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Managing vapor

no_way_jose | Posted in GBA Pro Help on

I have a roof assembly that is fully insulated at this point, and the question is whether or not to install a vapor variable membrane beneath the sheetrock. From inside to outside the assembly is:
7″ dense cellulose,
4″ closed cell foamĀ 
2″ of xps directly on top of rafters
1/2″ plywood
30# felt
2″ of strapping (ventilation channel)
metal roof.
We are in zone 5 Whitefield, Maine.
Thank you!

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  1. Expert Member


    I don't see any advantage to adding an interior variable-perm membrane with that stack-up. The interior surface of the foam will be too warm for any moist interior air that gets through the cellulose to condense on.

  2. no_way_jose | | #2

    Malcolm Taylor, t
    hank you for weighing in. It seems a bit redundant to me too. I was also considering a vapor variable membrane for the walls. Assembly there from in to out is:
    6" dense pack cellulose
    2" xps continuously on the outer stud face
    1/2" plywood
    1" rainscreen
    Cedar siding.

    Any thoughts? thanks again-

  3. Expert Member


    You are exceeding the minimum thickness of foam necessary to stop condensation on the walls too. I would keep the interior as vapour-open as you can. So no membrane there either.

  4. no_way_jose | | #4

    Great, thank you again for the advice!

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Good luck with your build!

  5. maine_tyler | | #6

    Whitefield is zone 6, not 5.

    For the wall, 2021 IRC says you can get away with interior class III vapor retarder if your exterior continuous insulation is greater than or equal to R 11.25 over a 2x6 wall. Use of a smart class II retarder lets you bring that all the way down to R 5.

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