Month: April 2021

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.