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Can I put up 2 kraft-faced batts in a shed roof?

Thomas Ennis | Posted in Green Building Techniques on

I have a room with a new roof and rafters, and want to install R-30 worth of insulation by installing 2 R-15 3 1/2 inch batts, is it kosher to put one kraft-faced batt on top of another? My buddy John says that is a risk for moisture buildup between the batts but I argue that the kraft paper is not a moisture barrier, There will be a baffle above the batts to allow an air channel from the front (bottom) of the shed roof (with a soffit vent) to the top (also soffit vented)

Thanks!

Tom in Westford, MA

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Thomas,
    1. The kraft facing in the middle of the stack-up probably won't do any harm. But it's easy to peel off -- so why not just peel it off and discard it before you install the batts?

    2. As you probably know, R-30 isn't much -- it won't meet minimum code requirements in Massachusetts.

    3. For more information on insulating this type of roof, see How to Build an Insulated Cathedral Ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    Leave the mid-depth facer on. Even though it's not an absolute air barrier, the facer is a LOT more air retardent than the fluff, so there is at least some benefit to having the extra layer.

    Kraft facers run about a half-perm when dry, but hit 5 perms or more when damp enough to support mold growth. They are "smart" vapor retarders, not vapor barriers. The extra facer will limit the rate of moisture movement in winter when the roof deck is cold and covered with snow, but if it ever needs to relive moisture (in either direction) it can.

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