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Community and Q&A

OSB on bottom of floor joist

jam1030 | Posted in General Questions on

Is it ok to put osb on bottom of floor joist on an elevated raised floor (about 4 feet off ground) to protect fiberglass insulation from critters and moisture?
Or will this cause a moisture problem? In zone 2.

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  1. Expert Member
    Michael Maines | | #1

    Jam1030, it shouldn't cause a problem, but the OSB might take on moisture and get punky over time. Last year I renovated a house in zone 6 with a setup like yours, with heights ranging from 5' to 1' above grade. The OSB had probably been in place for 20+ years and was soft but not moldy. A high-quality OSB such as Advantech will hold up better over time.

    1. jam1030 | | #2

      Since r13 is what’s required here in floor, I even considered using 2” polysio board under joist with no batts

      1. Expert Member
        Dana Dorsett | | #3

        >"Since r13 is what’s required here in floor, I even considered using 2” polysio board under joist with no batts"

        That would even exceed code performance on a U-factor basis, since there is no framing thermally bridging the joists. The maximum floor U-factor is U0.064:

        U0.064 is R15.6 "whole assembly", giving full credit for...

        the R-value of the subfloor (~R0.9)...

        the flooring (~R0.8 for most 3/4" hardwood)...

        the air film above the floor (R0.9)...

        the air film under the subfloor (R0.6)...

        the air film above the polyiso (R0.9)...

        the bottom side protective half-inch OSB (R0.5)

        With another R1 credit for the foil facer you would only need to R10 performance out of the polyiso, not R13.

        If using more moisture tolerant and insect resistant half-inch asphalted fiberboard rather than OSB as the protective sheathing you'd only need about R9 performance out of the polyiso.

        With the insulating sheathing approach to the bottom side the band joists would then need to be insulated to the same R-value requirements of fully above grade walls.

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