When it comes to insulation, can you have too much of a good thing?

In a recent post on a foam insulation system I noted that “I have been called an idiot so many times by people who point out that foam insulation works really well and that it pays back the carbon and greenhouse gas footprint in short order.” In fact, it is not so simple. Much depends on what the insulation is, and how much of it you use. (The house shown above is built entirely of foam)

In a recent article on Energy Vanguard, Allison Bailes builds on the work of Alex Wilson (here on TreeHugger) and Passive House designer David White. The basic principle is: certain foam insulations (Extruded Polystyrene or XPS is the worst) have blowing agents that are serious greenhouse gases, the HFC-134A in XPS being about 1300 times as bad as CO2. The blowing agents leak out of the foam over time, so the question is, at what point does adding insulation create more greenhouse gas than the energy saved?

clean electricity© Allison Bailes

Of course, this will vary depending on what you are using as fuel; if it is all clean electricity from solar, wind and water, then the greenhouse gases emitted by the insulation matter right away. If you are using natural gas or dirty coal fired electricity, then it takes longer.

But with XPS or sprayed closed cell polyurethane foam, you can hit a point where adding more insulation is actually worse for the environment, because of diminishing returns as you add more insulation, because the additional savings in fuel are so small in relation to the gases emitted by the foam. You can calculate the “payback time”- Wilson described this:

We want to know how many years of energy savings it will take to pay back the lifetime GWP of the insulation to figure out whether it's a good idea to use that insulation material in our low-energy buildings. Another way to think about this is how many years of energy savings will be required to "break even" on the GWP of the insulation.


Wilson calculated it to be as many as 120 years for XPS. Bailes disputes his calculation and thinks it is less than twenty and that it is not really a problem at all. You can parse his argument in this post.

If you buy into my argument, you should agree that Wilson's conclusion to avoid XPS and ccSPF wasn't warranted. If you don't buy into my argument, please let me know why. I'm not saying those two materials are neutral in all respects in comparison to other insulation materials. There are certainly other issues to consider. But when we look just at energy savings and global warming impact, XPS and ccSPF aren't as bad as it first seemed.

spreadsheetLloyd Alter/ David White's Spreadsheet/CC BY 2.0

I do not buy into his argument, because I don’t believe the payback question is particularly relevant; I had a look at the program and see that no matter what, XPS and ccSPF put out a lot of greenhouse gases, period, and the more you use, the more of a problem you have. Whereas mineral wool, cellulose and even EPS are way down there.

foundationLloyd Alter/ foundation section/CC BY 2.0

However it does confirm that from a greenhouse gas perspective, the Expanded Polystyrene insulation used in the Legalett system shown earlier is not that much of a problem.

None of this gives even the more benign foams a free pass. Polystyrene is made from the monomer styrene, which is made from ethylbenzene, which is made from the alkylation of benzene with ethylene. Benzine is a petrochemical and is carcinogenic. In short, EPS is solid fossil fuel (although to be fair, by volume it is mostly air)

Polystyrene, when burned, produces "a complex mixture of polycyclic aromatic hydrocarbons (PAHs) from alkyl benzenes to benzoperylene. Over 90 different compounds were identified in combustion effluents from polystyrene." The addition of toxic flame retardants like HBCD (hexabromocyclododecane) does not even keep it from burning; they barely do their job. However HBCD does do other things; “It is highly persistent in the environment and bioaccumulative in the food chain; it is believed to cause reproductive, developmental, and neurological impacts.”

wall system© Legalett at IIDEX

But again, on the other hand, you can do things in foam that you cannot in cellulose or rock wool, like totally wrap and seal a house so effectively as the Legalett system does. So when you look at the entire picture from the underside of the foundations to the roof, with all the thermal bridges and air seals taken into account, when you treat the entire building as an assembly rather than just look at the insulation, one might still be able to make the case that foam is a better choice.

I wish it was easier and one could just say “use this” ; I certainly used to be more doctrinaire and just say foam is bad, rock wool good. But it all has to be looked at as part of a bigger, more complicated picture. About the only thing that appears clear is that you shouldn’t use XPS foam.

Tags: Green Building | Insulation | Materials | Plastics


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