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Community and Q&A

Zip R Sheathing and Interior Closed Cell Spray Foam

lkoza | Posted in General Questions on

Hi we are building a house in the Chicago Suburbs – climate zone 5.  We have two questions. 

1 – We are looking into using the Zip R sheathing. Has anyone had issues with the zip system in terms of longevity? Our contractor says they have been hearing of some issues with it failing. I am seeing contradicting information on whether it is or is not a vapor barrier. By the literature – it is note, but I am seeing others say differently. 

2 – We are looking to to use the 1 1/2″ R Sheathing – so R 6 Continuous with 2″ closed cell spray foam for an R13 on the interior.  We want to make sure this wall assembly is not creating any moisture problems.  The closed cell will be creating a vapor barrier on the inside, so we want to ensure the wall is able to appropriately dry to the exterior. It is a brick wall so we have a drainage plane.  Thoughts?

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  1. Expert Member
    Akos | | #1

    The issues with Zip is mostly installation. Trades have a tendency to overdrive nails which damages and weakens the panel. Taping or liquid flash over the seams/rough openings also requires a bit of care.

    If your watch your trades and you don't have either of these issues, it should be fine.

    ZipR is a vapor barrier. If you are installing it over a 2x6 wall with R19 batts, you can skip the SPF if you installed a warm side vapor retarder (one of the smart vapor barriers is a better choice). If you go up to the R9 ZipR you can skip the interior VB.

    With the 2" of cc SPF over the foam, you definitely won't have any issues. With the cc SPF, you are fine with just painted drywall on the inside.

    1. JC72 | | #3

      "The issues with Zip is mostly installation. Trades have a tendency to overdrive nails which damages and weakens the panel. Taping or liquid flash over the seams/rough openings also requires a bit of care.."

      Concur. Taping is critical. I've read where owner-builders have had to go behind the framers to fix poorly affixed tape. The ZIP-Tape dispenser is a must, along with the J-Roller, and a nail gun set to the proper depth.

  2. Expert Member
    BILL WICHERS | | #2

    You should be ok with that assembly. Moisture can’t really get in, either, since everything will end up pretty much completely encapsulated.

    I’d advise against using spray foam in stud cavities though. The thermal bridging of the studs — which still applies in this particular case even though you’re using continuous exterior foam with the Zip-R — negates much of the performance advantage spray foam would otherwise have. I’d use mineral wool batts instead. You’ll save some money this way and end up with an assembly that performs nearly the same. You could even put the savings toward additional exterior foam which will gain you more insulting value.

    Do your air sealing by properly sealing the Zip-R panels instead of relying on interior side spray foam.


  3. JC72 | | #4

    ZIP-R is basically a vapor barrier because of the foam. However, regular ZIP has similar permanence to OSB + WRB (ex Tyvek).

    With a brick exterior you'll have an air gap between it and the exterior (OSB side) of the ZIP-R. The gap will allow the OSB-side to dry to the exterior. Interior wetting will NOT dry to the outside but will dry to the inside if you use cavity insulation other than close cell spray foam.

  4. Jon_R | | #5

    Center panel analysis isn't the whole story. Without the spray foam or other interior side air barrier, I'd be concerned that Zip-R could suffer the fate of some SIPS that weren't air sealed on the interior side - failure at the joints (where air can make it all the way to the cold exterior tape, deposit moisture and then return to the interior). Similar concern without an interior side vapor retarder - a large area for moisture to diffuse in and a small area for it to condense/sorb and accumulate. But no, I don't have proof that this can be a significant problem in a Z5 wall.

    Open cell spray foam + vapor retarder interior side paint is a lower cost option to block interior moisture from getting to the Zip tape.

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