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Covering and insulating the exterior floor of a raised pier/beam home

user-1063957 | Posted in General Questions on

Hi, our family owns a home on the South Carolina coast which was built on 18 foot piers. At some point in time, the prior owner sealed the exterior first floor (ie, the “underneath” of the home) with drywall. Although the drywall was painted for protection, a few portions of the drywall appear to be damaged from moisture and in some spots have mildew.

For insulation in this sealed space, the prior owner had fiberglass batts added to the joists before the sealing, but the batts appear to be upside down. In othe rwords, the backing of the batts were not placed against the exterior floor, but instead rest on the drywall used to seal.

We now need to have some electrical work to wires in this sealed space, which will involve cutting into some of the drywall. If necessary, I am considering taking this opportunity to correct any errors from the current set up. Consequently, I have two questions:

1. Are the fiberglass batts upside down as I suspect? I thought the backing should be against floorboards, but maybe I am wrong.

2. What materials should be used to seal this space? I suspect that drywall was not the appropriate material for this job, but am not sure what to use. Perhaps plywood or maybe even cement board?

Any thoughts or advice would be welcome. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    It doesn't really matter which way the kraft facing on the fiberglass faces. Assuming you don't find mold in the joist bays, here's my advice:

    1. Repair the fiberglass batts after doing the wiring work. (Remove the facing on the fiberglass in the exposed joist bays if you want, but don't worry about it if you don't.)

    2. Remove all the drywall.

    3. Install a continuous layer of foil-faced polyiso on the underside of the joists. Tape the seams with a high-quality tape, and seal air leaks at the perimeter.

    4. Install plywood or OSB on the exterior side of the polyiso.

    For more information, see How to Insulate a Cold Floor.

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