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Paper backing on polyiso rigid foam: Which way should it face?

AirstreamJake | Posted in General Questions on

I am building a new house in climate zone 3A – far northeast Texas.

My wall layers, from exterior to interior, are as follows: Hardie sheet siding (board and batten), housewrap (Tyvek Drainwrap), 1 inch of rigid polyiso foam, 1/2″ OSB.  Between-stud insulation will most likely be either cellulose or fiberglass batts, possibly blown-on foam.

The polyiso that I’ve bought is unfaced on one side and, on the other side, has a kraft-paper-like backing.  Should this kraft paper backing face the interior or the exterior of the house?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Stephan,
    The main criterion here is, "Which side of the polyiso is easier to tape?" (If the answer isn't obvious, conduct an experiment.)

    Once you have chosen which side of the polyiso is easiest to tape, that's the side of the foam that should face out. Needless to say, you should tape the polyiso seams.

  2. Expert Member
    BILL WICHERS | | #2

    I would face the kraft facing out (towards outdoors). I can't say I've ever worked with polyiso faced on only one side, but my thinking is you might want to look at this like drywall: the paper in drywall essentially provides strength and protects the gypsum core. The foam part of the polyiso may be more prone to damage if exposed without a facer, so I'd put the facing on the side most likely to encounter damage.

    My thinking in a wall like you propose, the highest chance of damage is on the exterior side, since the interior side is essentially already protected by the OSB which should be a fairly flat surface with nothing to put the polyiso at risk.

    Bill

  3. Expert Member
    Peter Yost | | #3

    Hi Stephan -

    When they make PI rigid insulation, they have to use a facer on each side so that as the PI sets, it does not bond to the laminator. So, pretty sure you don't have an unfaced side to your PI board. The facers are also a big part of giving this essentially rigid foam stress skin panel its structural integrity.

    Peter

    1. AirstreamJake | | #4

      Peter, you are absolutely right. After we broke open the pallet of foam, which had been covered by shrink-wrapped plastic, we discovered the paper backing on both sides of the foam. Mea culpa!

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