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.