Basement – Insulating against brick in rim joist bay

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I was hoping to air seal and insulate the rim joist in my basement. This is a balloon-framed house in zone 5, so the bays are open to the floor above. At some point brick and mortar were put in the bays as firestop.

This is what a typical bay looks like:
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And here it is with the insulation peeled back (about 14" x 7", and you can see a stud about 1/3 from the left):
enter image description here

Everything I read online about regular joist bays says to put rigid foil-faced foam against the rim board and then spray-foam it in place (and maybe with Roxul or drywall facing the interior for firestop). I am uncomfortable with this approach here because I fear trapping moisture between the rigid foam and the brick for several reasons:

  • The foam will not be tight to the mortar/brick due to surface irregularities
  • Mortar/brick can crack and let moist air in
  • Mortar/brick can wick moisture

I can't find a definitive answer. What can I do safely, if anything? I don't really want to mess with 2-part spray foam because I have about 6 bays to do so I don't feel it will be cost-effective or worth the setup/cleanup time, but if that's the only way, fine. And I really don't want to hammer the bricks out. What about using regular canned foam (which is super flammable) and then covering it with Roxul or caulked drywall? Should I just try to air seal the holes and leave them uninsulated?

Best Answer

I think you can do air/moisture/heat in one shot by using spray foam behind foam panels.

  1. Buy two cans of max-expansion spray foam. One may do, but it's best to have enough on hand.
  2. Cut 1-1/2" or 2" foam panel to fit each bay reasonably well.

The plan is basically this: Spray a layer of foam on the mortar / set the foam panel in place / let the foam cure / trim the excess

Here's how I'd probably do the hot work based on what I can see:

  1. Mark the thickness of your foam plus 1" out from the mortar near each end of the joist series. Mark that on the bottom of the joist (or mark an offset if the lower wall is in the way, or simply use some other reference, such as the inner edge of the sill plate).
  2. Snap a chalk line between the marks, then square the actual (foam plus 1") marks up each side of each joist. This will be the final position of the face of the foam.
  3. About 2" down the joist on each side of each bay, partially set a nail on each side of the bay. These will serve to retain the foam panels.
  4. Spray a layer of foam, as quickly as possible, over the mortar. Use a stripe or zigzag pattern.
  5. Slide the foam panel in place behind the nails, and set nails near the bottom in the same manner.
  6. Let the foam cure for an hour, then pull the nails and trim any foam squeeze-out with a sharp blade. A fillet knife would do well.

The outcome will be a clean, uniform, on-plane foam barrier over which you could adhere drywall (though I don't consider that a serious concern).

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