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Floor joists and insulation for vented crawlspace in a flood plain

user-1063388 | Posted in Energy Efficiency and Durability on

This is a rehab of a home in a FEMA floodplain in a suburban area. The house has never flooded and is not likely too. In any case, the county will not allow land disturbance of the existing foundation and we cannot enclose the crawlspace because of the floodplain. However it appears many of the floor joists are warped so we may need to replace the first floor deck and joist system.

Can I use Regular i joists in a vented crawlspace. It is 30″, so the joists would be well off the crawlspace floor (dirt covered by vapor barroier) and bearing on a pt sill plate. Is there a coated ijoist?

If I am stuck with dimensional lumber, is there a coating I can use to inhibit mold growth etc a reduce warping that comes with pt.

I intended to insulate the joists with closed cell. If so, what do I cover the underside of the joists and foam with to resist pests.

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  1. GBA Editor
    Martin Holladay | | #1

    You wrote, "The house has never flooded and is not likely too." I think you are being optimistic.

    FEMA floodplains (by definition) are likely to flood. The question isn't whether or not such a floodplain will flood; it will. The only question is when. It might happen next year, and it might happen in 15 or 20 years. But it will happen.

    It's a waste of money to rebuild your floor system unless you can be sure that the lumber stays dry. Without a site visit, it's impossible to tell whether you need to raise your foundation -- but you might. If your crawl space is only 30 inches high, that isn't much. If the soil is chronically damp, that's no good.

    Assuming that you can find a way to keep your floor assembly dry, you shouldn't have to worry about mold or rot.

    Pressure-treated joists shouldn't warp, as long as they are sound joists with adequate blocking.

    If I were you, I would install a continuous layer of foil-faced polyisocyanurate foam insulation under your joists. I would protect the rigid foam with a layer of OSB or plywood.

  2. rstex | | #2

    Would the foil faced foam be foil up or down? OSB or plywood would only need to be minimal thickness correct? Doing a similar crawl space build and had about decided to do this kind of installation.

  3. GBA Editor
    Martin Holladay | | #3

    Most foil-faced polyiso has foil on both sides. However, the foil facing isn't essential. You could also use polyiso with a different type of facing.

    The rigid foam should be protected by a layer of OSB or plywood that is rugged enough to resist local critters. I would use 1/2 OSB or plywood. Even with 1/2-inch material, you should inspect the protection layer occasionally to see if rodents have chewed any holes in your floor assembly.

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