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Community and Q&A

Rigid foam in enclosed crawl space

mvpeters | Posted in General Questions on

I line in Northern Virginia around the DC suburbs and have an enclosed crawl space.  I sealed up all the vents properly and have hired a company to install new 20mill plastic and rigid foam.  Our crawls space does not take on any water however the only moisture I see is under the plastic in the form of dew.  The Crawlspace company advised to install rigid foam board on the concrete block to insulate the space.  My question is, if they install the rigid foam board, how will my concrete block dry out in the event it gets saturated by heavy rains?  Won’t mold grow behind the foam board?  Then I have a worse problem.  I’m thinking to not install the foam board so that I can see when water is penetrating my crawl space.  Any advise on this??????

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  1. Expert Member
    Dana Dorsett | | #1

    With even 12" of above grade exposure on the exterior the CMU block will dry toward the exterior. If there is less exposure than that it may be important to have a good capillary break between the top of the foundation and sill plate, or improved surface drainage on the exterior.

    The soil on the other side of the CMU is full of mold. Why do you care if mold grows between the foam board & CMU? CMU is highly tolerant of both moisture and mold, and so is foam board.

    In termite zones leave a ~3" wide strip of space between the bottom of the foam board and the crawlspace floor as an inspection strip. You can insulate that with cut up rock wool batt if you like, to be removed & re-installed during inspections.

    It's normal for condensation to form on the bottom of a ground vapor barrier, even when the soil isn't super saturated wet, whenever the temperature in the crawl space is cold. Insulating the walls will keep it warmer, with fewer periods of visible condensation.

    1. mvpeters | | #3

      Thanks Dana for your answer. I'm concerned that if the CMU block gets wet and stays wet behind the foam board that it will never dry and eventually weaken the CMU block. Is that a possibility? Would it be better leaving the block uncovered so that it can dry out in the event of water infiltration? Not to mention, I'd be able to see water penetration without the foam board.

      1. Expert Member
        Dana Dorsett | | #4

        Concrete is extremely tolerant of water, which is why it's used even in fully submerged bridge foundations. It doesn't need to dry, and is not weakened by moisture even when saturated.

        When the drying path to the interior is blocked more ground moisture will wick up to the above grade section to dry toward the exterior. If the above grade exposure is severely limited it will affect the moisture content of any wood foundation sills, which ARE susceptible to moisture, in which case it would need some sort of capillary break between the CMU and wood. A foot of exposure is usually more than enough.

        In the extreme, if the ground is always very wet and there is lots of moisture coming through the wall creating efflorescence on the interior, that WILL degrade the CMU over (a very long) time. But blocking the path to the interior blocks the flow, and it's the flow that removes mineral from the structure, not the mere presence of water.

        If bulk water is coming through the wall it will puddle on top of the vapor barrier, usually near the leak point. You claim that the crawl space has never taken on any water, so the risk is really pretty low, and manageable if it does. If you're the type that would lie awake at night you could parge the interior side of the CME with mortar mixed with Zypex (or similar) to seal over any cracks and capillary flow prior to installing the wall foam.

  2. Jon_R | | #2

    Mold between the foam and block will matter much more than mold outside the block if you have *any* air movement from the potentially moldy gap to the crawlspace to the house. You can reduce such air movement with good air sealing or stop it with crawlspace pressurization.

    Unfaced EPS foam allows more inward drying which results in dryer walls (which reduces mold risk).

  3. BrianPontolilo | | #5

    It seems like you are doing the right things too improve your crawlspace, but there are still more details to consider, like airflow between the sealed up crawl space and the conditioned space of the house above. Here are some articles that may be helpful if you still have questions and to make sure you and your contractor have considered everything.

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