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Preventing air-washing in fiberglass batt insulation

[email protected] | Posted in General Questions on

I have had a number of situations where a wall insulated with fiberglass batt insulation is exposed to air spaces (a knee wall, e.g.). Since the movement of air through fiberglass greatly reduces its ability to insulate, would it make sense to apply something like a housewrap over the batt insulation to reduce the airflow (and increase the insulating value)? Or is there another way to resolve this problem? I have thought of plastic or insulating board (polyiso), but would be concerned about moisture issues (condensation).

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  1. GBA Editor
    Martin Holladay | | #1

    According to the requirements of the Energy Star Homes program, as well as the advice of building scientists, fiberglass batt insulation should be enclosed by an air barrier on all six sides. (The only exception to this rule is fiberglass insulation on an attic floor, where it's usually cheaper to just make the insulation deeper than it is to install a top-side air barrier.)

    Housewrap will work, although it is difficult to detail housewrap as an air barrier. Rigid foam (for example, foil-faced polyisocyanurate) is usually a better choice. Because polyiso has significant R-value, it usually doesn't get cold enough for condensation problems to develop. That said, 2-inch thick polyiso is better than 1-inch thick polyiso for kneewalls in cold climates.

    For more information, see Two ways to insulate attic kneewalls.

  2. [email protected] | | #2

    Thanks for the response. While I recognize that rigid foam would be best, in the situations I have, there is either limited access (small access holes) or limited space to work (behind the knee wall). I could use either thinner foam panels that were more flexible (1/2", e.g) or cut the panels into strips, but then that seems to be opening up a lot of seams (which, though sealed with tape, I would think reduce the quality of the installation). And, of course, the foam panels are not cheap. So I was thinking the housewrap would be better than nothing (worth doing?). As an alternative to house wrap, could I use a poly barrier with a few slits cut to allow moisture to escape, or would I need to cut so many slits to do that that it would make it ineffective against air movement?

    Also, one wall is a 2x4 wall w/R-19 insulation, so foam board would compress the insulation (which I would assume would mean I should not do it there). If I were able to use the rigid foam, what kind of mechanical fasteners should I use, or is the adhesive enough?

  3. GBA Editor
    Martin Holladay | | #3

    If you decide to install rigid foam, you should fasten it with cap nails (nails with large plastic washers attached to the head).

    If you can't manage to get rigid foam in the tight spaces, the housewrap is a good choice. It's worth installing housewrap.

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