R-value tests by the National Roofing Contractors Association and the Building Science Corporation have revealed that the thermal performance of polyisocyanurate is greatly reduced at cold temperatures. While the R-value of polyiso at a mean temperature of 75°F might be about R-5.7 per inch, the effective R-value of the polyiso drops to about R-4.8 per inch at a mean temperature of 25°F.
Over the past three years, GBA has published several articles on this problem. (See, for example, In Cold Climates, R-5 Foam Beats R-6 and Cold-Weather Performance of Polyisocyanurate.) As builders have learned that polyiso wasn’t doing as good an insulating job at cold temperatures as they had thought, they’ve started to ask questions, including:
In response to such questions, it’s fair to say that polyiso manufacturers have been tight-lipped. John Straube, a professor of Building Envelope Science at the University of Waterloo in Ontario, has had several conversations with representatives of polyiso manufacturers. In a 2015 interview, Straube told me, “Maybe the manufacturers … are not telling us the truth. But in apparently honest conversations with several polyiso manufacturers, they’ve told me that they do not fully understand why this is happening. PIMA [the Polyisocyanurate Insulation Manufacturers Association] doesn’t even believe the phenomenon is real. Some of their members … say, ‘I am not 100% sure it is a real effect.’ Well, it is real. … Here’s the thing about polyiso manufacturers: They do not share their proprietary formulations. Manufacturer A may not know what is in Manufacturer B’s foam. So now, when we discuss cold weather performance, they are kind of going, ‘I’m really surprised.’ But it’s possible that they are jiving me.”