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Community and Q&A

Air sealing or Insulating firebrick

user-901114 | Posted in General Questions on

In our new-to-us 1940s dormered cape just outside of Boston, we have some firebrick just above the sills. Looks like it’s stuffed in between the balloon framing to stop fires from going into the upper levels.

When we bought it three months ago, it had dirty glass batts between the joists covering the bricks. I’ve since ripped them out and I’m looking for options. Hope there are some that will let me inspect for termites and moisture later.

The rest of the house has had some air sealing details and there’s cert on the wall that says it’s got cellulose blown in with R13 walls and R38 attic. It’s got a block foundation, vinyl siding and vinyl replacement windows. Somebody did a pretty good job keeping up with the place.

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Replies

  1. user-901114 | | #1

    Pic

  2. GBA Editor
    Martin Holladay | | #2

    Stephen,
    If you weren't worried about termites, the best air-sealing methods would be either spray foam or rectangles of rigid foam with sealed edges.

    I'm not sure what level of inspection you need to spot termites, but to provide a better air seal, you'll have to put something on top of the bricks and mortar -- for example, plaster, drywall, or cement board. Don't forget to seal the edges with caulk.

  3. user-901114 | | #3

    Thanks Martin. Which of these two options would be the lesser of two evils from a risk and energy efficiency perspective to keep easy access for inspection?

    1. Stuff roxul batts in the joists

    2. Airsealing only the cracks with low expansion foam or caulk.

  4. GBA Editor
    Martin Holladay | | #4

    Stephen,
    I don't recommend using air-permeable insulation like Roxul at a rim joist, because that will allow moist interior air to contact the cold rim-joist area, leading to condensation which can drip on your mudsill.

    If those are the only options you are considering, then just seal the cracks.

  5. user-901114 | | #5

    ok, I'll start with air sealing this year. Thanks

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