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Rigid foam crawlspace insulation — vapor permeance?

HD S | Posted in General Questions on

We currently have an 18×30 crawlspace that is vented with batt insulation in the floor joists. The crawlspace also connects to a full basement that is 22×30. There is a 2’x2′ opening between the crawlspace and the basement that is blocked with plywood, plus a few openings for ductwork and such. There is a radon mitigation system installed on the crawlspace floor, plastic covering that goes up about 10″ on the crawlspace walls. HVAC supply and returns, plus plumbing all goes through the crawlspace to that section of the house. There is batt insulation around it, but falling down in places.

We live in SW Ohio, Climate Zone 5 and it is usually pretty humid during the summer months. During the summer, the basement will get up to 65% RH and we get a bit of a smell that tells us it is time to turn on the dehumidifier. In addition, we are dealing with mice in the crawlspace and probably other areas as well. Also have lots of spiders.

For the winter I sealed up the two vents (can spray foam/rigid foam) with a plastic vent cover. I put plastic between the spray foam and rigid foam to pop it out if needed later. With the mice, needing to do some plumbing in the bathroom, my thoughts were to pull the batt insulation and use rigid foam on the crawlspace walls.

My initial thought was to use either Thermax or John Manville’s CI Max since both have the thermal barrier built in. But reading Info-512 on buildingscience said the perm ratings are too low and I may have issues moisture buildup between the concrete foundation wall and the rigid foam. I’m not sure if the thermal barrier is code in my area, but wanted to use it to be on the safe side.

The John Manville installation instructions had the option of using 1x furring strips attached to the wall and the foam board attached to the furring strips. That would leave a small air gap, ~3/4″, would that alleviate the potential problem of moisture being trapped between the wall and foam board? Although the cold foundation wall could induce condensation? Would you seal the top of the foam board instead?

Or if a thermal barrier isn’t needed, go with the regular XPS? Seems like polyiso’s foil facing has the same perm rating as the thermal barrier versions and would still lead to the same problems.


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  1. GBA Editor
    Martin Holladay | | #1

    There is no reason that a damp concrete wall needs to "breathe" or diffuse moisture toward the interior of your home. Concrete can stay damp forever, without encountering any problems. Below-grade concrete walls are always damp; that is their nature; so you want to prevent drying toward the interior, not encourage it. After all, why invite moisture into your house?

    The Building Science Corporation used to advise builders that concrete basement walls had to dry to the interior. I opposed this advice for years, and Joe Lstiburek finally conceded, admitting that the advice was wrong. Read more here: Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.

    For good advice on how to insulate your crawl space walls, see this article: Building an Unvented Crawl Space.

  2. HD S | | #2

    Thanks Martin, it's great to get the updated information. I read through the "Building an unvented Crawlspace" and have one question in regards to the vapor barrier up the wall step.

    "Install a durable vapor barrier — for example, a 20-mil pool liner or Tu-Tuf poly — over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall. Leave a 3-inch-wide termite inspection strip at the top of the wall."

    The current passive radon mitigation plastic in our crawlspace floor only goes up about a foot. Should I add more vapor barrier down the wall and overlap it or just go ahead and add the rigid foam and skip that step?

    Thanks again,

  3. GBA Editor
    Martin Holladay | | #3

    Q. "The current passive radon mitigation plastic in our crawlspace floor only goes up about a foot. Should I add more vapor barrier down the wall and overlap it or just go ahead and add the rigid foam and skip that step?"

    A. Just add rigid foam to the walls. The rigid foam is a vapor retarder. Installing polyethylene under the rigid foam is an optional step.

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