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Is there a way to provide geothermal HVAC to a rim joist in order to keep the rim dry?

jonathanupr1 | Posted in General Questions on

I have a Rim joist that was just above grade sitting behind a 5″ concrete lip … presently i have waterproofed exterior foundation wall, rock from footing up to sill plate, sill is sitting 6″ above grade right now … though 2 feet out from wall the grade rises 1 ft. I am contemplating ways to run radiant tubing behind the Rim joist and sil plate in order to keep them at a desired emulated ambient temperature, pulling water from geothermal pump, in order to avoid condensation. I am going to flash a membrane between the sill plate/rim joist and the concrete, but figured there would be condensation on the membrane regardless. I will have 3″ of XPS from the foundation footing on up to the rafters. It is a conditioned crawl space on a high water table, figured the foundation wall flashing would have a bit of condensation finding its way between it and the Rim joist.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is there a way to provide geothermal HVAC to a rim joist in order to keep the rim dry?"

    A. Yes, of course. If you want, you can run hydronic tubing along side your rim joists to keep the joist as hot as you want. The question is, why would you want to do that?

    Your explanation is a little unclear. It sounds like you may have installed 3 inches of XPS on the exterior side of the rim joists. If so, that's a lot better than if your rim joists were uninsulated.

    In general, you don't have to worry about condensation on your rim joist if you have installed 3 inches of exterior rigid foam. The R-15 insulation on the exterior of the rim joist will keep it warm enough to avoid problems. If I were you, I would beef up the R-value in this area somewhat by installing mineral wool batts or fiberglass batts on the interior of the rim -- and you're all set.

  2. user-1072251 | | #2

    The exterior foam below the siding needs a solid coat of cement stucco to keep the critters out (ants & termites love to nest in this stuff), and a metal flashing break at the top of the foundation so that if/when they chew into the foam they cannot get to the wood portions of the house.

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