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Roni Collier | Posted in General Questions on

I am finishing a screened porch into a fully-insulated room, to be part of the house and used year round. I understand Roxul is an excellent choice for exterior walls. Is a vapor barrier necessary and, if so, can someone suggest how to install. Tks for your help.

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  1. Expert Member
    Dana Dorsett | | #1

    Your climate/location, sheathing, and siding types matter. Roxul is extremely air & vapor permeable, but depending on the rest of the stackup you may still be able to count on standard latex paint as the interior side vapor retarder.

    So, got ZIP code or city? What type of siding, and what type of sheathing?

  2. GBA Editor
    Martin Holladay | | #2

    No, Roxul is not a vapor barrier. In most situations, the concern over vapor barriers and vapor diffusion is overblown. More information here:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    If you aren't sure how to install Roxul insulation, I suggest that you call up an insulation contractor.

  3. Expert Member
    Dana Dorsett | | #3

    "If you aren't sure how to install Roxul insulation, I suggest that you call up an insulation contractor."

    ...or you can watch the instructional vidis:

    A couple of finer points- tuck the edges & corners to be sure they make it all the way in for a no-void snug fit, then tug gently until the batt is just proud of the stud edges, to guarantee that it stays tight to the gypsum too.

    Also, before insulating the cavities caulk the framing to the exterior sheathing with an acoustic sealant type cualk, and seal any lateral seams/gaps in the sheathing that can leak air on the exterior side. (There was a real gaping canyon of a seam in the sheathing in the Roxul "Above Grade Exterior Wall" vidi- might have even gone with low expansion can-foam there.)

    Don't forget to caulk the top plate seams and the bottom plate to the subfloor before the drywall goes in too. There's no such thing as "too tight", and all air leaks rob performance.

    There are also other instructional vidis for batt installation detailing what to do around electric penetrations, dealing with wider-than-spec stud spacings, etc, eg:

    There are no rocket scientist wasting their careers as batt installers, but attention to detail DOES matter. DIYers may take longer than the pros, but they have a vested interest in the outcome, and are at LEAST as likely to do it right if they know what they're shooting for.

    If you'd rather call an insulation contractor, consider going with a blown or sprayed product rather than batts, which has the benefit of a near-perfect fit, no gaps, voids or thermal bypass channels.

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