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Community and Q&A

Exterior ridgid polyiso, ok foil faced?

drewintoledo | Posted in Green Building Techniques on

Wondering if I can use polyiso foil faces on both sides in the following build:
Drywall->2×6 dense packed wall cellulose or fiberglass–>Georgia-pacific Forcefeild taped/sealed–>3.5″ foil faced polyiso insulation.
Ok to use polyiso faced on both sides here?

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  1. Expert Member


    Can you provide a location for where this would be built? The answer is somewhat climate specific. In any case, that’s a very thick wall. Have you considered the cost/payback on that, as opposed to other energy upgrades in a different spot?

    1. drewintoledo | | #3

      Thanks for the reply. I'm in climate zone 5A. I've been planning this build for quite a while and was going to land on Roxul but decided to take advantage of the high R value of polyiso. It's actually lower cost for me to purchase and ship 3.5" polyiso than it is 3". I could probably get Roxul at a slightly lower cost but the R value would drop significantly.

  2. Expert Member
    BILL WICHERS | | #2

    With 3.5" of exterior continuous polyiso you're pretty safe whereever you are -- that's around R23 worth of exterior rigid foam!

    It doesn't really matter if you have one or both sides foil faced if you're concerned about the polyiso being a vapor barrier. The foil facer is the part that acts as a vapor barrier, so as soon as at least one side is faced with foil, the polyiso sheet acts as a vapor barrier due to that foil facer. The second facer is no longer relevant.


    1. drewintoledo | | #4

      Thanks Bill. I figured that since I have to detail exterior foam, I might as well go all-out! :) I appreciate your feedback.

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