First time home owner, please go easy on me haha. I purchased my home March 2023. It's a 1947 bi-level located in Minneapolis, MN.

I was looking into options to air seal or insulate my rim joists. However, upon further inspection the joists seem to be partially embedded by concrete and sit directly on the foundation wall without a sill plate.

Images: https://imgur.com/a/1Dt1byf

I am finding mixed results in my research as to the best way to go about this. Some posts/guides say to not to seal with spray foam or caulk to avoid moisture penetrating the joist ends. Others say to stuff insulation above the concrete slope and use a vapor barrier.

Additionally, some areas of the joists are covered by a board with spray foam filling the gaps. I haven't tried to remove any of these yet to see what's behind them. I wanted to at least tackle the uninsulated bays first.


I don't plan to drywall or finish any of the walls at this time. I am primarily looking to reduce the cold air from hitting the floors above.

Any advice would be greatly appreciated!


1 Answer 1


My 1952 home in St. Cloud had the same foundation type. I simply fit fiberglass batts into the cavity, covering the rim joist. Since your walls aren't insulated that's about all that's called for. There shouldn't be air leaks there, so an air or vapor barrier is unwarranted.

If you're finishing the basement and adding energy walls, the insulation would roll down over the masonry and lap the top of the energy wall to create a continuous envelope.

  • Ah got it. To clarify, did you add the batt to the small area at the end of the bay? Or did you fill out the entire bay? I did purchase some mineral wool and considered just layering it in there. Commented Jan 22 at 20:09
  • I put two layers of R-13 or whatever I had on hand at the end of the bay, against the rim joist.
    – isherwood
    Commented Jan 22 at 20:26

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