hot feel (which may be good or bad – I have nearly burnt myself on hot-water radiators before). This means that it will take longer to heat your house up from cold – like when you come home from a winter vacation. But, as long as the heat output is matched to your heating load your rooms will stay warm, it will just take longer to get warm compared to what you are used to.

Are you thinking of using the UNICO high-velocity system with a heat pump so it would heat in the winter as well as give you AC in summer? We have a similar system in our rental house and I would consider replacing it with a heat pump if such a system exists. Let me know if you find one. Note, if you do this you will need to insulate the attic on the sloped part of the roof plus the gable ends and add window inserts to improve the insulation on any windows in the attic. If you don’t do this, the heat pump will be leaking heat into the attic in winter (no matter how well you insulate the ducts themselves) and this is not only wasteful but it will melt the snow on the roof which can lead to ice dams and water leaks into the house.

Q:  how does a heat pump work with radiators?

A: You can buy heat pumps specially made to work with forced hot-water systems, see other answers on this topic.

Q: If you have baseboard heating, wouldn’t it a bit of work with heat pumps adding/distributing ducts to all/most rooms in the house.

A: Yes, if you do not have ductwork for AC. If you have the ductwork for AC then the best solution is to connect your heat pump to that duct work. It will then both heat and cool via forced-air circulation, so your primary heating will now be by forced-hot air not forced-hot water. Not everyone likes forced-hot air heating, though I prefer it to forced-hot water because it creates better air circulation. If you do not have ductwork, you can add heat pumps that heat the water for circulating in the radiators. Please see the answer above.

Another alternative to adding ductwork is to install mini-split heat pumps rather than a ducted system. This is what we have at our rental property. Having experience with both ducted and ductless systems I prefer ducted systems. The air circulation is better, the installation cost is lower (if you have the ductwork in place) and the efficiency (COP) is higher, in our case the COP on our ducted system is about 3.0 and on our ductless system is about 2.5.

There are also hybrid solutions where you put new ductwork in the places that are accessible. Usually the first floor is accessible from the basement and so it is fairly cost effective to add ductwork to the first floor from below. Likewise it can also be cost effective to add ductwork through the attic to provide heating/cooling to the top floor. If you do add a heat pump to the attic the attic needs to be well insulated and the heat pump needs to be inside that thermal envelope. If you don’t do this you will be wasting a lot of heat in the attic in winter. This heat will melt snow on the roof and can cause ice dams to form and water leaks that can cause a lot of damage.

Heat-Pump Hot-Water Tanks

Q: How are you heating your hot water?

A: Using a heat-pump hot-water tank, see below.

Q: How is your water heated?

A: Our hot water is heated with a heat-pump hot water tank (sometimes called a hybrid water heater in the trade). Ours is made by State which sells the same tank under the A.O. Smith brand. It is very cheap to run, even on utility electricity but is almost free on solar panel electricity. In addition, it dehumidifies the basement which is a huge benefit that is seldom mentioned by the manufacturers. 

Q: My hot water tank has broken, what do you recommend?

I am guessing that you are heating with natural gas and currently have either a direct-water heater (which means it has its own burner separate from the furnace) or an indirect-water heater (which means it is heated from your furnace and is essentially just a heating zone on your heating system). I would suggest you look at a heat-pump hot-water tank. These are very cheap to run. Ours provides all our hot water for under $300 a year whereas our old oil-fired indirect water heater cost us over $500 a year. And that is with us paying 23c/kWh for electricity. If you live in Wellesley (just a guess from your email address) you are paying half that from the MLP, so your cost would be $150 a year. A heat-pump hot-water tank will also dehumidify your basement which is a big advantage because basements always get damp either from condensation, water leaks or rising damp. Because it takes the heat from the air in the basement, your basement will get a bit cooler, ours cooled by a few degrees F. Because of this it is better to also insulate the ceiling of the basement so that the heat from your house does not go down to your, now cooler, basement. This is very easily done by just pushing fiberglass batts in between the rafters on the ceiling of the basement. I did this and it cost me $1,000 in insulation and I installed it myself. It is saving us around $3,000  year. If you do this wear gloves, eye protection and a face mask (like your covid one) because fiberglass fibers can irritate. It is completely safe once you have installed it. 
Heat pump hot water tanks can be bought from Home Depot or Lowe’s for about $2,000 and installation will probably cost you $250 each for a plumber and an electrician. Alternatively your plumber can buy it for you and install it. We bought ours from State/AO Smith (they are the same company) but there are many other manufacturers. Ours paid for itself in about 10 years on the energy bill savings. An indirect tank will cost you about $1,500 and will have no savings from your current bill. I have no experience with a tankless gas-fired hot-water heater but I am guessing that they would cost about the same as an indirect tank and are probably slightly cheaper to run than your current tank. However you get no bill savings and no dehumidification, both of which are significant benefits to me. 

Q: Do heat pumps work with water heaters, are they efficient?

A: Heat-pump hot-water heaters are about 400% efficient or more than 4x as efficient as even the best natural gas furnace.  They are even more efficient than air-sourced heat pumps for heating your house. This is because they take the heat from the air in your basement which is probably at 50°F all year round. Air sourced heat pumps for heating your house take the heat from the ouside air and, in winter that could be at 10F. All heat pumps get less efficient as the temperature drops. Our basement cooled by about 8°F after we installed the heat-pump hot-water tank. But this included the effect of insulating the hot-water pipes and insulating the ductwork in the basement, so the effect of the heat-pump hot-water tank alone was less than 8°F. I am  guessing that it would be 2-3°F on its own. The way to avoid this cooling of the basement cooling the ground floor is to add insulation to the ceiling of the basement. I added 12” of fiberglass making it about R38.

Q: back in the 80’s when I had heat pump the exit temp was in the mid 90’s so even if you were sitting in front of the air duct, you perceived the warm air as cold – it was uncomfortable heat. Do the new systems provide a higher temp of the output air?

A: Yes. The output air temperature of my Bosch units of over 100°F. Also, that chilling effect of feeling even warm air rushing past your skin is less prevalent with modern heat pumps because they operate at much lower fan speeds (but almost continuously) rather than the on/off of the older fans. This makes the new fans much more energy efficient as well as making the heating system more comfortable.

Q: Heat pump hot water heaters and bacteria.

I tried to ask this tonight, but in addition to the hot water radiator situation, for Heat Pump Hot Water Heating (for your potable hot water), what temperature can those heat pumps keep your water at? If it can’t keep it above 120 continously, that would be a huge health hazard: “Water conditions that tend to promote the growth of Legionella include: temperatures between 20° and 50°C (68° – 122°F) (The optimal growth range is 35° – 46°C [95° – 115°F])” from OSHA

A: My HPHWT keeps the water at 140F which is hot enough to kill bugs. This is hot enough to scald skin so the 140F water is mixed with cold water at the valve to bring it down to a safe 125F. I have had no problems with this.

Can you put a ground loop for a geothermal system below the basement floor?

Q: For new construction, would it be possible or make sense to put in loops for a ground source heat pump below the structure? 

A: I have not seen this done, but I have thought about it for new construction. Since you are excavating for the basement anyway, why not just go down a few feet more and put in ground loops? I think this would greatly reduce the cost of adding geothermal. However, I do not know if this would reduce the cost enough to compete with air-sourced heat pumps. The downside I can see is that your basement would get colder which could lead to increased condensation in summer when it is humid. It might be a good idea to insulate the floor of the basement if you are going to do this and also make sure that the geothermal pipes are below the water table.

A heat pump for a greenhouse?

Q: What about installing the heat pump in a greenhouse? Maybe removable for the summer.

A: I have not done this in a greenhouse, but I think it is a good idea. Effectively I have done something similar by bringing plants indoors in the fall and keeping them in our sunroom. The sunroom receives a very small amount of heat from our house heat pumps because it is at the end of the ductwork and has no return vent. But, using LED grow lights (powered by my solar panels) I was able to get red ripe peppers at Christmas. They tasted great!

Heat pumps increase your house price 4-7%

A paper published in Nature Energy in October 2020* states that houses with air-sourced heat pumps sell for between 4% and 7% more than comparable houses without heat pumps. This research was done on 450,000 houses across 23 states in the USA. For many houses this premium exceeds the cost of adding a heat pump.

If you are looking for a house with heat pumps this research suggests you should do the opposite and look for a house without heat pumps and add one yourself because the house price premium exceeds the installed cost. This is true if you are paying the full cost of the heat pump, not just the incremental cost of the heat pump compared to replacing the AC unit. 

The authors put this seemingly  irrational behavior down to people not understanding heat pumps, plus the hassle of searching for one and installing it. 

This work on house price increases caused by reducing utility bills by using heat pumps is 100% consistent with prior work showing the house price increases by $20 for every $1 cut in utility bills. 20:1 implies a discount rate of 5% is being applied to the additional cash flow generated by the lower utility bills. 5% was the after-tax mortgage interest rate when these studies were done in the 1990s. The 4-7% house price increase observed with heat pump adoption implies an after-tax mortgage interest rate of 2.4% – almost exactly what it was in 2016 to 2018 when these heat pump studies were done. It turns out consumers are extremely rational when it comes to paying higher house prices in return for lower utility bills, whether the lower bills come from solar panels, heat pumps or just insulation. 

*Estimation of Change in House Sales Prices in the United States After Heat Pump Adoption. Nature Energy October 2020, Shen et al.

Q: For homes that already have solar, it seems like heat pumps would be a tougher sell, because they’re already getting the advantage of reduced electricity. How does the ROI of heat pumps change when the solar is already in place and electric bills are already very low?

All solar installers I have ever met install enough solar to offset the current electricity usage. Adding heat pumps will increase the electricity usage and decrease the heating fuel usage. Hence the ROI on a heat pump is going to be based on the cost of the additional electricity needed to run the heat pump not the current cost of electricity (whether from solar panels or just from the utility). Installing heat pumps powered by utility electricity at 23c/kWh will be about breakeven in terms of heating costs vs heating oil and will be more expensive than heating with natural gas. I think you said you were on propane, it will definitely be cheaper than propane, even if you have to use utility electricity. By far the best way to do it is to install more solar panels at the same time as the heat pump. If you don’t have the roof space to do this, the next best option is to get community sourced solar which is 100% solar at about a 12% discount to utility rates or about 20c/kWh. With a mini-split heat pump at COP (efficiency) 2.5 the heat in your house is going to cost you about 8c/kWh or about the same as heating oil. Propane is more like 11c/kWh of heat in your house. With a ducted system the COP is likely to be more like 3.0 and heat in your house would cost under 7c/kWh which is cheaper than heating oil but still more expensive than natural gas (which is about 5c/kWh of heat in the house). And remember, you have to buy the heat pump whereas you already have the furnace. This is why the most cost effective way to add a heat pump is when you need to replace your AC unit. Finance the new heat pump with the 0% interest Heat Loan and the cost difference between replacing two AC units and upgrading to two heat pumps is only about $3,000. 

Q:  Have you compared the actual cost difference between gas and electricity? . We are seeing higher unit costs with electricity, even factoring in COP advantage  with new heat pump system.

A: Yes, please see the table that I provided in answer to a similar question. Generally speaking, heat pumps need to be powered by cheap solar electricity to be lower cost than natural gas heating. 

Q: Is there a company you recommend to install AC heat pumps?

A: I have had good success with Rodenhiser. Clients of mine have had good success with New England Ductless. I have heard good things about Muirfield Mechanical but I have no direct experience. Other people on the webinar recommended Boucher as well.

Q: I want to put in a heat pump and have a gas boiler with a how water heater – is this recommended

A: You could replace the hot water heater with a heat-pump hot water heater, replace your AC unit with a heat pump for heating the house and leave your gas boiler in place for heating the house on bitterly cold winter days.

 INSULATION:

Q: My question for you is to offer your thoughts on a hot roof vs flat insulation.  If we do a hot roof, we do not have to worry about ventilation issues in the attic with the gambrel, but we are basically paying to heat/cool our attic since the thermal barrier and insulation is on the underside of the roof.  By contrast, by doing the flat (attic floor) insulation and air seal, we have to find solutions to properly vent the attic of the hot, moist air that accumulates in the summer.  The price difference for the hot roof is about 50% higher (~$22k vs $15k) and we can’t find a definitive answer to which solution is the best.

A: It sounds like you have two different problems: insulation and moisture. You need to deal with both. My fundamental question is where is the moisture coming from? The usual culprit is the basement. If the basement is damp (in summer when the humidity is high dampness in the basement is usually caused by warm moist air condensing on the cooler surfaces (especially cold-water pipes) and in the winter it is caused by water wicking up from the ground. This, in turn is often caused by leaking guttering and downspouts that dump the water right by the basement wall. Leaking pipes cause dampness year round. 
First, fix the source of the dampness. So get downspout extenders and clean out the clogged leaves in the gutters and maybe fit perforated plates on top of the gutter, and fill any cracks in the basement wall with exterior grade spray foam. I did this on my rental property and it greatly helped. Then I would seriously consider replacing your hot water tank (I am assuming it is heated from your furnace) with a heat pump hot-water tank. A Rheem 55 gallon hybrid heat pump hot water tank at Home Depot costs about $1200, you can get a $600 rebate on this. It will take an hour or two with an electrician and plumber to install it. These heat your water at less than the cost of heating oil and about the same as heating with natural gas. However they also dehumidify your basement and that is the big plus. Ideally you would also insulate the ceiling of your basement (just push fiberglass in between the rafters or ask MassSaver to do it for you) to prevent the heat from your house just leaking down into the basement and becoming the source of heat for your heat pump hot water tank. This is a good fix for you but if you also get solar panels you would be saving a ton of money on heating your hot water, even if you are on natural gas today. 
With the moisture dealt with, I think it would be fine to insulate the attic  either on the floor or the sloped sides of the roof. Which you do depends mostly on whether you use the attic as storage space. If you do, the sloped side is better. If ever you intend in the future to install a heat pump for heating the house with an air-handler unit in the attic you MUST insulate the sloped part of the roof or you will get icicles and ice dams in winter. You could either wait a season to see if the moisture fixes work or insulate the slopes with fiberglass or Rockwool because they are permeable to air and so will allow a small amount of condensation to evaporate. The best solution is to add a one-way vapor permeable membrane (but 100% air tight) on the inside of the rafters/insulation such as Intello which you can get from Building 475 in Connecticut. You need a pro to install this and I have consulted with Dolphin but I have not hired them yet because I need to deal with the water issues in my basement first! This membrane prevents moisture traveling from the loft to the roof surface but still allows any moisture from behind the membrane to evaporate to the inside. This insulation-plus-Intello-approach is about the gold standard in roof design and I am about to use it on my other rental property. This is going to be more expensive than just blowing in cellulose to the floor of the attic, but it will still save you loads of money on the bills and give you a completely air tight roof and get rid of the condensation problems in the attic. However, still fix the source of moisture first, because that moisture is causing mold elsewhere in the house, condensation on windows and probably even making the towels stay damp on the rack too.

Q: If you have vinyl siding, is it worth it to replace with insulated vinyl siding?

A: I have not done this, so I have no direct experience. I doubt that it would be worth it if you have no other reason to replace your siding. If you are going to replace your siding anyway, then I think it might make economic sense. However, I would compare the cost of the insulated siding to the cost of filling the wall cavities with dense-packed cellulose (which will add far more insulation value than the insulated siding) and then replacing the siding as is.

Q: Would you suggest installing insulation in basement ceiling even if the basement walls are insulated?

A: If the basement space is used as living space (and heated) then there is no need to insulate the ceiling if the walls are already insulated. It will be better to just install a thick pad under the carpet – and that will keep your feet warmer too! If the basement is not used as living space and is unheated then it makes good sense to insulate the ceiling so that you are not waseting your heat on your luggage.

Q: Do you have an EPDM rubber membrane roof with no attic/vent space?

A: Yes.

Q: What research is available concerning indoor air quality and various kinds of insulation? What effect on the air we are breathing in our homes? What outgassing, particulates and other pollution is caused by different kinds of insulation?

A: The easiest way to reassure yourself about indoor air quality is to measure it using an inexpensive indoor air quality monitor. Ours is from Foobot but there are many others. Our indoor air quality is consistently excellent. Our insulation is a mixture of fiberglass and ISO board panels which are essentially spray foam in a board. I do not know of any indoor air quality issues arising from any of the common types of insulation like fiberglass, cellulose, spray foam, ISO board or rockwool. Concerns over off-gassing from insulation usually relate to urea formaldehyde foam insulation. The use of this was banned in the USA in 1982.

Q: Our 100-year-old walls are very drafty–quite icy to the touch in winter. In multiple home energy audits, we’ve been told it is impossible to blow insulation into them. If we replaced the siding, would that allow us to add insulation outside the existing walls. Likewise, we definitely don’t have 10 inches to spare in the attic. If we replace the roof, is it possible to add a layer of insulation?

A: If you replace the roof, you can add a layer (or two) of ISO board to the roof on the outside of the plywood sheathing. This is what we did on our flat roof. The DOE recommends that roofs be insulated to at least R49 in MA. This would require 5 layers of ISO board. In my opinion this is spending too much money given how much you can save – insulation has diminishing returns to investment. After about R35-R40 the savings from the next layer of insulation rarely pay for the extra cost. Spend the money you save on a good air-sealing job.

Q: My attic floor is insulated but not the ceiling. I had heard conflicting things about insulating both. What is your advice?

A: Insulating contractors I have spoken with recommend removing the insulation on the floor before adding insulation to the sloped sides of a roof. This makes sense if the insulation has mold growing or has mice living in it.  But other than that, I do not see what harm in having more insulation. Often, the reason people are insulating the sloped sides of a roof when the floor of the attic is already insulated is because they are adding a heat pump air handler unit to the attic. It is essential to insulate the sloped parts of a roof when adding an air-handler unit to an attic or the heat from the air handler in winter will melt the snow and cause icicles or ice dams. The only circumstance that I could see that would warrant removing the insulation on the floor would be if the attic now gets warmer in winter than the house below it. Then it would make sense to allow the heat from the air handler to be conducted down into the house. These circumstances would be quite likely if you were adding a very thick layer of spray foam to the sloped parts of the attic. 

Q: How useful is blowing in insulation in the exterior walls?

A: Very. Especially if you have none today. 

Q: We have essentially no insulation in our walls, and relatively little space in the walls (brick outside, plaster inside). Is there any kind of insulation that will help there?

A: If the gap is narrow then the only solution is probably to inject spray foam through the outer brick wall. There is one company in MA, Building Envelope Materials that has developed a “pin hole” injector system for sprayfoam which means the holes are much smaller than the old way of doing it.

Q: We need a new roof.  We are thinking of metal.  What do you think about metal?

A: I think standing-seam metal roofs are the best long-term investment for making a roof. They play especially well with solar panels because the panels are clamped to the seams, so no penetration is required on a sloped roof. 

Q: MassSave would not touch our attic unless we first did very expensive removal of vermiculite. If not for the vermiculite, they would have blown in insulation over the existing fiberglass. Is there a practical way I can add insulation without hiring a contractor?

A: If you are OK with the vermiculite being left in place the easiest way to insulate is to just buy fiberglass at the local home improvement store and roll it out over the existing insulation.

Q: Might you only insulate the ceiling of the utility side of the bsmt if half of the bsmt is conditioned space (family room)?

A: Yes, this is a good idea – insulate the basement ceiling where you don’t mind the basement getting a few degrees colder. If the basemetn space is living space then it is better to not insulate the basement but instead insulate the walls and add a thick carpet with a thick underlay pad.

Q:  Are there paint additives with insulating properties you recommend?

A: No. Paint does not insulate. However, there are paints that can seal out the wind and perhaps that is what you are thinking of?

Q: Do the costs quoted include installation costs as well, eg $26k for heat pump, $1k for insulation.

All my costs include the cost of installation.

Q: Is house wrap critical to sealing a home’s exterior?

A: Wrapping a house in Tyvek or a similar membrane that allows moisture to dry to the inside but block wind and rain from the outside is a very good practice in cutting your house’s bills and carbon emissions. However, we got to a zero carbon and zero bills without one. 

Q: Once the air leakage is reduced, are you installing hrv or erv to provide fresh air?

A: We did not. It is almost impossible to seal an existing house so well that it gets stuffy and needs mechanical ventilation. However, on tightly sealed new construction you really do need one, it is not an option.

Q: mass save pays for any type of insulation?

They mostly pay for blown-in cellulose in attics and fiberglass in the ceilings of basements. They do not pay for spray foam.

Q: If the basement is conditioned space, is it wise to bother insulating the ceiling?

A: No, insulate the walls and floor of the basement instead of the ceiling. 

Q: What do you think of honeycomb shades for windows? I have seen these claiming R4.7. Any thoughts?

A: I think they are a very good idea. They are cheap, look good and are very effective. Also, thick, pleated, lined curtains that go to the floor (or window sill) can add R3-R5 to any window. Window inserts can add R1-R2 to any window, but they also block drafts which neither shades nor curtains can do. Many older windows, especially sash windows, leak like sieves, so for these the window inserts are probably the best idea, but if your windows are not drafty then I think either curtains or shades work great and are pretty inexpensive.

Q: What are your thoughts on flash and batt to save money insulating a new house?

A: Flash and batt (which is where about 2” of spray foam is installed to provide the moisture and air barrier and then the rest of the cavity is filled with fiberglass) is very economical.

Q: You insulated your basement ceiling because it’s unfinished and unheated? If you heat your basement, it wouldn’t make sense to insulate the basement ceiling, right?

A: Correct. Go to the top of the class. If you use the basement as living space, you need to insulate the walls and the floor of the basement.

Q: Another question please.  Our contractor is saying that spray foam insulation on the basement ceiling is more efficient. 
Do you have any thoughts about this recommendation?

A: Hi XXXXX, spray foam has R5 insulation per inch whereas fiberglass is R3, so in that sense spray foam is more efficient.
However, spray foam sticks to the joists like glue (that’s the idea) so you can’t get it out if you need to get access to a pipe or wire. For this reason I do not recommend spray foam for the ceiling of a basement. 
Also, spray foam is more expensive per inch and per R value than fiberglass. So, economically, sprayfoam is less efficient than fiberglass. 
And, finally, the gas used to blow the bubbles in spray foam is 1,000 times more potent than CO2 at causing global warming. 
And, finally finally, spray foam is flammable whereas fiberglass is not. 
For all these reasons, I prefer fiberglass to spray foam for insulating the ceiling of a basement. 

And, really finally, fiberglass is porous to air which means that if some water does get into the fiberglass (like from an overflowing bath) it will evaporate and dry out. Spray foam is water tight which means any water falling onto the spray foam from above will sit there like a puddle with no ventilation and hence will take much longer to evaporate and dry out. This is risk for mold, rot and asthma for the people in the house.

Q: What about a walkup attic with flooring installed, so insulation hard to access to add

A: insulate the ceiling of the attic, i.e., in between the roofing rafters on the sloped part of the roof. Sprayfoam does a good job here because it sticks to the rafters, but fiberglass is cheaper and very effective. 

Q: Did you consider drywalling the basement ceiling to complete sealing on all six sides?  

A: No. I actually like having the fiberglass exposed so that I can get to pipes or electrical cables if I need to. You do need to seal the drafts somewhere, but that does not have to be the ceiling of the basement. I sealed obvious holes in my basement walls with a few cans of sprayfoam.

Q: Does it make financial sense to add insulation to walls if you’re planning to replace the siding anyway?

A: Often yes. The insulation itself is cheap and effective. It is the cost of all the labor to take off the current siding and replace it that makes insulating walls too expensive to be financially feasible for most people. If you have very little room, Spaceloft is a good option. It adds R10 per inch, but it is expensive (at least as far as insulation goes). If you add anything much thicker, like 2” of ISO board you will need to move the windows out or at least extend the sills and frame. If you are going to go to the expense of taking off the siding, then you should consider adding an airtight but breathable membrane because this will seal out drafts. 

Q: Does Mass Save rebate for insulation reqiure contractor installation?

A: Yes, and a Masssave audit too.

Q: How often does insulation need to be replaced?

A: Spray foam will last as long as the building. Dense-packed cellulose and fiberglass tend to settle down over time and so become less effective. However, taking off your siding or drywall is expensive and the reduction in heating bills will likely not pay for the cost of this work even if your insulation is in poor shape. It is better to add insulation opportunistically see Chapter 2 in Zero Carbon Home.

Q: How toxic are the various kinds of insulation?


A: This question mostly arises in conjunction with formaldehyde in foam insulations. Formaldehyde is not used in making cellulose insulation which is made from shredded newspaper, fiberglass which is made from fibers of melted glass or rockwool which is made from fibers of melted rock.

Strictly speaking spray foam is not toxic. But, if you house catches fire, it does burn with a thick black smoke that can be toxic and this is one reason I avoid it if I can and use fiberglass or rock wool (neither of which can burn) instead.

Also, the gas that is used to make the bubbles in spray foam is usually a gas called an HFC (or hydrofluorocarbon) that, although, strictly not toxic itself, is 1,000 stronger than CO2 in causing global warming, so it is, in that sense, toxic.

Q: How can you find out what insulation your house has already if you are not the original owner? hope this is not a dumb question!

A: The easiest way is to drill a small hole (about a ¼” diameter) through the drywall in an inconspicuous place like in the back of a closet. Then take a piece of wire with the end folded over to make a hook and push it through the hole and pull it back out. If you see strands of glass it is fiberglass, if you see pieces of what looks like shredded newspaper it is blown in cellulose and if you hit something that feels slightly soft but nothing comes out if it is spray foam.

Q: How can you tell if you have insulation in your walls?


A: The easiest way is to drill a 1⁄4” hole in a wall in an inconspicuous place like in the back of a closet. You may be able to see the insulation through this hole or you can take a piece of wire, bend the end back to make a hook, insert it into the wall them pull it back out. If you see strands of fibers it is fiberglass, if you see what looks like old shredded newspaper it is cellulose and if you can’t get the wire in but it feels a bit spongy then it is sprayfoam. You can spackle the hole when you are done.

Q: How to insulate an attic room with limited space between ceiling and underside of roof?

A: The only option here is to add insulation to the underside of the existing ceiling. See the section called, “Insulating the walls – opportunistically” in Chapter 1 of Zero Carbon Home.

Q: What is/are your top recommendation(s) to insulate a home as a renter?

A: Please download my mini book, “Special Stuck at Home COVID19 Edition” which is available for free on my website. Use the code COVID19. This covers several different techniques that are so effective and so cheap that they often pay for themselves in under a year, which make them good things for even a tenant to do.

Q: I’m curious about basements – I find that more and more people are finishing their basements. Is that generally a bad call in terms of energy efficiency, because insulating the walls is so much more difficult?

It is easy to insulate a basement when you are finishing it. You just put roxul boards on the walls, cover them with drywall and paint. The only hard part about this is moving all the stuff out of the basement to do it, but since you have to do that anyway in order to finish it, it is actually quite easy. The floor is harder to insulate but even putting a layer of foam underlay on top of the concrete and then putting in thick carpet will greatly improve the insulation. Before anyone does this though I would recommend installing a sump pump and wiring it into the emergency panel and having a back up electrical supply (either a generator or a battery) because even a small amount of water in the basement would now mean that your carpet is ruined. This happened to us, 3 months after installing a beautiful new wool carpet that cost $3,000. We had to put the whole thing in a dumpster.

Q: Does spray foam have a high embodied carbon footprint?

Q: I understand the carbon footprint for the plastic foam insulation is pretty high. 475 Building Supply and PHIUS are recommending cellulose and wood fiber bd which can be carbon sequestering. Short of that they are also recommending ROXUL, much lower carbon footprint.

A: Spray-foam insulates well and hence helps to reduce global warming. However, the gas used to make the bubbles in the foam is often a gas called a hydrofluorocarbon or HFC. If these gases sound familiar it may be because of their cousins, chlorofluorocarbons, or CFCs, which became infamous for causing the hole in the ozone layer. CFCs were banned in 1996. They were replaced by hydrofluorocarbons, which don’t deplete the ozone layer but were later found to cause global warming.  Hydrofluorocarbons can be about 1,000 times as strong as carbon dioxide in causing global warming. Hence, installing spray-foam has a high carbon footprint even if it insulates well. Because of this, some companies are now using hydrofluoro-olefins or HFOs to make the bubbles in spray-foam insulation. HFOs have very low global warming potential. If you are going to use spray-foam insulation I recommend these HFO-blown foams. 

Rockwool boards are also great insulation and are fireproof, which is distinct advantage over sprayfoam which burns with a thick black smoke. 

Dense-packed cellulose is also a good insulator. However, I am unconvinced by the argument that it is sequestering carbon out of the atmosphere because that wall will eventually be demolished and either decompose in a landfill or get incinerated. Hence, its carbon is returned to the atmosphere. I think it is more accurate to say that using dense-packed cellulose delays the carbon-dioxide emissions compared to the tree falling down and rotting in the forest, but I think it is inaccurate to think that it is permanently sequestering carbon dioxide in the sense that other carbon-sequestration technologies (such as turning carbon dioxide into rock and burying it underground) do. 

All types of foam, whether sprayed or boards, have similar end-of-life issues as dense-packed cellulose and are likely to get incinerated (releasing their carbon dioxide) or buried in a land fill (where the carbon will be permanently sequestered) when the building is demolished. 

Overall, I think that fiberglass, rockwool and dense-packed cellulose (treated with a fire resistant coating) are the best insulating materials, because they do not have the global warming potential of releasing HFCs into the atmosphere like sprayfoam does, they are fireproof, and they can easily be separated when the house is demolished and recycled or reused. On a renovation we are going to be doing shortly, we plan to use rockwool boards outside the air-barrier membrane (as the thermal-bridging prevention layer) and rockwool batts in the 2”x6” cavity walls for the insulation. On a different renovation we are planning to use dense-packed cellulose and rockwool boards. See my other answers to related questions on walls, SIPs, and controlling moisture in general in houses. 

Q:  How cold does your basement get in the winter with the insulation in your joists and how did you airseal that area?

A:  The basement temperature is 60°F to 65°F in winter even with the heat-pump hot-water tank cooling and dehumidifying the air in the basement. The temperature used to be about 8F higher than this before I insulated the ceiling of the basement and installed the heat-pump hot-water tank.  I air sealed obvious gaps in the ductwork with aluminum tape (do not use duct tape for this) and also sealed gaps along the joint between the concrete and the walls with a few cans of sprayfoam.

Q:  If we need new siding on just one side of our home which has house wrap, can we add foam board on the outside side of the wrap and then replace the siding?

A:  Yes. My preference would be to add rockwool as the insulation board – see my answers on insulation, wall construction and condensation problems in houses.

I have no insulation in my walls

Q: So, your walls are insulated with 4″ fiberglass batting?  Earlier I had the impression that your house has no wall insulation.  What about homes much older that ’74 with NO wall insulation? 

A: Yes, our house has 2” by 4” stud cavity walls filled with fiberglass. The only answer, John, is to get wall insulation. You can do this without having to remove and replace all your siding by blowing in spray-foam insulation or dense-packed cellulose from the outside. This requires drilling holes in the siding, but these can be patched afterwards, or a small section of the siding can be replaced. If you have no insulation today in your walls you are losing money through the walls the way that water runs through a sieve. 

Adding insulation to a wall

Q: I watched a presentation of yours a few weeks ago, the one sponsored by Sherborn and Holliston. Impressive and informative…many thanks.
As a result of the presentation I’m interested in adding 2” insulation board, as well as Aerogel, when I re-side the house. My wife does not like foam off-gassing, so we will leave our fiberglass batting alone and add the insulation board outside of it.
There are LOTS of insulation panel types to choose from. Any recommendations on which are best?
In your presentation you mentioned the aesthetic problem with adding 2” or 4” insulation boards, i.e. that they strand the window a few inches inside the siding. I wonder if this problem could be overcome by cutting the insulation board surrounding the windows into picture frame dimensions by making – a 3-d mitre cuts of the insulation at the window corners. One would end up with thin insulation board at the edge of the windows, but gain a lot of insulation everywhere else.
Any thoughts much appreciated.

A: You can certainly do this kind of mitre cut to lessen the impact of installing the insulation. If you are adding only 2” it might still look good. I think it is best to discuss this with the window installer. 
The Aerogel product has the best R value per inch at R10, so 1” has the same insulation as a 2” ISO board, but it’s only an inch deep. It is more expensive psf than ISO board. I do not know what it’s flammability rating is though. 
A more fire-proof alternative than the foam boards is to pull out the fiberglass and replace it with rockwool batts (Roxul comfortbatt is the most widely available). It is about R4 per inch vs R3 for fiberglass. Rock wool cannot burn (it is literally strands of melted rock) so it is better fireproofing. You can add a 1.5” rockwool board (R6) instead of the ISO board outside the studs too. If you are going to go to all this trouble it is probably worth adding an airtight/waterproof membrane on the outside too. Siga makes these type of specialized membranes. They have to be properly installed (with the seams sealed) to create the air-tight barrier. After this your house will be far better insulated and far less drafty. 
I am glad your wife is concerned about off gassing, but I think there is an even bigger concern: fire resistance. Sometimes I feel like a lonely voice on the flammability of foam products. I intend to use rockwool when we add insulation on our rental property for precisely this reason. It is not just the flammability that bothers me, it is the thick black smoke created when foam insulation burns. I will also be adding the membranes for air tightness and humidity control too. 

Should I insulate my doors?

Q: What about insulated non-glass doors – are they am important adjunct to triple glazed windows?

A: A typical wood door is only about R1-2 or only slightly better than a single-glazed window. So, having insulated doors will help cut your carbon footprint. However, I do not know of any way to improve the insulation of a door other than by replacing it. So, you would need to do the calculations to see how much carbon footprint and money you would save. Very roughly, if you improved the R-value of a door by R1 (say from R2 to R3) you might save about $40 a year on heating bills. Before you do this though, I would make sure the door has no drafts around it. Sealing these drafts with weather stripping is cheap, easy and effective. A triple-glazed glass door can be R4-5 which is better than any wood door. Most of the external doors in our house are actually sliding patio doors and they are R4 triple-glazed doors. Our front door is wood and it is only about R1.5.

Is it worth adding insulation to a ceiling with 8″ already?

Q: Could you talk a little about financials of adding iso board to a pitched roof-cathedral ceiling with 8″ cellulose in rafter bays and shingles that otherwise wouldn’t be ready for replacement? 

A: If you have 8” of insulation already, then paying a contractor to add more insulation is unlikely to save enough money on the bills to pay back the investment in a reasonable period of time. However, it might make sense if you did it yourself, which is easy if you are installing fiberglass. I have found with my client work that even with 6” of insulation already, the savings on going to 12” is only about $150 a year. If you have no insulation today it is worth adding 12” but going from 6” to 12” will not save very much money. If you did this very cheaply, by just buying rolls of fiberglass and installing it yourself it would shorten the payback period. Please see also my other answer (below) on cathedral ceilings.

Insulating a cathedral ceiling

Q: Insulation recommendations for cathedral ceilings?

A: Cathedral ceilings are tricky to insulate. Most building codes require them to be vented to remove the inevitable condensation caused by warm, moist, air rising to the top of the ceiling and penetrating the ceiling through light fixtures, skylights, cracks and just from diffusion through the drywall. The moisture in this air then condenses when it hits the cold roof surface. If this does not evaporate you will get mold and rot. This is especially problematic on north facing roofs that do not warm up in the sun. You can now get vapor barriers specially designed for this situation. One supplier is Siga in Switzerland. I have not used this myself, but I intend to use it when converting unfinished space in our rental property to finished space. With a proper vapor barrier, you can add insulation behind the barrier. Check out Martin Halladay’s posts on this topic on Green Building Advisor (GBA). One of his posts states: “GBA gets more inquiries about rotting cathedral ceilings than any other type of building failure. That’s why I’m conservative about recommendations for cathedral ceilings. It’s important to get these details right. If you screw things up, everything gets damp and begins to rot.” See below information on condensation problems in general.

Q: mentioned insulating the ceiling of your basement, what about insulating just the sill?

A: The sill (the area above the concrete foundation and the floor joists) is a major source of drafts and heat loss in a house so sealing gaps and insulating it makes a lot of sense. However, don’t stop there. The ceiling of the basement losses a lot of heat from the house to the outside and insulating that will greatly reduce your heating bills and carbon emissions.

Should I insulate a crawl space?

Q: What about sealing/insulating crawlspace walls and plastic on soil to insulate crawlspace? 

A: Insulating walls in crawlspaces is a very good idea. It is a lot easier to insulate the underside of the floor than to try to insulate the soil because you can just push fiberglass in between the joists. 

Response: Hi David, Thank you for replying. I had noticed condensation problems in the crawl space due to uninsulated HVAC ducts sweating in the summer. Venting the space in the summer allows humid air into the space where colder pipes sweat badly. Researched this and found a revised opinion on what to do with crawl spaces. The advice I read (sorry, no references) was to seal the wall vents, insulate the inner walls and cover the soil base with heavier plastic. Wetness due to water inflow may require drains and a sump pump. I am moving in that direction (DIY) having previously insulated the joists under the floor and ducting but still experiencing the moisture issue. Thank you again for sharing.

Reply: It sounds like you have two sources of moisture in the crawl space. The vents and the soil. I think you will need to deal with both to stop the water condensation on the HVAC ducts when the AC is running. Sealing the vents will help, but without dealing with that wet soil it will probably not be enough. I think the plastic sheeting on the soil will help, but it is a band aid, not a cure. The real question is why is there so much moisture in the floor in the first place? Is the ground water high near you? If not, are your gutters in good shape? Overflowing gutters can put a lot of water in the soil right by the house and this will wet the soil under the crawl space. I have seen this at our rental property.  Repairing the gutters (I put a perforated metal plate on the top of the gutter to stop leaves blocking the gutters and causing them to overflow) and replacing split downspouts (and adding extenders to the bottom of the downspouts to keep the water away from the foundation), worked. I also sealed cracks in the basement concrete with a can of spray foam – cheap and effective as a water barrier as well as an air barrier. Once you have solved this problem, I think replacing your hot-water tank with a heat-pump hot-water tank will help. It will not only cut your bills and carbon footprint, but it will dehumidify the basement air too. Once you have done all this, I would insulate the HVAC ducts, but if you don’t deal with the moisture first, you will risk getting dampness and mold on the insulation.

What insulation should I use in a stone basement?

Q: What about solid stone walls/floors and insulation? Ensuring breathability. 

A: Probably the best solution here is to use a product like rockwool boards for insulation. They are breathable so you reduce the risk of mold behind the boards. They are also fireproof, unlike most foam insulation. Most building codes require a fireproof layer (like drywall) over a flammable material like foam insulation. Unless you want to paint the surface of the drywall I think it is easier to just install rockwool. Also, it is probably a good idea to also install a heat-pump hot-water tank. Unless you have very high electricity prices, these are cheaper to run than heating water with a boiler, even one powered by natural gas. If you use solar panels to generate your electricity it will be much cheaper than heating your hot water with natural gas. They dehumidify your basement which will reduce the risk of mold behind those rockwool boards. 

Q: How do you avoid the prob of getting condensation in the late fall and early spring with a spray foam insulated homes? Did you encounter any problems of this type?

A: We have not used sprayfoam so I do not have direct experience of this. However, the best solution to any kind of condensation problem (and many houses have condensation even if they don’t know it) is to control the indoor humidity. Please see other answers on this topic on my website under Webinar Q&A.

Condensation and moisture problems.

These last few questions raise issues related to dampness or moisture control. While moisture control is not explicitly part of a zero-carbon retrofit, I think it is very important to pay attention to it or you will get mold and eventually rotten timbers. This is not a trivial problem. A house in our town was condemned by the board of health because mold issues sent the occupant to the hospital with asthma. The risk of condensation/mold/rot increases if you do a lot of air sealing because air sealing reduces the flow of air in the house. It is this air flow that evaporates the condensation and prevents mold. Early-generation PassiveHouses, which rely on exceptionally tight building envelopes, often developed mold issues. These problems can be overcome with good humidity control.

In the winter, drafts rob you of heat (and dollars), but they are also a source of dry air that evaporates any moisture that has condensed in your walls, basement or attic. Condensation happens when the temperature of the air falls below the dew point. This is what causes dew on the grass in the morning. Overnight the temperature dropped below the dew point of the air. The dew point of the air varies with its humidity, but it is often around 40-45°F. This is why you see dew on the grass in October and April because we have fairly warm days with lots of humidity in the air, followed by cold nights. If there is dew on the grass, there is dew in your walls.

At these times of year (spring and fall), condensation in your walls and roof is inevitable because if the house is at 70°F and the outside is at, say, 40°F. Somewhere in the wall, the temperature is at the dew point and condensation happens. Air flow up your cavity walls and through your roof evaporates this condensation. This air flow is not driven by wind. It is driven by the stack effect which creates vertical air flow in the house caused by warm air rising. Condensation does not cause mold as long as it evaporates within about 24 hours. Condensation is inevitable, but mold is not. 

The best way to prevent mold in a zero-carbon retrofit (or any house) is to control the humidity. If the humidity in the house is lower than that in the walls (which it will be as long as the house is warmer than the dew point of the air, which it almost always will be unless you keep your house at 50°F) then condensation can dry to the inside of the house by diffusion. Even drywall allows moisture to dry through it by diffusion. This provides moisture a way out of the wall cavity even if there is very little air flow up the wall cavity. Air-sourced heat pumps come with humidity control. On mine, I can set the humidity level as easily as I can set the temperature. I set it to 40% year-round which feels comfortable because it does not leave your skin feeling dry, but the air is still dry enough to dry out the bath towels or the laundry.

In addition to the humidity control on the heat pumps, I also have a heat-pump hot-water tank in the basement. This dehumidifies the air in the basement. Humid air is less dense than dry air and so it rises. Warm air is also less dense than cold air, so it rises too. If the air is both warm and humid, it really rises. Basements are often damp and are often warm (at least in places) because of the furnace or boiler. The warm humid air in the basement rises through the house, reaching the roof where the moisture often condenses on the cold roof surface. The source of mold and rot in cathedral ceilings can often be traced to dampness in the basement. Hence, if draft sealing is part of your zero-carbon retrofit, I think it is essential to control the humidity in the house and the basement. 

Heat pumps with humidity control, heat-pump hot-water tanks and draft sealing play very nicely together. 

Q: Have you had any issues with internal condensation following air sealing?

A: I have not and I have never heard of it being a problem on a renovation because it is almost impossible to get the natural air leakage down low enough to get to the point of getting condensation problems. However, one of the benefits of doing a comprehensive “zero carbon, zero bills” program is that heat pumps control the humidity as easily as they control the temperature. I have only once had condensation on my windows since I have installed the heat pumps and that is in our mud room when I was growing plants indoors throughout the winter which created high humidity in a room with very little air flow. Opening the door to create more air flow solved the problem.

Q: Why do you insulate the ceiling of a basement, shouldn’t you just let the heat from the furnace rise to the floor above?

A: I have found that the physics of heat flow in basements is widely misunderstood even by people for whom it is part of their job. 
The most basic fact to understand is that heat flows from higher temperature to lower temperature. This is a fundamental as the laws of gravity. You don’t let go of a ball and see it rise. It falls. Always. The same with heat, it always flows from higher temperature to lower temperature, always and without exception. On average (I will deal with the area above the furnace in a moment) the house is at say 70F, the basement at say 60F and the outdoor ground below grade is at 50F. Heat flows from your house to your basement and from your basement to the ground. There is a lot of air circulation in a basement, sometimes deliberately because there are vents in the basement, and sometimes just because of convection currents caused by – you guessed it – that furnace. The heat rising from the furnace (lets say the air above the furnace is at 90F) stirs up the air in the basement causing the air to circulate around the basement and warm up the cold walls and floor of the basement (because they are colder than the warm air pushed up by the furnace). In the area right above the furnace the air, at 90F is indeed warmer than the floor of the house which is at around 70F and so heat does flow from this warm-air column to the floor of the room above the furnace. The further away you go from the area immediately above the furnace, the cooler the ceiling of the basement will get and at some distance, I am guessing about 15’, it will drop to 70F. Further away from the furnace the ceiling will be colder than the air in the basement and the flow of heat will be down from the house to the basement. So the ideal situation would be to insulate the entire ceiling of the basement except for a 15’ radius circle around the furnace. You could easily determine the size of this ring by just measuring the temperature of the ceiling of the basement using an infrared thermometer (such as the one I have, the Black and Decker TLD100 which you can get for about $30 at Amazon) during the time that the furnace is on.
I installed the fiberglass insulation on the ceiling of our basement in exactly this pattern. The ceiling of our basement is not insulated in a strip about 8’ wide and about 20’ long above the furnace. The long axis of this strip aligns with the long axis of the basement, which is where I think the convection currents carry most of the heat. Since, now we have heat pumps, we use the furnace only on about 20 days a year this has very little effect, but I think it is worth doing if you are still using your furnace year round.
However, what I did do that I think has a much bigger effect is to insulate the bottom and sides of the ductwork which run along the ceiling of the basement. I did this with 4″ thick fiberglass. This allows the warmth of the heated air inside the ducts to warm the floors above them while keeping most of their heat for the rooms to which the ductwork carries the heat like the bedrooms on the upper floor in the corners of the house. The heat in the ducts is no longer warming the basement. This is part of the reason the temperature in my basement dropped about 8F and the temperature of the air coming out of the vents in the bedrooms increased 5-10F. 
Overall, the trick is not to heat the entire house. It is to heat the parts of the house you live in.  
Let me know if you have any further questions.

Q: If heat rises (and the basement is warmest in winter due to stored energy in ground from summer) how does insulating basement ceiling help keep heat in the living space?

A: The ground below grade is at about 50-55F year-round. If the living space is kept at 70F year-round then heat is flowing from the living space down to the basement and then out to the ground. Heat (energy) flows from higher temperature to lower temperature. This is the most fundamental law of thermodynamics. Heat never moves in the opposite direction. This is why you will save money by insulating the ceiling of the basement. 

Air Sealing

Shouldn’t air sealing come first, even before insulation?

Q: since this is talking about what makes sense financially, it would be useful to add air sealing to your fab 4, and I think you would find that this is the best investment, even better than making your basement cold (insulating the ceiling). I think the conclusion that you hinted at is that you should not “wing it”, but do the shortest payback measures first, and this results in the least expensive net zero result. Anyone can be net zero by adding solar collectors, but the question is how to get there most cost effectively. So, things like doing air sealing, should be done first, always. Yes? 

A: I believe draft sealing is important, it just wasn’t a big issue on our house. This is because both our roof and our walls were already well sealed. However, our house is the exception, I have seen houses where draft sealing alone has cut the energy bill 25% and the Department of Energy states that air infiltrations through gaps and cracks can lose 30% of a house’s heat.

So why weren’t drafts a big issue on our house? Our house has a flat roof with a rubber membrane waterproofing layer on the top of the roof. Under this waterproofing layer are two layers of 2” ISO boards. This makes my roof air-tight as well as water-tight. This makes my roof far more draft-proof than a typical roof with sloped sides, shingles and a lot of small air cracks between the walls and the roof. 

Also, our siding is vertical boards with tongue and groove connections, and it is well painted. This makes my siding almost impenetrable to wind. This cannot be said of unpainted shingles or standard shiplap horizontal siding, both of which allow a lot of drafts.

Also, because our house is two stories high with a flat roof, the top of our house is about 20’ high which is much lower than the 30’ or so of a house with a pitched roof. The low height of our roof reduces the “stack” effect which is where rising warm air forces air to leak out of the top of the house and sucks cooler air into the basement. So, our house did not have many of the causes of drafts in typical houses. In contrast, our windows were terribly drafty. Our windows were all replaced with well-sealed and well-insulated triple-glazed windows. We paid particular attention to making these windows draft proof around the frames.

The biggest source of drafts through the walls in our house was along the sill plate, which is the place where the top of the concrete of the basement joins to the wood studs of the walls. I sealed obvious drafts with a few cans of spray foam and I weather-stripped the bulkhead door.  I also stuffed the fiberglass that I used to insulate the ceiling of the basement into the sill plate and this cut down the drafts.  Since this was done at the same time as I did the insulation, I could not separately measure the contribution of the draft sealing compared to that from the insulation. Hence, you can say that the money savings I attribute to insulation alone are actually due to both insulation and draft sealing. I just think the insulation was by far the bigger contributor because the drop in my energy bills was almost exactly what was predicted from my energy model which directly accounts for insulation but, back then, did not account for drafts. The current version of the energy model explicitly accounts for drafts.

Early on in my zero-carbon renovation, I added weather-stripping to my external doors, but I could not detect any change in the energy bills from sealing the drafts on the doors alone.  That is why I do not call it out as one of the fab four. That does not mean that draft-sealing is unimportant. In fact, on most homes with sloped roofs, shiplap siding (or shingles or shakes) and no sealing of the top or bottom of the walls, drafts can be a major factor in heat loss. Air-sealing is generally cheap, easy to do and highly effective. Unlike other things like heat pumps, solar and triple-glazed windows, it is something that you can do yourself, which makes it a very good return on investment.

There are quite a few other things I did that did not warrant being called out specifically (I wanted to keep it simple) such as: insulating the hot-water pipes in the basement, insulating the ductwork in the basement, replacing an old fridge and adding a heat-pump hot water tank. I think all of these had very good returns on investment, but they were too small for me to be able to quantify with any confidence (except the fridge which paid for itself in 18 months on the electric bill savings). So, I do think they are important, and they have high ROI’s, but they each only cut my carbon footprint by relatively small amounts.

David Green’s House Has 4.6 ACH50 – Proving That You Can Get To Zero Carbon Without Passive House Level Air Tightness

I recently (July 2021) had a blower-door test done at my house and the result was 4.6 ACH50. ACH50 is a common standard for air infiltration and stands for Air Changes per Hour at 50 Pascals. Pascals are, like pounds per square inch, a measure of air pressure. 50 Pascals is about the pressure caused by a 20 mph wind. 4.5 ACH50 is equivalent to 1,035 CFM50 (cubic feet per minute at 50 Pascals). This means that the natural air exchange on my house (i.e., at 0 Pascals) is about 0.23 ACH (sometimes called ACH0). This means that the entire air volume of my house is replaced every four hours due to drafts around doors, windows, walls and chimneys. The natural air infiltration rate in my house is 238 CFM0.

This proves what I have long suspected, which is that it is absolutely not necessary to seal your house to the level of air tightness required by the Passive House (PassivHaus) Institute in order to cut your carbon emissions to zero. 

The Passive House standard is often held up as the ideal standard for low-energy consumption houses. But I have never seen any financial analysis accompanying this conclusion. This data proves that you can cut both your carbon emissions and bills to zero (and I am making a 15% return on investment too) without the expense of creating a very tight building envelope. 

Very few builders can build to a the Passive House standard of 0.6ACH50 and doing so often requires many hours of skilled labor plus the addition of an ERV (energy recovery ventilator) which, alone, can add $5,000 to the cost of the house. I know one contractor who recently did the air sealing on a Passive House project. He gets paid about 3x what a typical laborer on a construction site gets paid. Labor hours add up real fast at those rates! Hence, the Passive House standard for air infiltration can only be achieved at considerable expense – an investment that will never earn a return.

Much like geothermal, solar hot-water panels and thickening your walls with insulation, a super-tight building envelope makes energy sense but does not make financial sense.

Q: Doesn’t improving air sealing mean that ventilation is reduced?

A: Yes it does. Unless the house was built very recently and was built to a very high standard for air tightness (such as less than 3 ACH50) air sealing is very unlikely to reduce ventilation to the point where you will get mold. The best way to avoid mold in houses is to control the humidity (see other answers on this topic). The best (and cheapest) way to control the heating bills (and carbon footprint) is to seal up the drafts and add insulation. See above.

Q: How hard is it to seal the drafts in a basement?

A: When I air sealed my basement there were many small holes and a couple of large ones. One was about one foot square and was just above the sill plate (where the concrete meets the wood framing). You could not see through it, but it obviously connected to the outside probably through hollow walls to an overhang. It was too big to seal with a can of spray foam (I tried) so I stuffed the hole with fiberglass and then coated the fiberglass with a layer of spray foam. The foam stuck nicely to the fiberglass and then also to the wood framing around the hole. One can of foam did it. The other big hole was a hole drilled through the concrete wall, probably for an electrical run but there was nothing in the hole. It was a great way for mice to get in. You could see daylight though the hole. This I just filled with spray foam. This strategy might work well for you. Most of the drafts in a basement are not caused by wind but instead by the stack effect which is where warm air rises sucking cold air in through the holes in the basement. Now we have cold weather this makes it easy to find the holes – you can just feel the cold air rushing in. Seal the biggest ones first and it will increase the flow rate of air through the smaller ones making them easier to detect. You can also detect them with an infrared camera. Libraries often have these available for loan. A cheap alternative is an infrared thermometer. Mine, called the TLD100 is available on Amazon for $30 https://smile.amazon.com/BLACK-DECKER-TLD100-Thermal-Detector/dp/B0044R87BE/ref=sr_1_5?crid=11SQIH456HZHU&dchild=1&keywords=tld100+thermal+leak+detector&qid=1605307783&sprefix=tld100%2Caps%2C237&sr=8-5
Once you have done this you can start a second career as an air sealer. One guy I know gets paid $75 an hour!

Q: Did you measure building envelope and duct leakage before and after air sealing and insulating?

A: We did not, but I intend to do a blower-door test once COVID is over.

A2: In July 2021 I did the blower-door test and the result was 4.6 ACH50 – see above.

Q: What mechanical ventilation strategy are you using? 

A: We do not have any mechanical ventilation. It is almost impossible to get the draft sealing done so well on an existing house that the natural air flow is so low as to cause condensation or mold (see other answers on condensation and mold).

Q: Per the International Mechanical Code (IMC), Mechanical ventilation is required when building envelope tightness is less than 5 ach (air changes per hour). very important to be testing and not guessing to ensure occupant health and safety.

A: I agree, and I will do the blower door test once COVID is over (the result was 4.6ACH50 – almost 8 times as much as is permitted by the Passive House standard. Despite this . However, we do monitor our indoor air quality using a small WIFI enabled meter (ours is from Foobot and cost something over $100) and our air quality is consistently excellent. Our indoor air never smells musty or stale so I think we have good ventilation. I think measuring the air quality directly is a better approach than measuring the ACH50 and relying on a standard to get reassurance. Since indoor air-quality monitors are very cheap this seems a better strategy to me.

A2: In July 2021 I did the blower-door test and the result was 4.6 ACH50 – see above.

Q: I heard that sealing your home too much is not good because your house needs to breathe.


A: Yes, that is true. Air circulation evaporates water that inevitably condenses in walls in spring and fall. Please see my other answers to people’s questions on moisture on this page. However, it is almost impossible to seal an existing house so well that you get moisture problems. It happens far more often on new construction, particularly on houses built to the Passive House standard with requires almost no infiltration of air. The best way to deal with condensation in a house is to control the humidity which is easy to do with heat pumps. Please see my other answers on condensation and moisture problems in houses on this page. Condensation in houses is the most insidious problem in houses today.

Q: Did you measure air quality after you insulated the house?

A: Yes we use an indoor air quality meter (ours is from Foobot, but there are lots of others). Our indoor air quality is consistently excellent.

Q:   What is air sealing?  Is that the same as duct sealing?

A:  Air sealing (or draft sealing) usually refers to sealing leaks in the building envelope rather than sealing leaks in the ductwork. Both are important in reducing the energy used by your house, see other answers on these topics.

Q:  Another question – we have a gas insert in the original brick fireplace.  It is a direct air unit with two tubes going up the chimney.  However, the chimney itself is not air sealed – the wind just plows down.  However, MassSave does not cover it.  Any thoughts on how to seal the chimney space?

A: I have never done this myself and there may be code issues that apply to fireplaces so I would consult an expert.

TRIPLE-GLAZED WINDOWS :

Q: What about adding film with low E coating to existing windows? We have beautiful old windows we want to keep.

A: I have never seen a low-E film. The films that are common are shading films that just block all light. They do not selectively block UV light or IR light in the way that low-E coatings do. I have used the sun blocking films on exposed south-facing windows and it works pretty well, but unless you remove it in winter it does darken the room in winter. A better solution would be to buy a window insert made from glass (such as from Innerglass in CT) and get the low-E coating on the glass. 

Q:  Do you need to special order the window inserts?

A: Yes. They are custom made.

Q: What do you think of honeycomb shades for windows? I have seen these claiming R4.7. Any thoughts?

A: I think they are a very good idea. They are cheap, look good and are very effective. Also, thick, pleated, lined curtains that go to the floor (or window sill) can add R3-R5 to any window. Window inserts can add R1-R2 to any window, but they also block drafts which neither shades nor curtains can do. Many older windows, especially sash windows, leak like sieves, so for these the window inserts are probably the best idea, but if your windows are not drafty then I think either curtains or shades work great and are pretty inexpensive.

Q: How much difference is there between double-glazed low-e windows and triple-glazed low-e windows? in terms of insulating properties?

A: Plain glass double glazing is about R2. Low-E double glazing is about R3. Plain glass triple-glazing is about R3.3 and low-E triple glazing is about R4-R6.

Q: Does the inner glass come out?

A: Yes they are easy to remove if you want to but I leave mine in year round.

Q: how do triple-glazed windows work in the summer with air-conditioning?  Which companies make these?

A: Triple-glazed windows keep the heat in in winter and the heat out in summer. You need them year round. I bought my triple-glazed windows from Lowen, Sierra Pacific and bought triple-glazed glass panels (that were installed by a carpenter in places where we had only a panel of glass that did not open) from a local glass supplier.

Q: Are the inserts custom made depending on variety of sizes of windows?

A: Yes.

Q:  Can you get a window insert with low-E coatings?

A: Yes, but only if the window material is glass. The only maker of window inserts that I know of that makes them in glass is Innerglass in CT, all the other companies I tested were made from clear plastic.

Q: Which company does he recommend for these window inserts?

A: Please see the evaluation I did of the window inserts from four manufacturers. It is in the “Stuck at home COVID19” edition of the book which is available for free from my website, use the code COVID19 at checkout. If it does not work email me.

Q: Do you take these off in the summer when you want to open windows?

A: You can but I do not. I leave mine in year round.

Q: Have you heard of Window Dressers in Maine?  They are an all volunteer group that manufactures indoor storm windows and teach people to make them.  It’s pretty cool. Started with a UU Church group about a decade ago.  They sell 70% and give 30% to low income families. https://windowdressers.org/   They also use foam around the frame, so there’s no sticky tape messiness.

A: Thank you I did not know about them, they could probably have made my windows for me!

Q:  Are you saying you do not open your windows in the summer? Have you tested your indoor air quality? Indoor air tends to be very dirty. Opening windows is the most effective way to breathe clean air in many locations.

A: We open plenty of windows and patio doors in summer, or more commonly in the shoulder season when it is around 70F outside, just not the ones with the window inserts.

Q: Are the window inserts used only in the winter?  Or are they permanently installed?

A: You can easily remove them for the summer if you like, but I leave mine in year round because insulation in the winter keeps the heat in, insulation in the summer keeps the heat out. 

Q: If you have good storm windows with good, tight fitting older single glass windows, would innerglass type be worth it?

A: Probably not. My calculations were done assuming that the window insert is added to an existing single-pane window. However, unless the storm window really is sealed (and many are not) then the storm window is not providing much insulation and then it would probably be worth it. An exception to this would be if the existing window (single glazed plus storm) is drafty, then the benefit of having the window insert seal the drafts would almost certainly make the window insert pay for itself.

Q: Do those inner glass windows come coated?

A: You can get them coated. I got mine with the low-E coating, it almost doubles the R value.

Q: Can you coat existing windows.

A: No. The low-E coating gets added to the glass at very high temperatures in the factory. You can buy films to apply to the glass from the inside, but these are not low-E coatings (i.e., they do not selectively reflect either UV or IR light) but they darken the whole window. They are good for blocking intense sun but they do not reflect the IR light that keeps you warm in winter.

Q: Are these inserts very customizable?

A: Yes, all the manufacturers require you to measure the window first, so every one is custom. 

Q:  who can replace large double pane sky lights with triple pane.?

A:  Ours were replaced by A&A Construction. Large triple-pane skylights get very heavy and ours had to be supported by metal rails, which made it rather expensive.

Q: should a window insert be added if the current window configuration is single glaze window with low e storm window?

A: Storm windows protect the window frame from the sun and rain but they do very little to insulate because the air is not trapped between the storm and the original window and it just circulates with the breeze. So adding a window insert will improve the insulation and cut down the drafts and noise through a existing window even if it has an exterior storm window.

Q: What about argon windows?

Argon is worth the extra price. Krypton is not.

Q: If you have 3 floors and could only afford triple glazed windows on one floor which one would you put them in? Guess is basement but wanted your thoughts.
A: The best energy performance would probably come from adding them to the upper floor as this is where most heat and air leakage occurs due to the stack effect of warm air rising. However, new windows look far better than old ones so putting them on the ground floor may make more sense aesthetically. Then adding cheap window inserts to the upper floor would greatly reduce the heat loss at very little additional cost. 

Q: If you are building a new house would you go only for Triple glazed windows?

A: Yes. Triple-glazed windows cost only a few % more than double-glazed windows. They cost exactly the same to install. Yet they will halve the heat (and dollars) lost through the windows.

Q: what brand triple glazed windows do you recommend?

A: I have used Loewen windows where we wanted the frame made in douglas fir. Douglas fir is the “weed tree” in Canada, similar to the white pine in New England, but its wood is a beautiful peaches and cream color. See the pictures in Chapter 4 in the book. Loewen is a Canadian company. I found that the major US manufacturers of windows like Marvin, Pella and Anderson did not understand triple-glazed windows. Where we wanted the frame made in pine (at our rental property) because we were painting the frame, we bought Sierra Pacific windows which are made in the USA. I think both Loewen and Sierra Pacific make excellent windows.

Q: You say window inserts are more permanent; I presume they come out in the summer so you can open the windows?

A: You can take them out in summer, but I do not. You need insulation in the summer as well in the winter. In summer you want to keep the heat out, in winter you want to keep the heat in. You need well-insulated windows in both cases.

Q: If we are new homeowners and inherited the windows and they all seem very nice and upgraded (150 year old home) can we assume they are triple glazed or are they likely double? If double do we need to do window inserts in ALL windows? There are a lot 🙂

A: In my experience triple-glazed windows are rare in the USA, so yours are probably double glazed. You can easily tell by looking at the bottom of the glass panel in the frame. If you see two spacers then it is triple glazed, if there is only one spacer it is single glazed. Adding window inserts will improve the insulation, cut drafts and cut noise on any window but if you don’t have the budget to do all the windows I suggest starting at the top and working down because heat rises.

Q:  I live in Phoenix, Arizona. Any suggestions for window solutions to decrease heat coming in? Home is only 12 years old, so not getting new windows yet. What about films? which are best?

A: there are several good options for you. Blinds are cheap and effective at blocking sun. Growing deciduous trees that shade the windows in summer are also very effective if you are prepared to wait the 5-10 years for the tree to grow. Adding window overhangs also works very well but they cost more to add. The sunblock films you can buy at hardware stores do work to cut down the sun’s heat. They are cheap so you can’t lose anything to try it and see if you like it.

Q : I’ve seen triple layer polycarbonate skylights.

A: This is a very good idea though I have not used them myself. They would be much lighter weight than glass ones.

Q: Can you add low-E coatings to existing glass?

A: No, it needs to be added in the factory because it takes place at very high temperatures. However, you can buy reflective plastic film that does a similar job (except it makes the room darker) and add that to the windows. I have used it but it does not last very long. 

Q: Do you remove inserts during summer or are they permanent?

A: They can be easily removed but I leave mine in year round. 

Q: Does Mass Save also provide a subsidy for window inserts or triple glazed windows in addition to insulation?

A: Yes, but indirectly. The Heat Loan is available for window replacements but not for window inserts. It is only available for upgrades from single-glazed windows to triple-glazed windows, it is not available for double-glazed windows. You can’t get the Heat Loan without first having a Mass Save audit. They do the audits remotely.

Q:  What type of weatherstripping or window inserts do you recommend for casement windows?  It sounded like all the recommendations pertained to double hung windows as opposed to casement windows.

A: Most casements seal very well, it is huge advantage of casement over double-hung windows. If your casements do not seal well (perhaps they are warped?) you can add fasteners that will pull the window tight into the frame. You can also add either the foam strips or the white plastic strips that spring up to close the gap onto the faces of the wood frame where it contacts the moving part of the window.

Q: Have analysed storm window impact? financial and carbon foot print?

A: No because I have not added exterior storm windows. However, the purpose of exterior storm windows and interior-fitting window inserts is very different. Exterior storms protect the window frame and its paint from wind, rain and sun. But the air between the exterior storm and the real window is not sealed and it blows away with the breeze, hence it has very little insulating value. With little insulation benefit there is very little cost saving with exterior storm windows hence the reduction in carbon footprint and heating bill is likely to be very small. The purpose of interior-fitting window inserts is to stop drafts, add insulation and block noise. Because they are very effective at blocking drafts (see Chapter 4 in the book) and adding insulation window inserts will both cut your heating bill and carbon footprint. The financial payback is about 5 years. The increase in comfort of the room is immediate.

Q: How much of the savings is from the windows being better and how much is from better air sealing around the windows?

A: I could not tell how much was from each. My windows were exceptionally leaky. There was 1” of daylight (which I had sealed with duct tape) between the frame and the window. So, even if I could tell, I do not think my results would be very helpful to anyone else. It is essential that windows are installed properly and that the gap between the new window and the framing of the house is filled with spray foam. Spray foam creates both an air seal and insulation. I have seen expensive windows installed by expensive installers that were not sealed properly, so watch the installer carefully!

Q: Since the air does the work, how thick does the air gap need to be?

A: The optimal air gap is about ½”. This is supported by both academic research and experiments I did on my own with small double-pane windows that I made in varying thicknesses.  Almost all double and triple-glazed windows use this spacing. If the spacing is larger the ½” the insulation value goes down because then convection currents start to move the heat from one side to the other. 

Q: Is the glazing on the windows always darK?

A: No. We have R4 triple-glazed windows on the lower floor and R5 on the upper floor. They are completely clear to the eye. There are photographs of four different types of low-E glass in the book in chapter 3, so you can see exactly what they look like. 

Q: If a triple glazed window breaks does the frame and glazing need to replaced or just the glazing?

A: The entire triple-glazed panel would need to be replaced, but not the frame. 

Q: For a fairly mild climate (I live in Melbourne, Australia: we rarely get to 0C and a few very hot 40+C days a year), is it worth triple rather than double glazed? How do I make the calculation?

A: In a hot climate you need to keep the heat out. In a cold climate you need to keep the heat in. The answer is the same in either climate: insulation and triple-glazed windows. There are examples of how much money you can save by installing double and triple-glazed windows in the book in chapter 3. In more remote places like Australia, the choice of windows is often less than we have in the US or EU. This often makes triple-glazed windows hard to find. I even found it hard to get them in the US, my windows came from Canada. If you cannot get triple-glazed windows I suggest looking for the best low-E double-glazed windows. If you use a low-E 272 coating on the outside and a low-Ei89 coating on the inside you will get about R4 insulation with only the slightest greenish tint. This is as good as a triple-glazed window with i89 on the inside.

Q: So what’s the difference between having a window insert and storms?

A: Window inserts are sometimes called interior storms because they do look a bit like exterior storms, however the function of window inserts is quite different to exterior storms. Window inserts provide insulation, sound deadening and draft sealing. Exterior storms protect the wood frame from weather but provide little insulation and little draft sealing.

Q: Alpen has R10 windows. Have you seen them?

I have no experience with these windows Raghu. R10 is exceptionally good. 
It is not clear how they make it. It is probably two sheets of glass and 2 more plastic films in between to create the 3 cavities. This works (I have made similar structures my self using stretchy film) but plastic film is not as clear as glass. If you look at the photograph of the window with the mountains in the background you can see the image is somewhat fuzzy and also slightly greenish. Personally I would want to see one live before buying. 
We used lowE 186/i89 triple glazed (all glass) which is R5 upstairs and lowE i89 triple glazed (all glass) which is R4 downstairs. If you add thick, lined, pleated curtains (or honeycomb shades) that go to the floor you can get these to about R10. Going beyond R10 is unlikely to make economic sense. In fact they don’t tell you how much this window costs. My guess is it is expensive. You would have to calculate if the extra savings of going to R10 would payback the extra cost in a reasonable time frame. Until the manufacturer proved this to me, I would remain healthily skeptical. 

Q: Can you comment on insulating curtains?

A: Thick, lined, pleated curtains that go to the floor can add between R3 and R6 to any window. If you add these curtains to a triple-glazed low-E window (which is about R4 or R5) your total insulation will be almost as good as a wall, which is often about R10.

Q: How does the triple glazed windows work in hot humid climate where heat is not required indoors?

A: Having a good thermal envelope prevents the heat from escaping in winter and keeps the heat out in summer. Hence both insulation and triple-glazed windows are necessary in both hot and cold climates. 

Q: Tripled glazed windows: How does the insert fit over the inside of the window? What about the lock? How does the insert fit over the lock?  Can you open the window in the summer?

A: If there is a wood trim around the window (reveals they are called in the trade) that extends beyond the lock mechanism by about an inch then you can fit the window insert. You need to remove the window insert to get access to the mechanism for opening the window. Some people remove their window inserts for the summer season but I leave mine in year round.

Are triple-glazed windows a waste of money in hot areas?

Q:  Just had a contractor tell me that triple glazed windows are a waste of $ in the desert southwest – – true?

A:  Replacing any windows is expensive. Windows are expensive to buy and expensive to install. The full cost of installing any windows is unlikely to pay for itself with energy-bill savings, at least not for decades. However, in our home the additional cost of triple-glazed windows above the cost of double-glazed windows paid for itself (with the additional saving from triple-glazed windows above the savings from double-glazed windows) in about 5 years for a return on investment of 19%. I think something similar would be true in Texas. See Chapter 3 in Zero Carbon Home.

Q: We replaced single glaze with triple glaze a couple of years ago, yet we still get drafts around the triple glaze windows. MassSave, using infrared, told us that we have no insulation in the walls and around these windows,… basically saying, such is life.

A: This is a good illustration of why getting modern windows installed properly is so important. I am guessing that the gap between the stud in the wall and the window frame of an inch or two was not filled with spray foam. It should have been, both to block drafts and to insulate. Short of taking off the drywall and installing the spray foam yourself (a can or two from a hardware store is all you need), there is nothing you can do about this.

Is it OK to measure the R-value of a window at the center of the glass?

Q: How are you calculating the R value? are you using the center of glass U value only? should you consider the frame? 

A: The R-values I measured for the window inserts are taken in the center of the glass or plastic. As you can see from the infrared photographs in Chapter 3 of Zero Carbon Home, the frames are all considerably warmer than the glass or plastic sheet. The frames also have far smaller area than the glass or plastic sheet. Hence, I think that the center of the glass measurement is a good approximation of the overall performance of the window insert. 

Q: Would adding acrylic panels to your existing windows have similar 3-pane performance and much less life cycle ghgs?

A: Adding a single pane of glass or plastic (these are called window inserts) adds a similar amount of insulation benefit (R value) to going from a double-glazed window to a triple-glazed window. You will not save enough money on the heating bill to payback the full cost of taking out existing windows and replacing them with triple-glazed windows in under several decades. This is why I recommend installing triple-glazed windows only when you need to replace your windows for other reasons like they are rotting or falling apart as ours were. In this case the additional cost of triple-glazed over the cost of double-glazed is a good investment. If your windows are in good shape I suggest adding windows inserts. 

How do I improve the insulation on historic windows?

Q: Do you know about the availability of high efficiency windows that will be acceptable for homes in historic districts with strict regulations to adhering to historic accuracy?


A: I think probably the best way to deal with this is to add the window inserts you can see in Chapter 3 of Zero Carbon Home. These are invisible from the outside so historical commissions usually are OK. If you use the glass ones and get the low-E version, you can add about R2 to an existing window. If the existing window is R1 you now have an R3 window, which isn’t great compared to modern low-E triple-glazed windows at R4-5 but it is 3-times better (meaning it loses one third of the heat and dollars) than a single-glazed window. They are also far cheaper than replacing windows and you can fit them yourself, you don’t need to hire a contractor. 

Q: Do you know if window inserts can be made for large windows such as in a church?


A: I have seen them used in churches, but the window will often need two or three window inserts to cover it completely.

Q: Can I use window inserts to reduce the noise from the street?

A: Your question on sound deadening has given me the perfect opportunity to test out the sound meter I have recently bought. Using a white-noise generator on my iPhone I measured the sound deadening of the four different window inserts (see pages 129-134 in the book). As I thought, the flimsy Window Inserts one is the worst, but it is not the glass Innerglass that is the best, it is acrylic-plastic sheet Indow. This is what my perception was, but I thought it was just due to the road noise being higher on the side where the Indow is. Cutting the noise level by 20 decibels is a big reduction. If noise reduction is more important to you than thermal insulation, I would consider the Indow or the Alpina one, which is considerably cheaper and has similar noise reduction and thermal performance.

I found that Alpina was not the best value for money, the Innerglass one pays back faster. It is slightly more expensive but it is a higher R value. The table with these results is on page 133 in the book. Here is the table on the sound deadening results:


All but the Window Insert one will cut the noise considerably. The InDow one is noticeably the quietest of all of them so if cutting down the sound is your top priority I would go with that one. It is the most expense though and does not have a very good R value. Overall, I think either the Innerglass or the Alpina probably offer the best all-around value for you. Yes, I think about $200-$250 per window is about right for these window inserts.

How do I fit window inserts if my windows aren’t square?

If the windows aren’t square then you may be better off going with one of the windows made from plastic sheet rather than the Innerglass one which is made from glass. The InDow one comes with a measuring kit that explicitly accounts for non squareness by measuring the diagonals as well as the horizontal and vertical parts. The frame of the Window Inserts one is so flexible (flimsy you might say) that it would probably accommodate any deviation from straight up to at least an inch, even without explicitly accounting for it. 

Do window inserts create condensation problems?


Window inserts will increase the amount of condensation on the inside of the existing window. This is because you are blocking currents of warm air in the room from moving past, and evaporating, the condensation. If you have a lot of humidity (you will know because there will be a lot of condensation on the windows and your towels will take a long time to dry on the rack) then it would be wise to invest in reducing it. Heat pumps control the humidity automatically but if you can’t fit them to your house, a good alternative is a heat-pump hot-water tank. This will dehumidify the air in the basement. Since humid air rises, condensation on windows is often caused by dampness in the basement. And, yes, you are right, all dampness problems get worse as the envelope is tightened up. Passive Houses, especially the early generation ones, often had dampness, mold and rot problems.  Mold often leads to asthma for the occupants of the house.

Q: What brand of triple pane windows are the most cost effective or are you better off installing a real good double pane?

A: I found that Sierra-Pacific was very competitive for pine-framed windows (good if you intend to paint them on the inside) and Loewen was very competitive for vertical grained Douglas fir frames, which look a lot nicer than pine if you want the wood to show. A double pane low-E window can get to about R3 whereas triple pane low-E can get you to about R5. If you can get only double-glazed where you live, or that is all the space you have room for in the window frame, then adding thick, lined, pleated curtains that go all the way to the floor can add R3 to R6 to any window.

Q: What percentage window to wall area do you recommend and should overhangs be utilized to prevent summer overheating?

A: The less the window area the better the thermal envelope will be. Often houses have about 15-20% window area compared to wall area (without the windows) but our house has 42% and we still have a zero carbon footprint. This is one of the main reasons we need so many solar panels to get to a zero-carbon footprint. Overhangs are great for allowing the sun in in winter and keeping it out in summer. The optimal width of the overhand is about 3’ at 9’ height and 6’ at 18’ height.

SOLAR PANELS:

Questions to ask a solar-panel installer:

  • For how many years does the manufacturer guarantee the panel-power output?
    • Can I make a claim using only the data on the monitoring app, i.e., without having to get the panel independently tested?
  • For how many years does the installer guarantee the array-energy production per year (in kWh / year)
  • What is the cost per kWh of the electricity guaranteed to be produced over the warranty period of the panels?
    • Cost should be after the federal and state subsidies
    • 5c/kWh is very good but even 10c/kWh is still a 55% cut (from 23c where we live) and it is fixed
  • What inflation rate did you use in the financial forecasts? If it is more than 3% ask them to redo the calculations. I used 0%.
  • Did you allow for taxes on the SMART (in MA only) subsidy? SMART is taxable income.
  • For how many years is my roof guaranteed against leaks?

Q: We already lease solar panels (10 years old). Should I replace them by buying new panels?

A: Your lease probably does not allow it. But call them and see if they will upgrade the panels for you. 

Q: were told we do not have the right sort of rafter to support solar panels. How expensive is it to correct this problem? I don’t necessarily care whether it pays for itself, just curious about the out of pocket cost.

A: Usually this is done by “sistering” the rafter. This means bolting a new rafter alongside the old rafter. This is not hard to do and I think any local carpenter could do it for you. Some solar installers will quote for this because they are probably bringing a carpenter to the site anyway.

Q: What solar panels do you recommend? Sunpower?

A: I have bought Sunpower for my house and garage and have just bought Solaria for new arrays on my rental properties. I also was about to buy REC before the installer quit the job. I have recommended Solaria, REC, Panasonic, LG for clients. I think these are all high-quality panels with manufacturers that are likely to be around to honor the 25-year warranty. 

Q: If power from the utility is cut (storm, trees, down lines), the utility will NOT accept power from your panels.  To survive an outage a battery or generator will then be needed; savings and eco-impact are then significantly different.

A: By law, solar panels must disconnect from the grid during a grid outage. This prevents the linemen from getting electrocuted by your solar electricity. If you have a battery you can, during a grid outage, reconnect that battery and the solar panels to your house “behind the meter” (meaning still disconnected from the grid). This is what I am currently installing at three properties. Now the battery and solar panels are acting like a back up generator.

Q: Completely disagree with your re solar hot water panels.  We’ve now had it for 12 months and have never ran out of hot water in a 2-family house with 6 residents.

A: If your hot water is entirely coming from your solar thermal panels then please let me know. Most solar thermal panel systems come with back-up electric heating element so that you never have cold showers even on bitterly-cold, sunless days in winter. So, it is possible that your year-round hot showers are created by using electricity in winter and not by using the sun’s heat. 

Q: Will leasing panels still be financially beneficial?

A: Paying cash is the cheapest way to buy solar panels. Getting a loan is the second cheapest and leasing them is the most expensive. In this sense it is just like leasing a car. Unlike leasing a car, you cannot return the panels at the end of the lease. Solar leases can be very expensive compared to getting a loan but if your credit score is not high enough to get a cheap loan then leasing can still make sense. 

Q: How hard is it to put solar panels on poles next to your house? Rather that on the roof?

A: It is easy. It is called a ground-mounted array.

Q: Can you say which batteries are more eco-friendly?

A: All batteries are more eco-friendly than using a fossil-fuel generator, but within existing batteries I do not know which has the lowest carbon footprint from manufacturing.

Q : What are your thoughts on “renting” solar panels (e.g. from a group like Trinity) versus purchasing?

A: it is all about the economics. Renting or leasing solar panels usually works out more expensive per kilowatt-hour of electricity than buying them. But the same is true of leasing a car vs buying it. If you do not have the cash available to buy solar panels (or do not have the credit rating to get a cheap loan) then leasing solar panels can still lower your electricity bill, just not as much as paying for them in cash.

Q: Can you tell us the actual DC KW size of your PV array and the inverter(s)?

A: On the roof of our house is a 15kW array that produces about 15,000kWh a year with a 10kW and a 5kW inverter. On the roof of our garage (which is partly shaded) is a 13kW array that produces about 7,000kWh a year with a single 10kW inverter. On the roof of our rental property is a 17kW array that produces about 15,000kWh a year through a 10kW and a 5kW inverters. 

Q: Do you recommend any companies to buy solar panels from?

A: I have had good experience with New England Clean Energy and Great Sky, I have had poor experience (including in one case a law suit) with Second Generation Energy and Solar Flair. Other people on the webinar have had good experience with NEC Solar, RevoluSun and ACE Solar.

Q: What happens if there is an issue with the roof, like a leak?

A: The panels need to be removed before the roof can be repaired. This is not as bad as it sounds as only one or two panels usually need to be removed and they just bolt onto the rails.

Q: Is it true that panels in the future are to look like a traditional roof with tiles?

A: Tesla currently sells a roof tile system like this. However, in a recent interview with a Tesla employee who had installed them on his roof he stated that they cost 35% more than traditional panels plus the cost of the new roof. That sounds like terrible value for money to me. 

Q: are there solar installers that will install on slate roofs?

A: Not that I know of. It is very difficult to seal penetrations through a slate roof.

Q: What about snow collection on a flat-mounted array?

A: The panels can easily bear the snow load, including  during that winter a few years ago when we had 6’ of snow. 

Q : What are the most efficient solar panels available as of now? 

A: SunPower panels are about 22% efficient. Solaria, Panasonic, LG, REC are about 20-21% efficient and there are many companies (mostly cheaper brands made in China) that are in the 15-19% range.

Q: Is net solar after ITC?

A: Yes. All my calculations of the investment returns on solar panels are net of (i.e., after subtracting) the federal Investment Tax Credit, the MA tax credit and the present value of the MA subsidies (either the SREC subsidy or the SMART subsidy).

Q; What if your roof has too many gables and “wrong’ angle for solar panels?

A: Complex roof lines reduce the number of panels you can fit on a roof. Almost all solar installers today use a software package called Aurora that calculates how many panels can fit on a roof using satellite data on the angle, shade and orientation of the roof. I have found it to be very accurate. If your roof cannot fit enough panels to cut your carbon footprint to zero, I suggest looking at community solar options. There are several companies in MA that offer this. They build large solar-panel array in big fields in MA and sell you the net-metering credits at a discount of usually 10% (one offers 12.5%) to the utility price of electricity. These net metering credits appear on your electric bill. This is a very good option if your roof is not suitable for solar panels. 

Q: What is your utility company?

A: Eversource.

Q : Do you recommend a solar panel installer?

A: Yes, but each installer is very local so there is no one recommendation for everyone. In general, I have found the national installers to be more expensive and not necessarily better than the local ones. I suggest using the EnergySage website so that you can compare bids all presented on the same basis. 

Q: Can you replace your roof shingles after you install solar panels?

A: Yes, but you have to take off the solar panels first. This costs more so if the roof is likely to need to be replaced in the next 5-10 years then I think it is better to replace it before you put the solar panels on. Once you have the solar panels, they will shade the shingles and increase their life – it is the sun that rots shingles, not the wind and rain.

Q: Have you looked at Tesla solar roofing tiles?

A: I have, and I have concluded that they are just too new to even be considered at this point. A roof is a long-term investment. Until there is some data from roofs other than that on Elon Musk’s house, I will be cautious of these new products. 

There have recently been a few reports from people who have installed the Tesla solar roof. The numbers are not encouraging. The Tesla solar tiles are only about 4-5% efficient compared to solar panels that are about 20-21% efficient. The cost of the installed power is about $4.62 per Watt compared to solar panels at around $2.80-$3.00 per Watt. The Tesla solar roof is about 50% more expensive than solar panels and about one quarter as efficient. I was right to be cautious of this new product.

Q: How many solar panels have you in total? And how many people are in your household?

A: There are 46 panels on the roof of our house and 40 on the roof of the garage. They are 15kW and 13kW arrays respectively. There are essentially 5 adults in our house because all three of our children are at home due to COVID. 

Q:  Do you recommend owning or leasing solar panels?

A: It is usually a much better deal (i.e., it is cheaper) for the homeowner to own the panels than to lease them. If you do not have the cash available to buy them today, you can get low cost financing. In Massachusetts it is called a Solar Loan and there are also federal loans available see here for details: www.dsireusa.org

Q: Also did you consider ground mount solar?

A: No, we did not. We have enough space on the roof of our house and our garage to get to a zero-carbon footprint without needing a ground-mounted system. However, clients of mine that have a lot of field space next to their property and have a slate roof, which makes mounting solar panels almost impossible, are considering them.

Q: What if you need to put on a new roof within a few/several years of installing solar.


A: I replaced my 20-year old roof before adding solar panels. Taking the panels off to install a new roof will cost you more money, so if the roof will need replacing within the next few years, I think it is better to do it now. Solar panels will extend the life of the roof because it is the sun that deteriorates the asphalt not the wind and rain, and with solar panels, the asphalt is now shaded.

Q:  Are there solar systems that will provide enough power for our home during a power outage?

A: Yes, but probably not the whole house, probably only the circuits you would power from an emergency generator. Since solar panels must, by law, disconnect from the grid during power outages (to prevent the linemen getting electrocuted by your solar power) you will need to install a re-connect switch at your electrical panel just like that for a back-up generator. This usually needs to be installed with a battery, but when the grid is out and the sun is shining you will be powering the house from the sun. Nice. I intend to do this on my house this year. I have just ordered batteries to be installed at my house to replace our ancient (and no longer working) propane generator.

Q:  solar panel disposal also is a problem as they are replaced with more efficient ones

A: A solar panel is made from silicon, glass and metals. All are valuable and easy to recycle. Solar panel recycling is starting to become more commonplace as older panels from the 1980s start to get replaced with more efficient ones. There are already solar panel recycling facilities in MA, such as CED Greentech.

Q. My house has solar, but because the company that installed the panels is now out of business, the monitoring of the solar is inaccessible.

Q: Is there a third party monitoring system that could be added to an existing system so that I can see more than just my net usage, but actually see if my solar panels are producing as I’d expect? Is there a way for people installing solar from the beginning to ensure that monitoring will remain available (open source access)? Is that common in single family homes?

A: Do you know who made your inverter? It is often SolarEdge or EnPhase. The app for your smartphone (and the web portal) comes from them not the installer. I think if you contacted them they could sort it out for you. If that does not work let me know. You will not be the only person in this situation as solar installers go bankrupt quite often.

Q: I assume the efficiency drops as the temp drops?

A: Solar panel efficiency actually increases as the temperature drops. It decreases at higher temperatures. However, the effect is quite small.

Batteries to go with Solar Panels

I get so many questions about batteries I have made a separate section for Q&A on batteries.

Q: how large of a solar array is needed for connected solution?  Is it “large” vs your load or large from Eversource’s perspective of having fewer batteries to draw from?

A: there is minimum storage requirement (in kWh) and a minimum power requirement (in kW) that combine to require at least 2 hours of storage at the rated power. These terms are not at all well defined in the regulations, but what I think they are saying is that if you have a 10kWh battery (about the smallest you can get) with a rated power output of 5kW then because it would provide electricity for 2 hours (10kWh/5kW) it would meet the requirements. 

Q: Already have a 6.3 KW photovoltaic array from Sunpower.  Any recommendations for adding batteries?

A: The economics of batteries is highly dependent on the local subsidies. In MA with Eversource as your electricity supply company the subsidies are very generous. I have just ordered a Sonnen battery to go with an existing array of SunPower panels. I have also just ordered a Generac battery to go with a new array of Solaria panels. Generac is not good as a retrofit to an existing array because you need to use a Generac inverter with a Generac battery. I have not bought a Tesla battery, despite it being the cheapest per kWh of storage, because Tesla is not currently able to deliver batteries. 

Q: My solar panels have been installed 5 years; would it be financially smart to get a battery when I am on the SREC program? I have 28 panels and have not paid an electric bil since they were installed even with my pool & EV.

A: Great news on your house, pool and EV – you are well ahead of the pack! Arrays that are in the SREC program cannot be later enrolled in SMART. Don’t be too sad though, the SREC subsidy is quite a bit more generous than the SMART subsidy. However, the Connected Solutions subsidy is not dependent on enrolling the array in SMART. The Connected Solutions subsidy is not well documented but, in my opinion, once an incentive is in the electrical tariff it is quite hard to get it out again. Since the Governor just signed a major climate change bill in MA that commits the state to substantial increases in renewable power, I am guessing that the Connected Solutions subsidy will be there for at least 10 years. This was what I assumed when recently I ordered batteries to go with my arrays. One of those batteries is not eligible for the SMART battery adder because the array is enrolled in SREC. The NPV on that battery is about     -$3,000 (the battery purchase price is close to $20,000) but I am doing it because my existing back-up generator is broken beyond repair and buying a new one would cost me about $3,000 plus $500 a year to get it serviced. There is no service cost on the battery.

Q: What installers would you recommend for getting solar that a) has a battery backup, and b) can disconnect from the grid and run off that backup when the grid electricity goes out?

A: I am doing this right now. Battery subsidies in MA are very generous now so this is a good time to be looking at this, but so far I can only tell you who not to use.

Q: Can the battery back up really run the heat pumps and for how long? 

A: A 10kWh battery can only power a heat pump for an hour or less. The better way to add a battery is to keep your fossil-fuel furnace and have the battery run only the emergency circuits on your house like the fridge, lights, burner circuit, circulating fans, water pump and a few outlets. Together these might draw about 10 amps, 1kW or about 1 kWh per hour so a 10kWh battery would last you 10 hours.

Q: battery disposal  a big problem?

A: This is very unlikely. Batteries contain a lot of valuable metals like lithium that is expensive to mine. Hence, just like aluminum cans, batteries will get recycled. However, the infrastructure to recycle batteries is not yet widespread.

Q: We are building a net zero house. do you have recommendations for battery systems?

A: Both battery technology and the subsidy schemes in MA are changing very rapidly. I have changed my mind several times on which battery to get. This is an area where you need to get current advice.

Q: We are building a net zero house. do you have recommendations for battery systems?

A: Both battery technology and the subsidy schemes in MA are changing very rapidly. I have changed my mind several times on which battery to get. This is an area where you need to get current advice.

Q: What is the ROI (return on investment) of a Tesla Powerwall battery?

A: In general batteries have two uses. 

The first is to take advantage of time-of-day (TOD) tariffs. A TOD tariff is where your electricity company charges different rates for electricity at different times of the day. You can use this TOD tariff to buy electricity at a cheaper rate and sell it back at a higher rate. We do not have a TOD tariff where we live so I have no direct experience of using batteries for this purpose. 

The second use is as an alternative to a diesel or propane emergency back-up generator for when the power lines go down in a storm. In December 2020 I bought batteries to replace my back up generators at three houses.

The Tesla Powerwall is considerably cheaper per kilowatt-hour of electricity stored than other batteries from companies like Sonnen, LG Chem, Generac and Simpliphi. However, at about $7,000 it is considerably more expensive than a diesel generator which costs $2-3,000. However, I have a propane back-up generator and it costs me $500 a year to get it serviced so that it actually works when we need it. I have learned this lesson the hard way – our previous back-up generator was not serviced, and it failed to come on when we needed it – during a power outage. Over 10 years this maintenance cost is $5,000. So, the full cost of a back-up generator over its lifetime is about the same as a Powerwall. 

Also, in Massachusetts, the new SMART subsidy program for solar panels includes an extra subsidy (an “adder” they call it) for a battery. This is currently about 4c per kWh generated by the solar panels. If you have a 10kW array generating about 10,000 kWh a year then this is worth $400 a year for 10 years which brings the cost of the Powerwall down to about $3,000 which makes it similar to the upfront cost of a diesel back-up generator. Only now you have no maintenance cost. This is why I intend to install a battery to replace my defunct back-up generator in the near future. The regulations on qualifying for this adder are very complicated!

Q: Please tell more about “batteries for free if you have a large solar array?

A: Please see the answer below on batteries in MA.

Q: What is the ROI for a battery to go with solar panels?

A: The answer to this question is very dependent on the subsidies available in your area and, in MA, it is also highly dependent on the size of the array you pair with the battery.

Where we live in Massachusetts the subsidies for batteries are now very generous. When a battery is paired with a large array the battery can now be free. I have recently bought a pair of Generac batteries to go with each of my three arrays. I did not buy the Tesla battery because, even though the Powerwall battery is cheaper per kWh of storage than other batteries, Tesla cannot deliver batteries for several months. Since we are heading into winter, which is when the power outages are around us, the subsidies decline with time, and with today’s subsides the new batteries either save me money or work out cheaper than replacing my ancient generators, I decided to buy the batteries now. The economics of these three pairs of batteries are explained below.

When paired with a large array (20,000kWh/year in my case) the batteries (after the federal tax credit, the MA tax credit, the state subsidy (called SMART) and the Connected Solutions subsidy from our utility), are more than free – I am actually paid about $3,000 to own them.


When paired with a medium sized array (10,000kWh/year) the IRR on the battery and the array is 6.6% compared to 10% with no battery. So the battery is not free but it still makes an acceptable IRR and I will have a reliable back up which I do not have today with my old generator.


When paired with a small array (6,000kWh/year) the battery ends up costing my about $4,500. But my alternative is to buy a new generator (the old one is broken beyond repair) and pay $500 a year to get it serviced. So I think it is worth it to have back up power again.

Back to solar panel questions…

Q:  If the roof is 15 years old, would you replace it before adding solar panels?

A:  In most cases, yes. It will be expensive to remove the solar panels in order to replace the roof. However, this is not true is the roof is flat and the solar panels are held on to the roof with weights (called a ballasted system).

Q: Are their solar panels that look like shingles for use on historic homes?

A: Tesla makes solar shingles, however they look more like ceramic tiles that you sometimes see on houses in the Mediterranean than either cedar shingles or asphalt shingles that are more common in New England.

Q:  If you have a flat roof and have solar panels, can one walk on them if one needs to remove tons of snow in a bad winter?

A:  You cannot walk on solar panels. They need to be installed on a flat roof with some walking paths between them for maintenance and snow shoveling. In the first winter with solar panels I shoveled the snow off, but in subsequent years I did not. I calculated that I was only making about $2 worth of electricity on those winter days and it was taking me over an hour to shovel the snow off. $2 an hour is well below minimum wage!

Q:  Tips on planning new construction for solar panels on the roof?  Metal roofing?

A: Metal standing-seam roofs are very good roofs that last well beyond the life of a shingle roof. Solar panels install easily on standing-seam roofs because the panels are clamped to the standing seams and no penetration of the roof is required. If I were designing a new house, this is how I would do it. I would also design it so that the panels fit exactly edge-to-edge on the roof, so the entire roof is just solar panels with no shingles or membrane showing. Keeping the sun off the waterproofing will make the waterproofing last much longer. It will also make the roof look much nicer because the solar panels will look like they are the roof rather than them looking like they are on the roof. If you use the all-black panels (most manufacturers offer them, but they are about ½ % less efficient than the ones with the grid pattern on the front) then it will be hard to tell there are solar panels on the roof. I think this is a better solution today than solar tiles.

Q: Is there a rule of thumb for adequate number of HDD to make solar panels worthwhile? 

A: HDD will not have much influence on the payback period for solar panels. The biggest determinants of the payback on solar panels are what you are currently paying for utility electricity and the state-level subsidies. 

Q:  Are you aware of solar panel company called FaFco they make a combination solar panel PV / Thermal….. The thermal output is about 10,000 BTU per 10 FT. SQ. These are making an impact in the swimming pool industry. Have you looked at these FAFCO ?

A: I have not heard of this panel, but it does not overcome the biggest issue with solar thermal panels for home heating which is that you cannot store the heat generated in summer and use it in the winter. With solar PV you can effectively store the energy because of net metering. However, using them to heat pool water is a good idea. I would have to check the economics compared to what we did which is just installing solar PV and using it to power a heat-pump hot-water heater for the pool.

Will a battery like a Tesla Powerwall last for 3-5 days?

Q: How much can a Tesla power wall store? Will it be enough for 3-5 days in case of an outage? 

A: One Tesla Powerwall stores 14kWh of electricity. If you are using fossil fuels for heating, then you are probably using about 20kWh a day to run all the lights and appliances in your house. If you are using heat pumps to heat or you are using AC in the summer, then you are probably using about double this. Hence it is not practical to use a Powerwall (or any other type of battery) to power your entire house electrical load. However, this is not how most people use a battery. Most people use them as an alternative to a diesel or propane back-up generator. So, the battery or generator is powering an emergency panel which usually runs just the fridge, the furnace-burner circuit and the circulating fans or pumps, plus some lights and a few outlets. This is typically under 1kW in total load so a Powerwall can last about a day. This is usually enough to get you through a power outage. To last 3-5 days you would probably need 2 Powerwalls and to reduce the load on your batteries to just the lights, the wifi, a few outlets and the fridge.

Q: Subsidies for battery storage with a solar system?

A: There are now three separate subsides for batteries in MA: if you have an array enrolled in SMART you get an “adder” for having a battery. The SMART subsidy in October 2020 is about 8c/kWh (note this deceases with time and varies with the utility company you use so please check what it is today). The adder for the battery is about 4c/kWh. For a typical 10kW array producing 10,000kWh a year the SMART subsidy is worth $800 a year and the battery adder is worth another $400 on top. Both are for 10 years so the adder is worth about $4,000. Both decrease with time – that is the structure of the SMART subsidy scheme. However, once you get your subsidy rate it is locked in for 10 years.
A Powerwall battery costs $10,000 to $15,000 fully installed. So the $4,000 SMART adder cuts the price by a third to a half. The SMART adder is (as is all SMART income) taxable income.  I have yet to see a solar installer admit this.

In addition, Eversource has a subsidy called Connected Solutions where ES pays you to have access to your battery during peak power demand. This payment seems to be about $1,200 a year but the it is not really clear exactly how this is calculated or for how many years it lasts. It is not clear if this is considered taxable income (like SMART) of if it is considered more like a rebate or a credit (the way net metering works, and that is not taxable income). If the Connected Solutions program lasts 10 years it would pay for the entire cost of the battery which seems excessively generous and therefore unlikely. However, it will be something else to reduce the cost to you.

Finally there is a brand new subsidy, called CPEC (which was just announced in August 2020) or Clean Peak Energy Certificates. Power stations are now required to buy these CPECs to reduce their carbon emissions during peak demand on the grid. They are a bit like SRECs which is what SMART replaced. Peak demand (say on a hot summer afternoon when everyone has their AC on) brings the fossil fuel “peaker” plants into operation. These are not only the most expensive power-generating plants on the grid but they are often also the dirtiest, i.e., cause the most pollution and CO2 emissions. Hence, the state has a strong interest in reducing the use of these peaker plants through encouraging batteries. CPECs are brand new and no-one can even tell me how much I can earn from them or whether they are taxable income. However, it is sure to be favorable to you. So, overall, and despite all the uncertainty, batteries are looking like a better and better bet. I have ordered battery systems for both our home and our rental property.


My final point is not about batteries but about generators. A diesel or propane generator costs about $500 a year to set serviced so it will work when you actually need it. This is $5,000 over 10 years. The generator itself will probably cost you $3,000 up front. So, the 10-year cost of a generator is $8,000. The 10-year cost of a battery is between $5,000 and $10,000 with the current SMART subsidy alone. Add in Connected Solutions and CPEC and batteries are looking like a really good investment. Oh, and did I mention that they have a zero carbon footprint and emit no asthma-inducing soot, nitric oxide or sulfur dioxide?

Note this post was current as of October 2020. The subsidies for batteries are a moving target and one that changes by state, by utility, over time and even by zone you are in within a utility’s service area. Please check the latest information for your home before deciding to install a battery.

Q: With net metering always changing, do you recommend battery storage yet to store excess instead of only receiving avoided cost payments?

A: Batteries are currently much more expensive than a diesel or propane back-up generator. However, that is very dependent on the local subsidies. In MA now, if you pair a battery with a fairly large array (about 20kW) and get all the subsidies, then the battery can pay for itself in just a few years.

Q: Why do you not recommend solar hot-water panels?

A: Solar photovoltaic panels are sometimes called solar P.V. panels to distinguish them from solar thermal panels, or solar hot-water panels, which use the heat from the sun to directly heat water. Solar thermal panels can be over 70% efficient, which sounds great compared to solar P.V. where the maximum commercially available efficiency is 22%. However, if you are using that solar electricity to power a heat-pump hot-water tank (please see page 66 in Chapter 2 of Zero Carbon Home), with its 400% efficiency, you get a total heating efficiency of 84% for the solar P.V. panel that is heating your hot water with a heat pump. This is better than the efficiency of a solar thermal panel. 

Because of net-metering (please see page 82 of Zero Carbon Home), solar P.V. panels can generate the electricity in the summer, and you can use it in the winter. This is not possible with solar thermal panels, which don’t generate enough hot water in the winter and generate far too much in the summer.  Also, the solar-P.V.-plus-heat-pump-hot-water-tank option has no pipes and hence cannot leak. Better overall efficiency, energy “storage” via net-metering, and no burst pipes make solar P.V., in my opinion, a far better solution than solar thermal panels. 

How long do solar panels and inverters last?

Q: What is the life span of the PV array panels and inverter? 

A: Most solar panels are warrantied for 25 years to produce at least about 90% of their initial power production. This varies a bit by manufacturer. They will probably last for many years beyond that. My inverter is warrantied for 15 years, but new ones today come with a 25-year warranty. 

What about buying 100% clean energy from my utility? Is that better than solar panels?

Q: Do you still prefer installing solar panels rather than purchasing 100% clean energy from National Grid through suppliers like Eligo Energy. 8.9 cents/Kwh for 6 months. Have you done a financial analysis of paying 9 cents to 14 cents/kwh to receive 100% clean energy from National Grid compared to installing solar panels on roof?

A: I am assuming that the 9c per kWh is the cost of generating the electricity. Utilities charge separately for distributing that electricity, often about 12c per kWh in MA. This means that you actually pay about 20-24c per kWh after you add in all the other charges, including the $7 a month they charge you for being a customer. When you add solar panels you eliminate the entire bill (except the $7 a month which is effectively what you pay for maintaining the option of drawing power from the grid when you need it) so your cost drops from 24c/kWh (what I am paying today for Eversource electricity) to between 4c and 11c depending on how much shade you have on your roof and which subsidies you get. So, generating your own solar power is far cheaper than even the generating cost of electricity from Eligo, let alone the full cost of that electricity.

If your roof is so shady that solar panels on your roof will generate electricity at more than the full cost of electricity from your utility (21c/kWh in the above example) then buying 100% clean power may make sense for you. However even a half-shaded roof (I have one) generates electricity at 11c per kWh which is less than half of what I pay Eversource today.

Q: Does it make sense to use solar panels to charge an electric vehicle?

A: When my Tesla is charged from my solar panels (which generate electricity at about 8c per kWh) I can drive for 2c per mile. A gasoline-powered car getting 30 mpg on gas at $3 a gallon costs 10c per mile. It is 80% cheaper to drive on solar power than on gasoline!  And that is without even mentioning the zero-carbon footprint of driving my Tesla on solar power.

What is the embodied carbon footprint of a solar panel?

Q: Can you discuss the embodied energy and natural resource use of the active systems, such as solar panels, that are necessary to achieve a “net zero” status? 

A: The embodied carbon footprint of solar panels (i.e., the carbon dioxide released by making the panel) is about equal to 18-months’ worth of the carbon footprint avoided by generating zero-carbon electricity. So, from a carbon-footprint perspective, you can think of the solar panel paying for itself in about 18 months. Financially, ours will pay for themselves in about 6 years. 

Q: If I want to increase my solar arrays beyond 10kW so I have to pay commercial net metering rates. is it worth it?

A: Below 10kW in MA you get 100% net metering. Above 10kW (up to 25kW) you get “market net metering” credits at 60% on the amount produced by your panels less the amount used by your house. This is calculated per billing period so it is complicated. However, on average the credit amount for an array of say 15-20kW works out to be about 18c/kWh, rather than the full price of about 23c/kWh.

Q: How do you calculate so many cents per PV-generated KW? Once a panel is installed, isn’t the cost per KW zero?

A: In one sense the cost is indeed zero. You do not write checks for the electricity produced by your solar panels so in that sense it is zero. However, a more common definition is the Levelized Cost of Energy (LCOE) which is the upfront cost of the array divided by the amount of kWh it produces over its lifetime. This works out to be about 5c/kWh if you have a sunny roof and about 10c/kWh if your roof is in about half shade.

Q:  Solar 1) my house already had solar, it has 28 panels but it looks like about 10 more would fit. do you need a lot of clearance to the edge of each roof section, or did they not install as many as might fit?      2) this is just on the south-facing side. Is it ok/worth it to install panels on east/west facing roofs?

A: Most installers require some clearance just for the safety of the installers. This is usually about 2’. Solar panels on east and west facing roofs perform very well at about 90% of the electricity output of a south-facing roof.

Q: How hard is it to monetize the solar credits in Massachusetts?  Will credits be rising in value in coming years?

A: The tradeable credit scheme called SREC was eliminated in 2018 and replaced with new SMART scheme. SMART is less generous than SREC but your SMART rate is locked in for 10 years, you no longer depend on the market rate for SRECs.

Q:  For those who can’t get solar panels on their roof, can you recommend community solar options?

A: Firstly, I would do the calculations again, or have the installer do so using Aurora software which accurately maps the sun’s power on your roof. My garage roof has half shade and yet produces electricity at 10c/kWh or less than half that of utility electricity. If that still does not work for you there are several community solar installers in MA who install large solar panel arrays in big fields in MA and sell you the net-metering credits (which directly offset your electricity bill) at a discount of 10-12.5% off the Eversource rate.

If you sell your SREC are you double counting the carbon footprint reduction?

Question: Hi David, I really enjoyed your talk last week on your zero-carbon adventure. I did have a question, which didn’t get answered. I have a friend that works at National Grid and she told me if you sell your solar RECs, which I believe you did, you cannot claim to be carbon free as those rights go with the REC? After your talk, I went back to her and she felt that this was definitely the case and that you would need to buy RECs to offset the ones’ you sold, to make the carbon free claim and you should also tell people you sold your RECs and brought other ones. 
Just wondering your thoughts on this 
Thanks and thanks again for a wonderful talk. I plan to share this with some folks 

Hi Rob, I have occasionally heard this perspective before, and I think that reasonable people can disagree on this one.


I have two concerns with your friend’s perspective. 


The first is that I have not counted either RECs or SRECs in my calculation of my carbon footprint. I am not offsetting any of my carbon footprint with credits such as SRECs. I have actually cut my carbon footprint to zero by using the fab four. It is Eversource (our electric utility company) that is claiming it has cut its carbon footprint when it has not done so. So, I think it is more accurate that I say I have cut my carbon footprint to zero and it is Eversource that has been allowed by the state regulators to claim it has cut its carbon footprint (by buying my SRECs) when it has not done so. But I have genuinely cut my carbon footprint and it is zero. The SRECs have not been counted in any part of my carbon footprint reduction. My carbon footprint is zero because I no longer burn heating oil and I generate all my own electricity, including that to heat my house, using heat pumps, powered with zero-carbon solar panels. 


My second point is that, where I do count the SRECs is in the financial subsidies. I genuinely receive this cash, so I do not think it is right to remove it from the financial forecasts. There are many subsidies involved in going zero, from net metering and zero-interest loans (which are essentially funded through all of us in MA paying the highest price for electricity in the US, other than Hawaii) to federal and state tax credits. SRECs are calculated per kWh I generate. The federal tax credit is calculated based on the price you pay for the solar system you install. Why should one subsidy not count just because it is based on kWh generated rather than on the price of the solar system? In 2018, SRECs were replaced with SMART and so it is only owners of legacy systems that receive them. I do not see why a subsidy should be dismissed for those who installed systems prior to 2018 but should be allowed for those who installed systems after 2018. 

In my opinion, SRECs are just a form of subsidy, paid from a regulated utility to an owner of solar panels, which means they are ultimately paid by all electricity customers because that regulation has been imposed on the utility company by the state. . This is essentially a regressive form of taxation because it takes money from everyone who uses electricity (including poor people) and funnels it to those who can afford to install solar panels which tend to be wealthy people or investors. I think this is rather unfair, but I do not make the laws Rob, I just abide by them.

Please share these thoughts with your friends, I am interested to hear what she says. 

As of November 16th 2020 I have not heard back.

More on who owns the claim to having a zero carbon footprint, the owner of the array or the owner of the SREC?

Hi Bob, 
This point is often debated but it does have a bit of a “how many angels can dance on the head of a pin” quality to it. At the end of the day, RECs and SRECs help to fight climate change and I am not about to argue against them.


The generalized version of the debate over who owns the zero carbon claim on a REC is: whoever pays for the carbon footprint reduction is the rightful owner of the claim that they have cut that carbon footprint. This has some logical sense to it, if I pay for a car I own it including the right to brag about how good I look while driving it. The federal government subsidized 30% of the cost of my arrays. The SREC subsidy was for about another 30%. Net metering subsidized more than the remaining 40%. Net metering is the biggest of the subsidies because I am paid 23c/kWh for my electricity when the wholesale price for electricity is about 12c/kWh. I paid very little of the overall cost of my arrays. An economist would argue that since the NPV is positive at a 3% discount rate that I have actually paid nothing for my arrays. The total subsidies exceed the total cost. I agree with the economist. I have not paid anything for my solar panels, I been paid to own my solar panels.


By this logic, the cut in my carbon footprint should be owned 30% by the federal government, 30% by the purchaser of the SRECs (whoever that may be) and about 40% by Eversource because it pays the excess price to me for the net-metering. Logic then, requires that Eversource claims it owns 40% of my carbon footprint reduction, not 100%. And the 40% claim is because of the net-metering subsidy not because I have sold the SREC. Unless Eversource can prove that is has actually bought my SRECs (and not somebody else’s) it has absolutely no basis for claiming that I cannot claim my zero carbon production.


So, logically the argument that Eversource owns 100% of my claim to zero carbon electricity is invalid. At best it has a claim to 40% and not because it bought SRECs.
An exception to this would be if the various parties had contractually agreed to split the bragging rights. Some contracts do explicitly say who has the bragging rights. But mine do not. The federal government could have claimed to have 30% (or 100%) of the bragging rights in return for the tax credit, but I have not made any such agreement with the federal government. Neither have I made any such contractual obligation when selling my SRECs through my aggregator, nor with Eversource in my net-metering contract.


Furthermore, the argument that Eversource owns the bragging rights to my zero carbon electricity is not only logically flawed, it does not pass a basic legal test of ownership. A restaurant does not claim it owns half my meal if it gives me half off the usual price. The fact is that I get to eat the meal no matter who pays for a subsidy in order to buy it for me. By analogy, the owner of the array (not the one who subsidizes it) is the one who has the right to make the claim about cutting the carbon footprint. I am indisputably the owner of my solar panels. I wrote the check to the installer for the full $75,000 purchase price. My solar panels are not owned 30% by the federal government, 30% by the owner of my SRECs and 40% by Eversource. I own my array lock, stock and barrel, including the right to sell it to someone else, and all bragging rights associated with the zero carbon electricity it produces. 


So, in my opinion, the argument that the purchaser of an SREC (in the absence of an explicit agreement to allocate the bragging rights) has the right to claim that it has cut its carbon footprint fails both the logical test and the legal test of ownership. I own my solar panels and I have the sole right to claim that I have produced electricity with a zero-carbon footprint. No one else has any such claim, even those who have subsidized the cost of buying the solar panels. 


Again I look forward to your response Bob. 

Q:  Are there solar systems that will provide enough power for our home during a power outage?

A: Yes, but probably not the whole house, probably only the circuits you would power from an emergency generator. Since solar panels must, by law, disconnect from the grid during power outages (to prevent the linemen getting electrocuted by your solar power) you will need to install a re-connect switch at your electrical panel just like that for a back-up generator. This usually needs to be installed with a battery, but when the grid is out and the sun is shining you will be powering the house from the sun. Nice. I intend to do this on my house this year.

Q: Can you walk on solar panels?

A:  You cannot walk on solar panels. They need to be installed on a flat roof with some walking paths between them for maintenance and snow shoveling. In the first winter with solar panels I shoveled the snow off, but in subsequent years I did not. I calculated that I was only making about $2 worth of electricity on those winter days and it was taking me over an hour to shovel the snow off. $2 an hour is well below minimum wage!

Q:  If the roof is 15 years old, would you replace it before adding solar panels?

A:  In most cases, yes. It will be expensive to remove the solar panels in order to replace the roof. However, this is not true is the roof is flat and the solar panels are held on to the roof with weights (called a ballasted system).

Q: I have heard that there are reliability issues with solar panels, is that right?

A: I think these reports are aimed at commercial MW-scale solar installations where small differences in field reliability are important if you have sold all your power on a 20-year contract. 


However, to the homeowner these issues make very little difference. All the major panel manufacturers give 25-year power production warranties to the homeowner so the manufacturer, not the homeowner, is taking the risk and responsibility for any degradation in power output above the warranty limit which is often 0.5% per year. This degradation is usually taken into account in the energy forecast and financial forecast by the installer.

On top of the manufacturer’s warranty, the installer will often give an array-energy warranty such as a minimum of 10,000kWh per year. You often pay a bit extra for this warranty but, in my experience, it is worth it. It is also worth restricting your choice of panel manufacturers to only those that are large companies that are likely to be around in 25-year’s time to honor the warranty if you need to make a claim. Today, I restrict my recommendations to homeowners to: Sunpower, Panasonic, LG, REC and Solaria. 

Electric Vehicles

Q: Are you charging your EV from your PV as well and getting zero carbon footprint? 

A: I do not have enough solar panels today to cover the electricity needed for both my house and to charge 3 cars. Later this year we are planning to build an extension on our house and we will put solar panels on that and then I project that we will have enough to have a zero-carbon footprint on both our house and our cars. 

Q: Will electric vehicle play an important role in zero carbon emission?

A: Yes. According to a study at MIT authored by Prof. Jessica Trancik, the lifetime cost of owning an EV (which means the cost to buy it, plus the cost to repair and maintain it plus the cost of fuel) is now at or below that of owning a gasoline-powered vehicle. Her study assumed that you were paying utility rates for electricity. If you are using cheap solar power from your roof the lifetime cost of an EV is now lower than that of a gasoline-powered vehicle. If you power your EV from your solar panels then your transportation with have a zero-carbon footprint too and emit no pollution.

My wife recently drove in our Tesla to New York City and back in a day, a distance of about 400 miles. The whole trip cost $8. And had a zero-carbon footprint. The Greyhound costs $26, the seats are uncomfortable and it drops you at the Port Authority Bus Terminal.

Q: does the $8 include the prorated cost of the Tesla?

A: This is referring to the time my wife drove our Tesla to NYC and back in a day (which is a 400 mile trip) at a cost of $8 when the car was charged from my solar panels at 8c/kWh. And the answer is no, it did not include any depreciation on the car, only the “fuel” cost.

Q: You mentioned that you  have an EV.  Do you find it performs well when it gets really cold?  Are charging stations easy to find, or do you mostly charge at home? 

Yes we have an EV, I bought a Tesla model S about a year ago. We have three kids and they are all taller than my wife so we needed a car big enough for 5 adults. Also, I wanted 4WD given the winters we have in NE. This made the model S about the only option. We bought the long range 100kWh battery and that gives it a range of about 400 miles. The performance in really cold weather (like this week) has been completely normal except that the mileage goes down. If I drove it today it might only get 250 miles on a full charge. Unless the weather is really cold the range stays at about 400 miles.
It is charged about once a week from home (we had an electrician install a Tesla wall charger which charges much faster than just plugging it into a 110V socket) almost always overnight. When we do long trips (like the 400 miles round trip to NYC and back) we stop and recharge at a Tesla supercharger station which can add about 200 miles of range in about 20 minutes. There are many supercharger stations and the car’s software tells you which ones have spaces free and directs you to them. They are usually just off the highway in a shopping mall or something similar. So road trips are not a worry, in fact taking a break every 3 hours or so is actually quite convenient. 

Green Hydrogen and the Hydrogen Economy.
Q: I keep on asking the questions but no one gives me an answer:-
The are three grades of hydrogen:
Grey (high levels of CO2), Blue (low levels of CO2) and green (no CO2) 
Based on what I have read so far the Grey and Blue are made from natural gas while the Green is made from breaking down water.
There is also a maximum amount of hydrogen that can be piped though regular old fashioned gas pipes.

My big question is “How much does Blue hydrogen cost above natural gas?”

A: I do not know the answer but you are asking the right question. 
Energy is a total commodity, only the lowest cost player (or fuel) will survive. This is why coal displaced wood for heating homes, then natural gas displaced coal. Natural gas is now displacing oil for both heating and electricity generation. Now solar and wind are starting to displace natural gas for heating (solar PV plus a heat pump is half the cost to heat your home than natural gas) and for transport (solar PV plus a Tesla costs 2c per mile vs. gasoline at 10c per mile). It is game over for fossil fuels. Unfortunately it will take 20 years. But the outcome is now inevitable.

I doubt that even green hydrogen will be low enough cost to be able to compete with renewables and heat pumps for heating or renewables and electric vehicles for transport.

The only way to make green hydrogen today is to use solar or wind electricity to split water molecules into hydrogen and oxygen. This is fundamentally less efficient (i.e., more expensive) that just using the renewable electricity directly. There may be some niche applications, such as using excess solar electricity (that would otherwise be wasted) during sunny days to make hydrogen and then burning the hydrogen at night to run a power plant. But other than temporary energy storage, I just do not see hydrogen being competitive with solar, heat pumps and EVs.

Q&A From High-School Students after seeing my webinar for homework:

 QUESTIONANSWER
Out of all of the jobs you have had, which has been your favorite, and why?My career was in biotechnology, not energy. We produced a new product that combined stem cells from the patient with a tube made from plastic nanofibers to replace the trachea or esophagus of a patient with something awful like throat cancer. My favorite moment was being in the operating room, dressed in surgical scrubs, with the first patient in the US, a baby girl, who was born without a trachea.
I’m aware of when you got this idea, but when you were getting your two degrees did you know that you wanted to enter into the environmental realm?No, not at all. My career in biotechnology combined science and business so it built on both of my degrees.
What made you first begin to capture data and really invest your time into making these changes in your home, as well as your lifestyle?Two things. First my kids were interested in it and second, I had to take time off work to recuperate from a nasty road accident. That was what gave me the time to do the research.
How feasible of an option is solar given that our area of the country doesn’t get as much sun as areas like Arizona or New Mexico? Is it more difficult to have more solar panels installed onto houses without flat roofs?Solar panels work in MA, many homes have them. It is sunnier in AZ but the subsidies from the state government are better in MA. Houses with south facing sloped roofs generate about 10% more electricity per square foot than a flat roof. Sloped roofs that face east or west generate about the same electricity per square foot as a flat roof.
Are there severe detriments to your solar panels because of the snow/New England weather?No, the snow just melts off within a few days.
You mentioned in the video that you bought a Tesla earlier in 2020, and that the cost per mile dropped to only 2 cents per mile. What might be a more cost-friendly option to teens and/or people who are looking to cut down on their carbon footprint related to transportation that can’t afford the up front cost of a Tesla?Leasing a car is a good option if you do not have the up-front cash.
Do you think the reason why people don’t change to a more environmentally friendly lifestyle is because it is too much money economically?This used to be the case, but today it is more that people do not know they can save money by cutting their carbon footprint, which is why I frequently do webinars to educate people that you can. The webinars are very effective – 72% of webinar attendees say they intend to install at least one of the fab four within a year.
What was the biggest hardship that you and your team had to overcome during your process to produce a zero carbon home?We have big windows in our house that look out over a large grassy area that attracts deer and even coyotes. My whole family was concerned that low-E triple glazed windows would look green and hence distort our wonderful views. I ordered sample panels of several different types of low E glass in double and triple-glazed glass and set them up on our deck so that everyone could see them before we bought them. The first round of samples all arrived broken so it was a very long and involved process before we decided on the final windows. We chose ones that were very clear and transparent on the ground floor and ones that were slightly tinted green on the upper floor. Now they are installed no-one thinks they have even the slightest green tint – but convincing everyone took a lot of work!
What is the hardest part/most difficult change to transitioning to a zero carbon footprint?Installing the windows was a very long process. By contrast the heat pumps, insulation and solar panels were fairly quick.
What are some ideas you have for ways to convince people that being carbon neutral is an economically advantageous decision as opposed to more expensive than they are willing to pay when there is such a strong narrative out there that it is incredibly expensive?I do the webinar for free 2-3 times a month and give the book away free to all attendees.
These savings seem incredible to me, so I’m wondering why this knowledge isn’t more widespread. I have never seen advertisements for any of these things.I honestly think I am the first person to do both the financial analysis and the energy savings analysis with such a scientific approach. I wrote the book precisely because there was no guide book. I would have been very happy to use someone else’s guidebook, but there was none.
How long would it take to accomplish a zero carbon home/footprint (timeline to implement the fab four start to finish)?It took about 2.5 years from start to finish. But this was because I was doing everything for the first time. With homeowners who I advise on how to save money by cutting their carbon footprint it takes about a year.
Do zero carbon homes require lots of maintenance to keep the solar panels and other components performing well?No.
Do you in any way feel that, by your neighbors seeing what you’ve done for your home and how much money it is saving you each year, that they may follow in your footsteps?I am encouraging them to do so!
Why are geothermal pumps more costly than air pumps?It is mostly the cost of drilling the wells. The heat pump itself is about the same cost as an air-sourced heat pump.
What made you study finance but shift gears in your focus of your studies to environmental science and applications like these?I spent 25 year in biotechnology. It was only after the road accident that I focused on cutting our carbon footprint.
What is it like seeing your work referenced on a national stage?It is always nice to get recognition like that, but I honestly feel better when a homeowner tells me about how much money they are saving because of installing one or more of the fab four.
What are your thoughts on windmills for people who have large fields in towns like dover and sherborn.I looked into a wind turbine at our home. But with trees around it is not viable.
What do the Rs represent in the grade of insulation? Are you able to get an insulation that is higher quality than R50?R comes from resistance, as in resistance to heat flowing. You can always get more R value by increasing the thickness of the insulation. However, more than R50 is probably not worth it. 
What would you say to someone if they still don’t want to switch to the fab four? How could you convince them further?I have found that wanting to save money is almost universal. If even that does not get people interested I find the health aspects like reducing the risk of asthma for a family member is often a good way to spark interest.
Does the fab four also work for schools and other community buildings are does it have to be modified?Yes. The fab four recipe is rooted in the laws of physics and those are the same everywhere. The subsidies are a little different for commercial properties, schools and homes but the basic recipe is the same.
Our winters can get bitterly cold, sometimes below zero for long periods of time. Can your heat pumps support a large home in this scenario?No. In my experience heat pumps cannot keep a house at 70F when the temperature outside is below about 20F which happens on about 20 days a year in New England. This is why I always recommend that you keep your old furnace as a back-up heating system.
Do you think Britain is better at cutting down carbon emissions than the US?Britain has done a very good job of closing coal-fired power stations and building offshore wind farms. The US is usually ahead of Europe in all things technological (how many European internet companies can you name?), but in this area, especially wind farms, Europe is far ahead of the US.
My house is Surrounded by lots of trees and doesn’t receive much sunlight. Are there any alternatives to solar that are just as beneficial.You can get your electricity from a community solar installation. There are several companies offering this in MA. Their electricity is 100% solar, usually from large arrays in rural areas. They sell you the electricity, usually at 10% off the utility rate and one offers 12.5% off.
There seems to be a focus on heating, what is the contrast with cooling? Is the fab 4 solution still the best?In MA, winter heating costs far more (and has a far higher carbon footprint) than summer cooling. This is why I focus on heating.  In the winter you want to keep the heat in. In the summer you want to keep the heat out. The answer is the same in both cases – insulation, draft-sealing and triple-glazed windows.
Living in a house that doesn’t seem very sustainable and is far from zero carbon, what is the best place to start moving toward a zero carbon home? Even with bringing awareness to the idea/ the first stepsAlmost always the best place to start is with insulation and draft sealing. MassSave will pay for almost all of it. Go to their website www.masssave.com and book a no-cost, virtual audit.
I would like to know more about the benefits of a carbon neutral home when you are only a few years away from selling it. My parents like the idea of solar panels but say its too late to get them.According to Zillow, houses with solar panels sell for about 4% more than comparable houses without solar panels. Academic studies show that houses with heat pumps sell for between 4-7% more than comparable houses without heat pumps. These price increases often exceed the cost of adding the solar panels and heat pumps. Hence, adding them is a good way to position your house for selling it and you will probably get more money out of the sale than you invested in the fab four.
How can I save money and cut down on my carbon footprint while living in a dorm while going to college? What techniques can I use while living in a dorm. (obviously I won’t be able to install solar panels or triple glass windows ect.)In a dorm room there are still things you can do: block drafts under doors and around windows; make your own “fit from the inside” triple glazed windows (I have done this) using a simple wood frame and the stretchy plastic film you can buy at any hardware store, and advocate for solar panels. The recipe for the windows is in the book, “Special COVID-19 Edition of Zero Carbon Home” which you can download for free from my website www.greenzerocarbonhome.com or it that does not work send me an email to dgreen@greenzerocarbon.com and I will email you the pdf file.
Is there a way to use the fab four if you live in a smaller residence like an apartment or a condo since it might be harder to use solar panels?Yes. Focus on insulation, draft sealing and window inserts. Also see if your condo association is interested in putting solar panels on the roof.
Do you think it’s going to take people a long time to realize that going zero is one of the best things for the environment and actually try to do so? Or do you think there will be a rapid response with the new generations?I am trying my hardest to make it happen fast!
How do you recommend we spread word of the fab four?Talk to your parents over dinner. Say things like, “Did you know you can save a lot of money on heating bills by cutting your carbon footprint? We studied it in class today. Someone in Dover, you won’t believe this, his name is Mr. Green, he has cut his house’s carbon footprint to zero and he pays nothing for heating or electricity. He does webinars on it. Can we watch the next one together?” Sign up for the free webinar on my website, all attendees get a free copy of my book Zero Carbon Home too.
Can you elaborate on the ways tax breaks and subsidies contribute to the success of the fab 4?There is a lot here. I cover the subsidies for each of the fab four in the book. But, in short, MassSave will pay for insulation and draft sealing. Solar panels are heavily subsidized by the federal government, the MA government and your utility company. Heat pumps are subsidized by your electric utility and with the 0% interest Heat Loan from Masssave. Triple-glazed windows can also be subsidized with the Heat Loan.
Do you think it is possible for people to have their own windmills instead of solar panels?Yes, but it is far more expensive than solar panels.
How can we as teenagers do this… it’s not always an easy transition for adults/homeownersTalk to your parents over dinner. Say things like, “Did you know you can save a lot of money on heating bills by cutting your carbon footprint? We studied it in class today. Someone in Dover, you won’t believe this, his name is Mr. Green, he has cut his house’s carbon footprint to zero and he pays nothing for heating or electricity. He does webinars on it. Can we watch the next one together?”
How long do solar panels last and do they become less effective over time?The power output is guaranteed by the manufacturer for 25 years and they will probably produce electricity for 40 years. They become about 10% less efficient after 25 years.
What was your role as a strategy consultant?I advised large companies on how to improve their competitiveness. I also worked in South Africa in 94/95 which was when apartheid gave way to democracy and I advise the new government, headed by Nelson Mandela, with its industrial strategy.
What led you to be studying this? When did you realize this was your passion?The scientist in me has always been interested in energy. As a kid, I was fascinated by the potential of making hydrogen and oxygen from water with electricity. So, I have always been passionate about science and spent 25 years in biotechnology. I only got interested in zero carbon stuff when I was doing it on my own house. When I realised that I had cut the carbon footprint of my house to zero, which no-one thought possible, and I was saving so much money on heating and electricity bills that I was making a very good return on my investment, I decided I needed to spread the word and wrote the books.
How many members of our community have gone carbon neutral? What else are you doing to spread this amazing message?My sister, who lives in England, is interested and my sister-in-law, who lives in Needham, has added heat pumps and solar panels. I personaly know of at least 12 people who have cut their carbon footprint by using the fab four after seeing one of my webinars, and if the 72% of webinar attendees who say they will add at least one of the fab four in the next year actually do so, then over 1,300 homes will have done something major to cut their carbon footprint. 
Around how long will it take for you to make back the money spent on the fab four from cheaper heating?About 6 years.
If people only wanted to or could afford to use 1 or 2 components of the Fab Four, would it still be worth it and effective even though they’re not completely zero?Yes. It is not necessary for everyone to get to zero. One homeowner I know cut his carbon footprint 38% with just insulation, draft sealing and home-made triple-glazed windows. He spent $1,000. You do not need to go zero. Do what is right for your family. But do it.
How should we, as non-homeowners, contribute to lowering our carbon emissions?Talk to your parents over dinner. Say things like, “Did you know you can save a lot of money on heating bills by cutting your carbon footprint? We studied it in class today. Someone in Dover, you won’t believe this, his name is Mr. Green, he has cut his house’s carbon footprint to zero and he pays nothing for heating or electricity. He does webinars on it. Can we watch the next one together?”
What is a piece of advice you have for others who want to find innovative / entrepreneurial ways to help the environment?Whether you become a scientist or not, I believe a strong grasp of the fundamentals of science (such as gathering the data, carefully observing the real world, doing experiments and analyzing the results) will give you a strong ability to be both innovative and entrepreneurial. 
Do they offer Triple- Glazed Windows at hardware stores such as Home Depot?Probably not. I got all my triple-glazed windows directly from the manufacturers. 

Saving Water

Q: On the efficiency front, I was surprised there’s not much mention so far of water reduction items, like installing aerators on faucets or low flow showerheads. This would not only reduce water usage (and cost!), but also energy usage on hot water heating. And, water savings ARE energy savings in the big picture.

A: You are 100% right Colleen. It is just that I have never done this and so have no direct experience of it. Personally I like a lot of water in a shower (I think of it as a carbon treat) and I would not like a low-flow shower, but if you are OK with that then go ahead. I think Masssave will install low-flow adapters for you. 

Swimming Pools

Combination solar PV and solar thermal panels for swimming pools

Q:  Are you aware of solar panel company called FaFco they make a combination solar panel PV / Thermal….. The thermal output is about 10,000 BTU per 10 FT. SQ. These are making an impact in the swimming pool industry. Have you looked at these FAFCO .?

A: I have not heard of this panel, but it does not overcome the biggest issue with solar thermal panels which is that you cannot store the heat generated on a sunny day and use it on a cloudy day. With solar PV you can effectively store the energy because of net metering. However, using them to heat pool water is a better idea than using them to heat a house. I would have to check the economics compared to just installing solar PV and using it to power a heat-pump hot-water heater for the pool.

Free AC in my house from heat pump heaters for swimming pools and heat-pump hot -water tanks

Q: I haven’t yet read your pool topic & wonder if you’ve seen the pool heater that uses waste central AC heat?

A: You are well ahead of the pack on your thinking on swimming-pool heaters. My pool heater is a standard issue AquaCal heat pump – it is very good and very economical to run. On warm days when we are heating the pool, (say a warm day in May) it exhausts cool dehumidified air into the atmosphere, truly global cooling! My pool is too far from my house to capture this and use it as AC in the house. But if ever built a house with a pool I would design it so that the heat pump for the pool would dump its cool air into the house. Free AC! I do not know of any heat pumps designed to do this but it would be fairly easy (i.e., you would need to hire an HVAC tech to do it) to run the refrigerant line from the heat pump to the head of a mini-split unit in the house. If you do this, please let me know, I would very much like to publicize stories like this. 


While I have not done this on my pool I have done something similar on my house. I do this by opening the vents (where the air filters slot in) in my air-handler units in my house in about May through September. This is when it is warm enough outdoors that I need AC indoors. Rather than turning on the AC, I open the vents on the air handlers in the basement. This draws air out of the basement and into the circulation of the house. I lean the filter pads against the open vent so that the air circulation is still filtered. The air in my basement is cool and dry. Why? My heat-pump hot-water tank cools and dehumidifies the air in the basement. This works because the ceiling of the basement is very well insulated at approx. R38 because I added 12” of fiberglass in between the floor joists. Using this source of cool, dry air allows me to avoid using the AC units for about 4 weeks in the year when previously I had to use them.  It is not a major cost saving but it is a nice one. I like anything I can get for free! If houses were properly designed, things like this and the integration of pool heating with house cooling, would be built in from the start. Sadly these people seem to not talk to each other. 

If you are seriously thinking of putting in a pool then the $15 you spend on Zero Carbon Pool could be one of the best investments you will ever make. I am saving about $3,000 a year. My pool is big, but even on a standard sized 20’x40’ pool you would save about $1,000 a year by following the pool fab four recipe. 

Designing the pool to be efficient from the start

Q: We currently have a Hayward H250 natural gas heater on our 16’ X 28’ In ground pool and have thought about replacing it with a heat pump. Several years ago I installed a 2 speed pump on the pool and we now save a lot of kilowatts by using the lower speed. However when we want to heat the water we must run the pump on high speed to meet the gas heaters minimum pressure requirement. It’s not really much of a problem because we only use the pool from mid May till mid September and with the solar cover on it holds the heat in pretty good unless we have some unusually cool weather. We also have what I call an indirect solar heating system on the pool that adds a little extra heat as well so we don’t have to run the gas heater very much once we get the water up to temperature. As such I was wondering if the heat pump pool water heater that you use requires you to run your pool pump on high speed to keep your water warm? 

My indirect solar pool heating system consists of 400’ of polybutylene tubing that I put in the concrete that surrounds the pool. I feed it with pool water coming off a tee fitting that I installed after the filter but before the heater. Of course I have a ball valve installed so that I can shut it off at night. Once the water exits the 400’ of tubing that is buried in the concrete it just gravity flows back into the pool. I can send you som photos of it if you’re interested in learning more about it. Another benefit of using this indirect solar system or maybe we should call it a reverse radiant system is that it keeps the concrete a little cooler on those very hot sunny days. My grandkids really appreciate that.

A: This sounds like a well thought out installation – I like the passive pool heater which heats the water and cools the deck, a very nice two-for! I think this would work on most pool decks, but obviously you need to do this from the start. If I were designing a pool from the start I would also connect the pool heat-pump heater to the house AC system, but that is a different subject, you can read more about it here if you like: https://greenzerocarbonhome.com/2020/07/free-ac-in-my-house-from-heat-pump-heaters-for-swimming-pools-and-heat-pump-hot-water-tanks/
Our pool heat pump is made by AquaCal and it has proven to be both reliable and efficient. When we installed it, we left the old propane heater in place but we no longer use it at all. If it did not cost me money to take it out, I would have taken it out by now. 
The heat-pump pool heater does have a minimum water-flow requirement to work and on our pool and it is at about 1,400 rpm on the variable-speed pump motor for the water-circulating pump. I usually set the water pump at 2,000 rpm so that it circulates the entire pool volume once per 24 hours, which is necessary to keep the pool water filtered. So it is usually enough flow to allow the heater to come on. However, if the skimmers and filter are clogged, the heat pump will shut down because the water flow is insufficient even at 2,000 rpm.  
The pool-water circulating pump now runs 24/7 at 2,000 rpm compared to the old fixed-speed motor that ran at 3,450 rpm about 12 hours a day. This alone is saving me about 75% of the electricity used to run the pool. If you already have a 2-speed motor it is probably not going to save you a lot more money to go with a variable-speed motor. But when you need to replace the pump, I would recommend a variable-speed one as then you can set it to the lowest flow rate needed to circulate the entire pool volume in 24 hours. There is a lot more information on how we got to a zero-carbon footprint on our pool in the book Zero Carbon Pool which you can order here: https://greenzerocarbonhome.com/shop/

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