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Community and Q&A

Insulating rim joist sitting on brick veneer

Aburkhardt | Posted in Energy Efficiency and Durability on

Hi All,

Looking for advice as to how to handle insulating the rim joist on a CMU-Brick Veneer assembly.

House is a 70’s split level. Lower level is 6″ CMU with Brick Veneer. The floor joists rest on 2×6 top plates, and extend past the cavity to the rim joist which sits flush with the exterior face of the brick. Fiberglass batts are haphazardly stuffed in the space now. The plan was to do the often-prescribed rigid foam sealed with spray-foam, but i’m now unsure as to how to handle the open cavity separating the CMU wall from the brick veneer.

I can’t imagine that the assembly is relying on the cavity being open to the interior floor system of the house to keep dry, but i might be missing an important principle.

Should i add an horizontal layer of rigid foam, then proceed as usual with fitting foam against the rim joist and sealing appropriately?

Is this type of assembly common?

thanks in advance,

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  1. Expert Member
    Dana Dorsett | | #1

    Yes, go ahead and cut'n'cobble foam board over the foundation sill all the way out to the band joist, and seal it well with can-foam.

    If need be, a few tiny vent-holes drilled/cut into the top course of the brick veneer to vent the to the exterior helps, but it also needs weep holes in the bottom course to convection-purge the cavity. I've seen it done a number of ways- 1" round holes cut with a masonry hole-saw capped with a screen vent, or slots cut in the vertical mortar lines.

    There are even whole-brick sized brick veneer vents, if you REALLY want to move some air through that cavity.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana's advice about air-sealing the top of the cavity between the CMUs and the brick veneer with rigid foam and canned spray foam.

    Ideally, the below-grade sections of your split-level home aren't having any water entry problems, and the below-grade walls are insulated properly. In general, you don't have to worry as much about ventilation and drainage behind brick veneer when the brick veneer is backed up by CMUs. The ventilation and drainage details are far more important for above-grade wood-framed walls with brick veneer than for below-grade CMU walls.

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