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Zip Sheathing and interior vapor barrier

monkeyman9 | Posted in General Questions on

I’ve been residing chunks of my house.  I started pulling the sheathing off. Much is rotted due to nonexistent window flashing and incorrectly lapped Typar from 20+ yrs ago.

I started using Zip sheathing and taping all seams.  As I do this, I pull out the R11 fiberglass batts, leaving the faced vapor barrier from the fiberglass batts on the interior.  Then, I put in R15 Roxul batt insulation ,Zip Sheathing and zip tape.

LP smartside will go on top (was cedap clap).

My question is, should I be leaving the faced bat vapor barrier on the wall interior with taped zip sheathing in Massachusetts?

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Replies

  1. MattJF | | #1

    You should be fine, zip is vapor open at 12-16 perms. The wall will dry well to the exterior.

    If it was a kraft facer on the old batts, that acts much like a smart vapor barrier, so you are good on that side too.

    https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

    1. Jon_R | | #4

      Zip is nowhere near that vapor open (IMO, they deliberately obfuscate this) and this wall probably won't achieve the preferred 1:5 interior to exterior perm ratio. The wall should work but would be more robust with a rainscreen and/or exterior foam. Or plywood+DrainWrap replacing Zip.

    2. Expert Member
      Michael Maines | | #5

      Matt, the phenolic coating on Zip sheathing is vapor open, but the OSB itself has a much lower perm rating. Huber uses different types of wood at their various plants so the perm rating varies, but for 1/2" material it's around 1.6-2.0 perms dry and 3.0 perms wet.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Monkey Man.

    It sounds like you are stripping the fiberglass from the craft paper facing, tossing the fiberglass and leaving the facing as you install new insulation. If that is working for you, then I believe your wall will jibe with the 2018 International residential code, which calls for a class I or class II vapor retarder in your area (unless you are using adequate exterior continuous insulation or venting your siding).

    The missed opportunity here is to air seal the drywall when the walls are open. Air leaks introduce a lot more moisture into walls than vapor drive and degrade insulation performance. So, be diligent about air sealing the sheathing, but also the top and bottom plates when the walls are open, and do whatever you can to air seal the drywall from the inside, as well.

    1. monkeyman9 | | #3

      You got it! That is exactly what I have been doing.

      There is no exterior foam board unfortunately.
      There are no gable end overhangs and only small eave overhangs, which made it tricky to do foam board:(

      Good point on the air sealing. I have been dealing electrical and plumbing penetrations with spray foam. But I should run a bead around the top and bottom plates as well! Thanks!

      One room I did closed cell spray foam instead ( wing the coldest room)

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