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Attic vapor barrier (humid climate)

Jared Watson | Posted in General Questions on

I’m planning a renovation of an old attic space in New Orleans, La.
I have to install continuous soffit and ridge vents first.
After this, I am wondering about how to install a radiant barrier.

The roof is an asphalt shingle roof on 3/4″ ply.

Does the radiant barrier atticfoil get stapled directly to the underside of the ply and between the roof joists, or do I put up baffles first for a 1″ air gap, then install the radiant barrier between the roof joists.  Or do I use rigid insulation as a baffle, and install radiant barrier to the rigid insulation?

Also, because it is a humid climate, does the paper of the batt face the exterior, because the exterior side is the more humid side here, or is it best to use unfaced batts to insulate this attic conversion.

Thanks so much!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jared,
    You call this work "a renovation of an old attic space." Are you converting the attic into finished space -- a bedroom or an office? Or will it simply be an unconditioned attic?

    If it will be a finished space, you don't need a radiant barrier. You need real insulation -- something with R-value.

    If you plan to install insulation (for example, fiberglass batts), you need ventilation baffles, but you don't need a radiant barrier.

    For more information on this work, see these article:

    "How to Build an Insulated Cathedral Ceiling"

    "Site-Built Ventilation Baffles for Roofs"

    "Radiant Barriers: A Solution in Search of a Problem"

  2. Expert Member
    Dana Dorsett | | #2

    Using foil-faced polyiso as the exterior-side vent channel baffle gives a small boost in overall insulation performance. It is also an exterior side vapor barrier, which is useful in a location like Louisiana. To prevent wintertime moisture condensation on the interior side facer of the polyiso, use 3/4" or 1" goods (R5 minimum).

    Asphalted paper facers are vapor retardent but not vapor barriers, but they are also not adequate air barriers. It's fine to use them in your application, but it doesn't really matter which side they are on.

    That said, using high density unfaced batts is generally better- they're more air retardent and higher R/inch.

    In areas that won't get ceiling gypsum the batts can be held in place using PERFORATED aluminized fabric radiant barrier stapled to the underside of the rafters. While not a true air barrier it's sufficient for keeping the performance of even low density batts at their rated levels, and if perforated won't create a moisture trap between the foil-faced foam baffles and the radiant barrier. Perforated RB is cheaper and easier to install than gypsum board, and imparts a modest thermal performance improvement.

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