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Sealed crawlspace layers?

Tim L | Posted in Green Building Techniques on

Q: For a sealed crawlspace with interior insulated foundation walls. What is the best sequence from exterior to interior:

A) Foundation Wall, Poly, Rigid Foam


B) Foundation Wall, Rigid Foam, Poly

If the answer is “either one” then which of the two would be most preferred?

Tim L

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  1. John Clark | | #1

    IMO it wouldn't matter as long as the foam was exposed for inspection since I reside in a termite zone.

    In an ideal world I would build a crawlspace with a rat slab and have the poly underneath it.

    Interestingly I never see drawings which have the poly go all the way up to the mud sill, but whenever I look a photos of crawlspaces that have been sealed up the poly always goes up to the mudsill.

    In reality I think it depends on whether you're talking new construction or retrofit.

  2. Expert Member
    Dana Dorsett | | #2

    With continuous & sealed rigid foam on the foundation wall the polyethylene isn't necessary.

    If it will be installed anyway, if the foam is polyisocyanurate (rather than polystyrene), put the polyethylene between the foam & foundation, since polyiso is somewhat hygroscopic, and would be trapped next to the potentially damp foundation. If polystyrene (EPS or XPS), it doesn't much matter.

    If the polyethylene wraps over the top of the foundation wall, under the foundation sill it provides some function as a capillary break, protecting the foundation sill from taking on moisture from the concrete.

  3. Tim L | | #3

    Thanks Dana. That answers it exactly.

    It will be option A for me, since I do intend to use Thermax. Also yes, the poly will wrap around over the top of the foundation wall to make the connection between the ground vapor barrier and wall air barrier.

    Also found that Martin details option A as well in the following article:


  4. Jon R | | #4

    Don't be surprised if more mold grows behind the poly than would with EPS or XPS alone. But any odor problems this causes can be addressed with crawlspace pressure (eg, a crawlspace exhaust fan).

    A cooler crawlspace receiving air from warmer areas can create humidity problems - I'd always monitor crawlspace humidity.

  5. Tim L | | #5

    Jon, would this be a problem when using vapor permeable air barrier membrane. For example Solitex Sento 1000/Intesana/Intello

    Assuming with the smart vapor control of these membranes would allow interior foundation wall to dry inward?


  6. Jon R | | #6

    From a moisture vapor standpoint, that would be similar to the often recommended use of EPS/XPS directly against basement walls.

    IMO, the evidence is pretty clear that somewhat breathable to the interior foundation walls are typically dryer. And dryer is less likely to grow mold.

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