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Foil-faced polyiso on both sides of a 2×4 exterior wall?

user-1037041 | Posted in GBA Pro Help on

Hello all —

I’m about to begin work with my plumber on my remodel and have a bunch of scrap 1-1/2″ and 3″ polyiso laying around the site. I’ve installed 3″ on the exterior of the building and need some insulation in the stud cavities. Tomorrow, my plumber will be installing a tub and I’ll need to insulate the two stud cavities before we install the tub. As my walls are of the dry-to-the-inside variety, I’m thinking that it’s probably not a good idea to cut my scrap foil-faced polyiso and pack it in there? My ultimate plan is to fill the stud cavities with fiberglass when the time comes, but tomorrow, I was thinking I might be able to use some of the polyiso scrap I have.

My walls, from outside to in: Hardie Plank > 3/4″ strapping > Tyvek (taped) > 3″ polyiso (taped) > OSB (taped) > 2×4 framing.

What say you all?


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  1. mackstann | | #1

    This would put the OSB in a "foam sandwich" which seems to be pretty universally a bad thing.

    This search should shows lots of such advice in the past here at GBA:

  2. user-1037041 | | #2

    "Foam sandwich!" That's the "term" I was trying to remember... I pretty much knew it was a bad idea.
    Thanks for the links, Nick. Appreciated.

  3. GBA Editor
    Martin Holladay | | #3

    Nick is right. If you have installed 3 inches of rigid foam on the exterior side of your wall sheathing, you shouldn't use rigid foam on the interior or between your studs.

    Instead, you want to choose a vapor-permeable insulation. Choices include cellulose, blown-in fiberglass, mineral wool, fiberglass batts, or open-cell spray foam.

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