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: enter image description here

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?

2 Answers 2


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).

  • "...over which you could adhere drywall (though I don't consider that a serious concern).". Not disagreeing, just curious. Why no concern? Is it because it's in a basement with no source of flame/ignition?
    – SteveSh
    Jan 29, 2020 at 15:24
  • Because all the exposed wood is nearly as flammable. If a person was to address that issue in general, a full ceiling (drywall or fire-retardant poly) is the best course.
    – isherwood
    Jan 29, 2020 at 15:30
  • Ooh, that's a very interesting approach. I was planning on using 2-sided foil-faced polyiso due to its vapor barrier properties; I may be wrong but it seems like that would be a bit more fire-resistant than exposed canned foam (the appeal of the messy 2-part foam is it's fire-rated but canned foam isn't). Though I would likely still use Roxul or drywall in front of the panels. So ~1 inch thickness of canned foam in a zig-zag/stripe, keep the pattern tight so the foam would expand enough to fill the gaps between the stripes, right? Any concerns with brick holding moisture where it touches wood?
    – autonomy
    Jan 30, 2020 at 14:44
  • Well, not much of the canned foam would be exposed with this plan--just what squeezes around the edges of your panels. Was I not clear on how it goes together? I'll revise if something's confusing. Polyiso sounds good. It offers a better R-value, too, but it's more expensive. Standard extruded foam can have a poly sheet on it as well.
    – isherwood
    Jan 30, 2020 at 14:50
  • You can get one part spray cans of fire block (orange) foam in many hardware stores, it's just more pricey than the regular foam. Feb 1, 2020 at 4:41

Since you're in a cold zone (zone 5), I think your biggest problem is going to be (relatively) warm, moist air in the basement condensing on the cold rim joist.

If that's the case, then you want to put the insulation against the brick/stone/mortar, then a vapor barrier facing the basement. This is to prevent any warm, moist air from mirgating through the insulation to the cold rim joist.

Another alternative might be to use polyisocyanurate foam board with the vapor barrier facing the basement. The edges would have to be taped or sealed with foam. I am not sure if this requires an additional fire barrier or not.

Finally, just to confuse you even more, here's an interesting read:


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