Polyiso sheathing
bdrfab
| Posted in Energy Efficiency and Durability on
Most if not all references to polyiso sheathing on here seem to be towards the foil-faced variety as opposed to felt covered typically used in roofing. I’m curious: why? Is it because of the foil helping with radiation, tape sticking better, or what? Both types have the same insulating values/permeance, or no?
I was hoping to use it on my exterior walls, over the existing 1″ XPS, or is this a bad idea?
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Replies
Aaron,
1. Foil facing has two virtues: it acts as a radiant barrier (meaning that if the rigid foam is installed adjacent to an air space, the foil helps raise the R-value of the air space by about R-1 or at most R-2), and it helps prevent the evaporation of useful blowing-agent gases trapped in the foam. One other virtue of the foil-faced polyiso: it's usually easier to get, because it's the kind of polyiso that is most commonly available at lumberyards.
2. As long as you aren't installing the foam adjacent to an air space, the roofing polyiso performs just as well. Even if the foam is adjacent to an air space, the difference in R-value between the two products is small.
3. Installing polyiso over XPS is unusual, but there is no reason you can't do it. Just be sure that the combined R-values of your foam sheathing are adequate for your climate zone and wall thickness:
Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thanks Martin.