Zero Carbon ® Blog

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.

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.

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.

The energy model I use I built myself. There are lots of software packages out there 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 a couple of years ago:

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 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 bills 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: 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: his 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/

Q1: 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 leaves people using their AC in winter, which is insane. 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. 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. 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/

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

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

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

A: 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.

Q4. Any other thing that you would like to highlight and add from your experience.

A: 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.

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 that ISO board. I do not know what it’s flammability rating is though. 
A more fire-proof alternative to adding the foam 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 their 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 Siga membrane for air tightness too. 

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 big 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.

On Jul 1, 2020, at 2:29 PM, Chris wrote:I enjoyed the presentation & am looking forward to reading your book. I’m surprised air sealing wasn’t emphasized more. A new version of an old product looks very interesting: AeroBarrier. Instead of replacing 3 basement windows (12 x 32in); I’m getting quotes for Innerglass. Thanks!I haven’t yet read your pool topic & wonder if you’ve seen the pool heater that uses waste central AC heat?

Hi Chris, I am beefing up what I say about air sealing. I have seen houses where air sealing cut 25% off the energy bill. This was not true on my house, where my roof is a rubber membrane and my siding is tongue and groove. I did use a couple of cans of spray foam around the sill plate and weatherstripped the bulkhead door. However, this was not enough for me to be able to measure the cut in the energy bills. However since my house was an exception I will be mentioning it much more in future. See my recent blog post on the issue:
https://greenzerocarbonhome.com/wp-admin/post.php?post=1130&action=edit

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.