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Community and Q&A

Faced direction in kneewall

Champak | Posted in Energy Efficiency and Durability on

Hi, I’m installing the faced pink stuff in my knee wall. Which direction do I point the face? I know generally its on the heated side, however, im in NY and the crawl space gets super hot in the summer and i might be putting an AC in, so technically the knee wall would be the heated side, no? And the humidity in the house hits can hot 60 at times, averaging 50 on the first floor (which shouldn’t be an issue once the a.c. is in). But then what about the winter?

Thanks.

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #1

    Champak,
    The kraft facing should be installed to face the interior. That said, it wouldn't cause any problems if you got it backwards. Vapor drive doesn't matter much in this application -- what you need to pay attention to is air leakage, not vapor diffusion.

    Remember that you'll need to install an air barrier (drywall, housewrap, OSB, or rigid foam) on the attic side of the fiberglass batts. For more information, see “Two Ways to Insulate Attic Kneewalls.”

  2. Champak | | #2

    Thanks, it's a good thing you said I need to install an air barrier. I was going to leave it open since the area is just being used for storage. Took me a minute to process why I would need to put up a wall or something but I get it, to stop the draft from the soffit...but would it REALLY make that much of a difference? With that asked, only one side has the soffit open because of a previous owner renovations...You think it's necessary to put the wall up on that side as well?

  3. User avater GBA Editor
    Martin Holladay | | #3

    Champak,
    Q. "Would it REALLY make that much of a difference?"

    A. Yes. Fiberglass batts are close to useless unless they are enclosed on all six sides by an air barrier.

    Q. "Only one side has the soffit open because of a previous owner renovations...You think it's necessary to put the wall up on that side as well?"

    A. Yes. I'm not sure what you mean by a "wall." The kneewall is the wall. Between the studs are the fiberglass batts. The fiberglass batts won't perform well as insulation unless there is an air barrier on the interior side of the batts (usually drywall) and an air barrier on the exterior side of the batts (in this case, drywall, housewrap, OSB, or rigid foam).

  4. user-7049627 | | #4

    Is it cost effective to retrofit an air barrier to a knee wall that is fiberglassed?

  5. User avater GBA Editor
    Martin Holladay | | #5

    User-7049627,
    Probably, but the answer depends on (a) your climate, (b) your fuel cost, (c) whether you are doing the work yourself or paying a contractor to do the work, and (d) if you are paying a contractor to do the work, what the labor rate is.

  6. User avater
    Dana Dorsett | | #6

    In a vented crawlspace attic go ahead and install the kraft facer on the attic side of the assembly, but air-seal the wallboard and any electrical penetrations to the framing before installing the fluff. The facer is an adequate air barrier for preserving the thermal performance of the fiberglass, but it's never tight enough to prevent air infiltration from passing through the wall. But air-tight drywall IS tight enough to prevent the air infiltration.

    The kraft facer is semi vapor permeable, more vapor permeable than latex paint on wallboard, and if exposed directly to a vented attic space it will never accumulate enough moisture to create condensation or mold inside the kneewall (in any climate south of the arctic circle anyway. :-) ) In most of NY state (zones 4A & 5A) you could even use something as vapor tight as OSB or plywood on the exterior side of the kneewall and not trap moisture. But even in zone 6A there is no moisture risk if the exterior side is asphalted kraft facer.

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