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Air sealing rim joists

mfleck | Posted in General Questions on


getting even closer to finishing my attic insulation. I pulled up some of the sub floor boards around the perimeter. On a cold and windy day I can stick my hand in there and feel the air flowing!

whats the best way to seal this?  Keep in mind that it’s a little tight in there. I tried just spray foaming the gaps in a few bays and didn’t feel air flow afterwards. Could I just do this for the rest of them?

before I put those floorboards back, would it be worth stuffing some old rock wool insulation batts against the brick?  I have a bunch of old batts that I pulled out of the rafters that I’d love to repurpose. 


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  1. Expert Member
  2. mfleck | | #2


    I read this article but I’m not sure I can apply it to my situation. I don’t want to spray foam in there (not even with the two part kit—it’s just beyond my comfort level). After this the next best option seems to be foaming in rectangles of rigid foam, but this isn’t really a natural fit for the space I’m working with since the material above is a subfloor and below is plaster, I can’t really get a good seal all around the board and sealing off the section of exposed brick.

    I feel like the approach that I am describing above and have tried in a few cavities is my best bet but I’m waiting for an expert to weigh in. As I mentioned, I spray foamed great stuff between the top of the brick and the 2x10 sill plate. I also sprayed along the wood joists where they enter the brick and along the opening on the bottom where air can pass up between the brick and lathe from the floors below. Dipping my hand in the cavity that I tried versus one that I didn’t shows a big difference. I feel no air flow in the cavities I treated and noticeable craftiness in those I didn’t. My hand is calibrated by the way to be a highly refined measurement tool :)

    Is there a better approach? Any downside to what I’m suggesting?


    1. Expert Member
      MALCOLM TAYLOR | | #3


      You need an interior air-barrier to stop moisture from making it's way through the batts and condensing on the brick behind. Right now the bricks probably stay close to temperature of the inside air, or certainly warm enough to stay above the dew point. Once the batts are installed, the brick is going to be a lot colder.

      Depending on your climate you may want an interior vapour-retarder.

  3. mfleck | | #4

    Do you mean that I need a vapor barrier if I am going to put fluffy insulation against the brick?

    I should add that the house has stood 90 years now with nothing against the brick in that space. My guess is moisture likely condensed on the brick, but that the open air flow took care of it before any damage could be caused.

    I’m still not sure how to air seal this area other than just spray foaming (canned) in the gaps as much as I can.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      "My guess is moisture likely condensed on the brick, but that the open air flow took care of it before any damage could be caused."

      Yes, or the lack of insulation mean the brick stayed warm enough so that nothing condensed in the first place. If you add batts you need an air/vapour barrier to stop the warm, moist air travelling through the batts and condensing on the now cold brick. That's why in similar situations, like concrete basement walls or rim joists, they usually use rigid foam to provide both insulation and the combined AVB.

  4. maine_tyler | | #5

    Matt, this is in an attic? So you have above grade brick walls? Is the attic conditioned?

    I think stopping some air, even if you're missing some spots, is an improvement. It looks like you're essentially approaching it like a spray-foam job, but using great stuff and pinpointing the leaks rather than blanketing the area with foam.
    A big difference, however, is you're not addressing dew-point control since you won't have a built-up, vapor ratarding layer of foam to protect against possible condensation if you install batts (this is what Malcolm is saying). In this scenario, you may be best off leaving the batts out, or installing an air/vapor retarder on the interior of the batts.
    I'm not sure how you would do that. It seems like you'd have to seal something (plywood, membrane product) to the plate, the joists, and then down towards the ceiling (is the plaster air-tight?). Difficult.

    I think if you want some insulting value, cut n cobble may make more sense than batts.
    "but this isn’t really a natural fit for the space I’m working with since the material above is a subfloor and below is plaster" I'm not sure precisely what you mean, but you would want to be sure to seal all 4 sides of the foam. If you don't think you can accomplish this with cut-n-cobble, I think you'll find it only harder for something further out and interior to batts.

    A long way of saying: if you don't want to spray foam, and you don't want to cut-n-cobble, you may want to forgo insulation and just call it an improvement to have air-sealed the area some (which that really is the bigger improvement).

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