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Community and Q&A

Adding Exterior Polyiso Foam Insulation to Zip Sheathing

Sealed | Posted in Green Building Techniques on

Hello all,

I am currently building a home employing some techniques I’ve learned here. For insulation, I put R20 closed cell under basement slab and garage slab, R20 continues up basement walls where it meets ZIP R6 that is on all exterior walls and roof. In order to put the R6 on the roof, I unfortunately had to first put an entire deck of OSB down so the ZIP R6 would not compress on the rafters.

Now for the terrible part. I didn’t put the R6 on the the entire front wall of the home because I am putting a natural stone veneer on the front of the home and thought the R6 ZIP would have posed problems, so I put up regular Zip instead. This has bothered me immensely the whole time since I went through the trouble and spent a lot of money creating an entire circle of insulation around the home to avoid thermal bridging. The ZIP Huber rep stopped by my house and informed me that the R6 insulated sheathing can indeed support natural veneer stone. I beyond frustrated that I wasn’t smarter earlier. So now I need to put insulation on top of the regular zip wall that is on the the entire front of home. Do I put R3 ZIP over the zip, or put a piece of 1″ insulation right over the zip. I am worried about condensation. I can only add 1″ of material which is why I can’t put R6 up. Is R3 over zip going to create a swampy mess with condensate since it might not be able to dry to inside or outside since this would be literally a Zip sandwich? a ? Will putting up 1″ of rigid insulation be better so that it can dry to the outside?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    1" of polyiso will get you R6, and be a sort of DIY version of Zip R6 since they use polyiso on their panels. As long as you have drying ability towards the interior, you should be OK putting the polyiso on the exterior.

    It sounds like your plan is to put a layer of polyiso over a layer of "regular" (non-R) Zip sheathing that is against the exterior side of a studwall. I don't see a problem here as long as you don't have an interior side vapor barrier.

    Bill

  2. Sealed | | #2

    Thank you Bill for the insight. I will try using 1" of polyiso and see how that goes. Hopefully the weight of the stone veneer won't cause problems down the road from sag.

  3. Jon_R | | #3

    When using exterior wall foam, you should follow the code here (and whatever local code you have):

    https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements

    Get it wrong, and yes, condensation/sorption can be a problem.

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