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Hot roof… potential moisture issues…

Multiplexer | Posted in GBA Pro Help on

In the process of building new home. 2×6 Gambrel roof. This is an Amish built loft barn… converting to house. From the outside in… my roof assembly is structural asphalt shingles, synthetic roof underlayment, 1/2 OSB, then 2×6
roof truss. Inside the roof cavity I have about 4 inches of professionally installed closed cell spray foam, followed by R-13 faced fiberglass batts. Facing is towards the heated space. Lastly I have 5/8 mold resistant drywall on the ceiling.
I’m concerned that the fiberglass batt facing may cause me a trapped moisture issue within the fiberglass.  There is no air flow or ventilation in this roof anywhere… soffits and ridge vent are completely closed off. Of course the fiberglass batts are “compacted”.. into the 2 inch space in the rafter cavity.  The batts were installed for soundproofing, some R value, and for the facing as a vapor retarder.   Thanks for thoughts and comments.

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  1. Expert Member
    BILL WICHERS | | #1

    You won't generally have an issue with a kraft vaper retarder facing on fiberglass batts. Moisture can move through that material both ways, and since it's not a vapor barrier, it's not really a mositure trap -- you do have drying potential with a kraft facer.

    What you do need to do is to make sure your ceiling drywall has been air sealed at the perimeter and any penetrations (hopefully you don't have any recessed can lights here). Air leaks are much more of a problem than vapor migration through vapor retarders.


    1. Multiplexer | | #2

      Thanks Bill, I was very particular about not putting lights in ceiling.. only one light ... not a can light but a regular 4" plastic circle box. This has been foam sealed shut. I assume standard mud n tape joints will suffice for an air seal ?... thanks for your input.

  2. Jon_R | | #3

    The big problem is a lack of R value. Causing higher bills and, in many climates, ice dams.

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