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Contractor insists air gap between batts and basement brick mean no prob for humidity, with or without polyethylene vapor barrier.

Melville2 | Posted in General Questions on

If humidity is accumulating in an air-sealed air gap (humidity coming thru brick), an air gap won’t save you.  Surely an air gap is only good if you have air leaks.  Imperfections in sealing will happen inevitably unless installers take extreme care, including at electrical and plumbing penetrations, but I could additionally add a pinhole or 1 mm hole to the interior every 6 feet or so.  There are no known water leak issues and there is large overhangs with proper detailing so little rain gets to the brick, although some snow can accumulate at the base.  Should I apply a water seal over the exterior of the brick+mortar around the bottom say 18″?  Would an air gap then make batt insulation + poly okay?  

(double-brick / air-gap / batt / poly / drywall, + insulation in rim joist gaps + insulation under the-floor-above at top of air gap, so cold air doesn’t flow over the basement ceiling drywall)

Better with poly?  Without poly, condensation would happen on the brick, and may run down and pool at the base, but with sufficient air gap and leaks, condensation would evaporate before pooling became considerable. No weeping tiles or anything similar, so should use exterior-grade wood for bottom plate of wall framing, or add waterproofing to interior-grade bottom plates. With poly, humidity doesn’t pass to cold side of insulation, so there’s no condensation from outward flow.  From inward perspective, moisture that comes in thru brick would not travel thru poly but would escape thru leaks as humidity levels seek to be equal.



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  1. Expert Member


    For decades most Canadian basements were insulated with batts and interior poly. Some with gaps between the stud wall and the concrete, some not. Many of them experienced moisture and mold problems. The answer unfortunately wasn't to just eliminate the poly, or move the various layers around, it was to use an impermeable insulation against the foundation walls. That's the only sure way of insulating them. You may get away with what your contractor is suggesting, but it's a risky assembly.

    1. Melville2 | | #6

      It's risky, but so is the alternative. Some people say if I spray foam a brick basement wall the brick that can't release moisture inward due to the spray foam will disintegrate from freeze-and-thaw cycles. "tens of thousands of dollars damage" "within 3 years", I was told online. On the other hand, my 110 year old brick certainly seems like tough brick - feels much 'stronger' than the brick used on a 1980 house with a spalling chimney - but perhaps the characteristics that make it seem tough actually make it vulnerable: bigger, no holes - might mean the interior didn't bake as much as bricks that are smaller and/or have the standard 3 holes. A mason said my bricks have a patina on them which he thinks was there from the start, so he thinks they're high quality - but he has a 1.3 star rating on

  2. Expert Member
    Akos | | #2

    I have pretty much exactly this setup at a place in Toronto. As Malcolm said, it mostly works. What happens is in the spring when the foundation is still cold and the weather and humidity starts raising, I get condensation and a bit of mold on the foundation (you can smell it right away). Running a dehumidifier keeps it mostly in check and the smell goes away by about June.

    Unfortunately when it comes to old brick/stone foundations, exterior insulation or an inch or so of closed cell spray foam is the only thing that is guaranteed to work.

    1. maine_tyler | | #3

      What's the reason interior rigid foam doesn't work (but does for concrete)?

      1. Expert Member
        Akos | | #4

        Brick/Stone surface is too rough, there would be too much air volume behind the rigid. There is no way to get it air tight.

        Unlike a new concrete foundation, older foundations like this generally don't have any damp proofing so the surface contains much more moisture than anything new built.

        Combine the two issues and there is a good chance of problems down the road.

        1. Expert Member
          DCcontrarian | | #5

          Also brick wicks moisture from the soil more than concrete.

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