Musings from a Deep Energy Retrofit: XPS v Polyiso

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This week, I’m sharing a few observations, some technical, some not, from a deep energy retrofit of my own home in Burlington, VT. Part 1 of the series.

“Why aren’t you using polyiso?” So was the question from the foam guy, explaining that I could get almost R-7 per inch instead of only R-5 per inch with the XPS I was using to wrap my 1920s Colonial style home.

XPS intead of Polyiso exterior insulation

Setting aside the inflated claim of R-7 per inch, I replied, the R-value of polyisocyanurate goes down when it gets cold. But the R-value of XPS goes up when it gets cold, to on the order of R-6 per inch at 10-below zero (Fo). It gets really cold here, and the colder it gets, the more I want to slow down that heat transfer.

“No way”, he replied. “That’s wrong, polyiso has a higher R-value.”1 Well, it does, and it doesn’t.

xps-v-polyiso-deta-TR-value is temperature dependent. And the temperatures at which ASTM standards (for example) require testing don’t line up with the real world. But Building Science Corp has explored this with good research in support of the Thermal Metric Project. The Building Science paper on the temperature dependent performance of polyiso will get you deeper. Or you can cut to the chase with a couple of slides from a presentation I saw at Building Science “Summer Camp”, pictured here.

When it gets colder, the R-value of XPS increases. When it get colder, the R-value of polyiso decreases. It was 10 degrees below zero outside (Fo) when I got up this morning. I want insulation that works better when it’s colder. It seems XPS outperforms polyiso exactly when I want it most, days like today.

Now, I could compensate for this by using more polyiso, keeping the layers of polyiso toward the  inside warmer. But that’s expensive. (I chose 3.5″ inches of exterior insulation in part because of the cost, but also because of the easy availability of 6″ screws to attach strapping to stud through the insulation over the 7/8″ sheathing.) But, the wall cavities were already insulated, and thus the exterior insulation would be that much colder.

I don’t think there’s a right or wrong with the insulation material choice here. But it does help to know the performance characteristics to make whatever material you choose perform!. (I’ll note that when I did a deep energy retrofit on my last house by in 1998, before I knew the term “deep energy retrofit”, I used 1.5″ – 2″ of polyiso. I didn’t know what I didn’t know. I still don’t know most of what I don’t know.)

1.Another question, or comment rather, that I get, especially from those APA grammarians, is that the comma goes inside the quotation marks. So does the period. Example: This is the correct “style,” according to American usage. So is “this.” But what if we have a question at the end of the “sentence”? (Yep, question mark goes outside, there. I prefer the more logical British style, which places the comma based on the context of the quote in the sentence. I’m a fan of the Oxford Comma, too.

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About the Author:

Mike Rogers is the President of OmStout Consulting. A nationally recognized expert in residential energy-efficiency, he works with contractors and programs to scale sustainable market approaches to improving homes. More on Google+

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