Month: June 2020

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 air 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. I have seen houses where air-sealing alone has cut the energy bill 25%.

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 air tight.

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 air-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.

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 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.

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 7c 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. Did I mention the zero carbon footprint of driving my Tesla on solar power?

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 cost 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, vapor penetration, drafts, gain rigidity and 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 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 the back facing south with no shade then the roof can generate all the electricity needed to go zero carbon zero bills. 

The moderator on our call, Bruce Sullivan, 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 is excellent.

Q: Please comment on the utility/ROI of Tesla power walls. 

A: In general batteries have two uses. 

The first is to take advantage of TOD (time of day) 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 application. 

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. I have not bought a battery for this application but I am planning to do so. 

The Tesla Powerwall is considerably cheaper (at about $500 per kWh of storage) per kilowatt-hour of storage than other batteries from companies like Sonnen and Simpliphi. I had a quote from Simpliphi that was $1,333 per kWh). 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 stopped working right when we needed it. Over 10 years this maintenance cost is $5,000. So the full cost of a back-up generator 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 Powerwall to replace my defunct back-up generator in the near future. The regulations on qualifying for this adder are very complicated!

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 weatherstripping is cheap, easy and effective. 

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: 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: 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 adding more insulation is unlikely to save enough money on the bills to pay back the investment in a reasonable period of time. 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. Please see also my other recent post on cathedral ceilings.

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. You can now get vapor barriers especially designed for this situation. One supplier is Majrex 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 either add insulation behind the barrier or in front of it (lowering the ceiling). Check out Martin Halladay’s posts on this topic on Green Building Advisor.

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 50% better than a plain double-glazed window. They are also far cheaper than replacing windows and you can fit them yourself, you don’t need to hire a contractor.