At the North American Passive House Conference in Philadelphia in September, I gave a presentation on the global warming impact of insulation, an issue I’ve discussed a few times since Alex Wilson wrote his paper on the topic back in 2010. In my presentation, I covered two different aspects of the issue: (i) the problem with Wilson’s payback calculations and (ii) using David White’s global warming impact calculator to make more informed decisions.
I’ll discuss these two aspects of the issue in two articles, giving you the payback part here and the calculator in two weeks.
Diminishing returns and payback
I don’t know why it took me so long to figure this part out, but I didn’t see it when I first wrote about Wilson’s payback calculations. Now it seems pretty clear. Let’s see if I can convince you.
To calculate how many years it takes to pay back the cost of something, you need to know what the savings are per year. In this case, we’re not talking about money. We’re talking global warming emissions, but the principle of payback is the same. In Wilson’s calculations, he used a calculation method for energy savings that made two of the insulation types look really bad. (Those two were extruded polystyrene and closed-cell spray polyurethane foam.)
That calculation method he used was to split the energy savings due to the highly insulated walls in their study into two parts. The initial energy savings went to the cellulose insulation in the 2×6 wall cavities, with a whole-wall R-value of R-14. Then they added more insulation of the various types they studied and calculated how much energy that “extra” insulation saved.
That might make sense if you have an existing home with…