The Case of GWP, XPS, and SPF
This week I read and commented on Avoiding the Global Warming Impact of Insulation by Alex Wilson" target="_blank">Avoiding the Global Warming Impact of Insulation by Alex Wilson over at Green Building Advisor. I'd avoided the article for a while since climate change isn't what motivates me to do what I do, and I happen to think it may be trumped by a bigger problem anyway (i.e., peak oil).
The article has gotten some building science types in a tizzy, though, mainly because of the recommendation to avoid extruded polystyrene foam (XPS) and closed cell spray polyurethane foam (SPF), two products that are used in a lot of high performance homes. Suddenly, builders feel that they've been misled about using these products.
But are XPS and SPF really the problem the article makes them out to be? I don't hold any pretenses that I was a great scientist during my short stint in academia, but I understand the process of doing scientific research, and reading the article raised some red flags for me.
In case you aren't familiar with the name, the award-winning Alex Wilson is the editor of Environmental Building News (EBN), a great resource for green building information that's been around since 1992. He's well respected, has a reputation for providing in-depth, objective information on various products used in construction, and can recite arcane details about products on demand. I heard him speak at Greenprints this year, and I can tell you, he knows his stuff.
I'm not a regular reader of his blog, but the other articles of his that I've read have been informative and based on solid information. This one, however, seems shaky to me. Please understand that I'm not attacking Alex Wilson; I'm criticizing a particular report that gives the name of building science a black eye.
Briefly, the blog article is based on a longer report in EBN and is a look at the global warming potential (GWP) of different insulation materials. Wilson and his team calculated a 'payback,' the amount of time it would take for the material's GWP to be offset by the GWP of the energy savings they generate over their lifetime. They conclude that you should avoid XPS and closed cell SPF because they have high payback periods.
Here's my take on the problems with the article.
Ask the right questions.
When I read the article, the first question that came to my mind was, why are you looking at the payback over the lifetime of the product? It seems to me that it should be all about the net flow on a year-by-year basis. Does it really matter if the payback is 46 years when the GWP that's offset due to annual energy savings may be higher than the GWP of the offgassing?
They seem to have assumed that the blowing agent is released uniformly over the lifetime of the foam insulation, so if that's the case, all that would matter is that the payback be less than the lifetime. If, however, the entire amount that's released happened immediately, then payback would be relevant. That brings me to my next point.Show me the data.
Wilson admits in the short article on Green Building Advisor that he's "not 100% sure that XPS is made with [high GWP blowing agents]." He decided on the basis of "various hints in technical literature" to use the high GWP materials in his calculations. In the full article, he says, "Note that the values are highly dependent on assumptions," and "Assumptions are key in this analysis."
To get their results, the EBN team had to assume that:
- The manufacturers used the high GWP blowing agents.
- The offgassing profile is uniform.
- The lifetime of the product is somewhere between 50 and 500 years, though the article doesn't say what numbers they used.
Science asks the right questions and is based on solid data. If you've done some calculations and created some nice looking graphs, it means absolutely nothing if there aren't any data behind them. It's a house of cards.
One of the commenters on the full article at EBN praised the article as a "rigorous inquiry." If you don't understand how science works, it may look like it is indeed rigorous because it's easy to overlook those statements about the assumptions and focus on the discussion about calculations and the professional looking graphs of payback.
In science, there's this nice little thing called peer review. The way it works is that you can't publish your research without it being looked over by other scientists. This prevents researchers from getting lost in a bubble and believing they've done something magnificent when in fact, they've just concocted a giant fantasy. If this EBN report had had to go through scientific peer review, it never would have seen the light of day.
The term 'building science' is in vogue with green builders and home energy auditors these days, and that's a good thing. But let's remember that science has certain requirements. I was never enough of a theorist to really understand string theory, but I knew enough to see that it was BS (and I'm not talking about building science) because too much of it was based on assumptions that couldn't be tested.
What they should have said
Based on the data available, the EBN team had no justification to recommend avoiding XPS and closed cell SPF. About as far as they could have gone would be something like this:
Some insulation materials MAY use blowing agents with a high global warming potential, and IF those chemicals escape, it COULD be bad news for climate change, depending on how rapidly they're released and what kind of construction they're used in and where the building is. Then again, maybe it's NOT a problem and using them actually helps mitigate climate change. Based on the Precautionary Principle, however, we advocate avoiding these materials until it's certain that the blowing agent has a low GWP.
If they'd said that, the state of building science would be better, and I wouldn't have had to write this article.
In case you're wondering, I have no connection with the XPS or closed cell SPF industries. I like polyisocyanurate and open cell spray foam and probably recommend them more than the other two anyway. I'm speaking out here solely in defense of science.