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Community and Q&A

Brick house rainscreen space ventilation

Tim Buzz | Posted in Green Building Techniques on

1950s slab on-grade single story house in Southeast Pennsylvania

From outside in: brick, cinder block (I believe mortared to the brick totaling maybe 8 inches), black asphalt-type barrier paper, 2×2 studs, then drywall. There are weep holes on the outside of the brick.

My question is about the ¼ inch space between the cinderblock and the drywall that vents into the attic. Can that space be sealed at the top with foam without problems (mold, brick spalling, etc)?

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  1. GBA Editor
    Martin Holladay | | #1

    You refer to "the ¼ inch space between the cinderblock and the drywall." But that doesn't make any sense. Maybe you meant to write, "the ¼ inch space between the cinderblock and the studs" or perhaps "the ¼ inch space between the cinderblock and the asphalt felt."

    If the 1/4 inch gap is between the interior face of the concrete blocks and the asphalt felt, then there are pluses and minuses to your plan to seal the 1/4 inch crack at the top of the cavity.

    Pluses: Sealing the crack will probably reduce the rate of air leakage through your thermal envelope (because the stud cavity probably communicates with the interior of the home).

    Minuses: Sealing the crack will reduce the rate of drying in the gap between the concrete blocks and the WRB, making it more likely that the walls will accumulate moisture.

    I'd vote for leaving the crack at the top of the ventilation cavity, and finding another way to address air leakage -- for example, by filling the 2x2 stud bays with spray foam insulation.

  2. Expert Member
    Dana Dorsett | | #2

    Weep holes are usually an indication of a cavity-wall, which in 1950 was typically a 3/4" or 1" cavity between the brick and CMU blocks. If that's what you have it would be good news, since the air gap is an excellent capillary break, preventing rain/dew moisture from wicking into the interior. While mortaring the brick to the CMU was still done in the 1950s, many, maybe even most were built as cavity walls by then. More recently the cavities were made 2" or deeper, with 1-2" of rigid foam insulation plus at least 1/2" of air gap, but that isn't likely in your house.

    If you're not sure, see if you can't find place where you can inspect it top of the wall all the way out to the brick, which may involve moving some attic insulation or maybe even a facia board. You can always drill a hole in a mortar seam from the exterior to get a look at it (which is fine to leave open, venting the cavity to the exterior under the soffits), or worst-case, find a closet or other obscure location to do some drilling & banging to get a peek at it from the interior.

    Take the time to figure this out- knowledge is power, and the answers affects how you treat the gap between the CMU and the finish wall.

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