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Community and Q&A

Concrete Foundation Walls and Vapor Drive

lance_p | Posted in Energy Efficiency and Durability on
Building Science Corp. just released an article on foundations and they detail a number of options for interior insulation:
Can anyone elaborate on why the concrete foundation walls need to be able to dry to the interior?  I am under the impression that, even after fully cured and the excess construction phase moisture has evaporated, a concrete foundation wall will always have some elevated moisture content since it is buried in damp soil.
I am starting a build (Ottawa, Ontario) and was planning: 8″ poured foundation wall -> poly vapor barrier -> 2″ rigid foam -> R12 stud wall -> gypsum sheething.  The idea was to keep as much moisture (latent) load out of the basement as possible, and to tie the foundation wall poly barrier to the sub-slab soil gas barrier at the bottom and the wall assembly poly barrier at the mud sill (rim joist has thick exterior insulation).  If the concrete foundation walls are always damp I would assume a constant moisture drive inwards through the insulated basement walls if they are not vapor tight.
Can you please indicate why my idea is not a good approach?  Is there a concern for liquid water to form between the poly and concrete?

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  1. user-2310254 | | #1


    I'll give you post a bump. My recollection on the "new" consensus is that concrete foundation walls don't need to dry. In fact, they never dry if in contact with the ground. You have to be careful about bulk water, however.

    If this is a new build, you want to follow best practices for the slab (with a drainage layer, insulation, and heavy duty poly under the pour). GBA just posted an article that should be helpful. Go to

    1. lance_p | | #6

      Yes, the drainage layer is specified in my section on soil gas control. A 4" layer of crushed stone with no fines, if I remember correctly, then insulation and HD poly that will be sealed to the interior poly on my foundation walls.

  2. Jon_R | | #2

    > why my idea is not a good approach?

    As Joe L just reiterated (hopefully ending the "he thinks it doesn't matter" claim) and various others have also written, "mold". And even "liquid water". In some cases, moisture accumulation causes pink stains (from bacteria) and adhesive failure.

    > The idea was to keep as much moisture (latent) load out of the basement as possible

    8" of concrete plus 2" of foam allows so little vapor to move that the latent load is insignificant. Follow the expert's advice and use unfaced EPS or XPS or spray foam to allow a little inward drying (for walls and often floors). Similar for crawlspace walls.

    Re the BSI drawings: I'd lap the under slab plastic sheet inside of the basement wall foam, extending just above the sealant (see Fig 2). This is additional insurance that if water ever comes through the wall, it will be diverted under the slab (vs into the room).

    Note the drain pipe through the footing. This means that the under slab gravel and the "free draining backfill" are depressurized by a radon mitigation system.

    I would have shown rebar.

    1. lance_p | | #7

      I'm more concerned with gaps between the foam allowing moisture flow than the permeability of the foam itself. The consensus seems to be that all foam shrinks with time, so maintaining a continuous low-perm foam sheet that's air sealed between my sub-slab poly and the poly from my wall system above is the main concern.

      They say your air and vapor control layers should be continuous when looking at a section of your construction assembly, and that tracing them should result in never lifting your pen from the paper. My gut feeling is that foam on a wall, even with taped seams, should be treated more like a dotted line than a solid one.

  3. Expert Member
    BILL WICHERS | | #3

    Concrete itself doesn't need to dry. Concrete can remain wet forever, and actually prefers to be wet. The only downside is with hollow block walls, where there is a small possibility of the hollow spaces filling with water, freezing, then fracturing the block. This isn't really an issue of "drying" though, it's really a bulk water intrusion problem that needs to be corrected in other ways.

    The one issue that DOES come up is that if the interface between the concrete and insulation is damp, it can potentially grow mold. That's not usually much of an issue in practice though, and if you completely seal the walls anyway (such as would typically be done in a crawlspace encapsulation project), then any mold is on the "outside" of the barrier and can't really affect the living spaces.

    My solution if I suspect possible mold issues is to paint the concrete wall with a mold killing paint or primer prior to insulating as a bit of extra insurance. I wouldn't worry about allowing any drying with the insulation. Many, many people have insulated their basement walls with 2" or thicker XPS, which is pretty much a vapor barrier, and there are few reports of any problems doing that.


    1. lance_p | | #8

      I like the mold-inhibiting paint idea a lot! Even though I'm putting poly against the concrete I would rather play it safe, and for the relatively low expense and effort that paint is likely a good idea. Just in case.

  4. PLIERS | | #4

    There are also many people who use closed spray foam to insulate their basement walls and they are fine, there is very little potential for drying with spray foam. I believe Joe L. pointed that out somewhere when he retracted his statement about letting basement walls dry to interior. The only time I think you need some drying to the interior is when materials in contact with the concrete can’t stay wet and moisture needs to escape such as a carpet. That’s why in some instances carpet could be laid on concrete and then allowed to dry to interior with a dehumidifier. Although in that case you need maintenance and often you end up just tossing the rug eventually. Best case scenario is to keep the moisture out to begin with in my opinion. Again the drying to the interior debate is often argued both ways. I guess I’m on the it depends fence

  5. lance_p | | #5

    Thanks for all the replies.

    I absolutely remember Joe L. indicating that drying to the interior was no longer required in a basement wall which is why I was confused when BSC released that article containing insulation options that indicated vapor permeability was required.

    My local code calls for a minimum of R20 interior foundation wall insulation, including a minimum 2" of foam against the foundation wall (presumably to keep the vapor plane above dewpoint). I will also be insulating the portion of foundation wall exposed above ground with 2" of foam, so in practice the dewpoint of my foundation wall assembly shouldn't be an issue as the concrete shouldn't be much below the ground temperature.

    Agree that if finishing a basement floor soon after construction it will need to dry to the interior. We won't be finishing the basement for at least a year after moving in, so the slab will be plenty dry before we cover it. With 2"-3" of foam below the slab and a full vapor barrier for soil gas and moisture control I don't foresee any issues there.

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