Replies

1. Expert Member
   BILL WICHERS | Jul 17, 2022 01:37pm | #1

   Polyiso is commonly used on roofs, and it’s better in terms of R value per unit thickness, with 3” giving about R19. You should be able to order this through the pro desk at Home Depot, or any commercial roofing supply house.

   Is there any particular reason you think you’re limited to EPS and XPS?

   Bill

2. 
   littlebearinthewoods | Jul 17, 2022 03:05pm | #2

   Because in cold climates polyiso doesn't fare well. https://www.ecohome.net/guides/2254/polyisocyanurate-foam-is-a-hot-new-building-product-learn-where-and-where-not-to-use-it/ and articles on GBA as well.

1. Expert Member
   BILL WICHERS | Jul 17, 2022 03:16pm | #3

   That "polyiso doesn't work when it's cold" is really overblown in many ways. The usual derating is to put it at R5 per inch, which is the same as XPS when first installed, and better then EPS at about R4.2 per inch. Derated polyiso won't really perform significantly worse than those materials at colder temperatures, but it might lose it's R-per-inch advantage. All the rest of the time, polyiso will outperform those other materials. A lot of those concerns come from one particular article and a graph in that article, and it showed varying amounts of R value reduction for different manufacturers, but NONE of the manufacturers were listed. In past discussions on GBA between myself and Dana Dorsett, he liked to call that bad-performing example "world's crummiest polyiso". Everything else did better.

   Not all polyiso performs the same, either. The cold temperature issue is due to certain blowning agents, and not all polyiso manufactures use the same blowing agents. Dow Thermax, for example, claims to not suffer from this phenomenon, and older polyiso didn't have this problem, either. There is some reason to think this issue will improve in the future too, since manufacturers do have an incentive to correct this.

   Another thing that comes up is that it is the median temperature of the panel, not the temperature on the cold side alone, that effects this. 20 below on the exterior does not mean the entire panel is at that temperature, only the outermost part is. That means it's not as bad as you might think.

   Basically if you want to worry about cold weather performance, just derate the polyiso to R5/inch, which is the same as XPS starts out at (XPS degrades over time to the same as EPS), and don't worry about it. There is no reason to dismiss polyiso as not suitable for colder climates, just apply it appropriately as you would with any other product.

   Bill

1. Expert Member
   Akos | Jul 17, 2022 06:22pm | #4

   Fiber faced roofing polyiso tends to be the cheapest in terms for $/R. You can also sometimes find it reclaimed for significantly less than new.

3. 
   tdbaugha | Jul 17, 2022 08:44pm | #5

   Check out R-Shield. They make EPS very very thick.