I am in the beginning phases of finishing my basement, newer house with poured cement foundation and engineered I-beam floor joists. No moisture issues in the basement.

Rim joist currently has a batt for insulation and that's it. My plan is to replace that with 2" XPS and foam spray and then another batt (probably just reuse what's already there).

On the foundation walls, I am going to use a thinner XPS all the way up to the joists, frame out with 2x4 and insulate with batts, then drywall.

I think I have a good understanding of these 2 parts, but am not sure what to do where they intersect. It seems to me that if I don't somehow seal the top to the foam wall insulation it will allow air to flow in behind the rim joist insulation. I've not seen this particular issue addressed anywhere, so maybe it's not really a problem? I'm trying to err on the side of extreme caution since I am doing this myself, and dont want to headaches down the road.

Found this:

enter image description here

at BuildingScience.com

Which seems to be recommending a strip of XPS between the rim joist wall insulation.


  • When you say 2" XPS and foam spray, are you just referring to spraying around the edges to ensure a seal or are you planning on spraying over the entire thing? I guess I don't see the rationale to use both unless you are putting in the XPS to cut down on the amount that needs to be sprayed in.
    – Comintern
    Apr 14, 2014 at 22:43
  • you are correct, sorry for any confusion. heres an image i.imgur.com/toGSFy4.jpg
    – Joe Riggs
    Apr 15, 2014 at 1:22
  • That's one option. As long as you cover all cold surfaces possible with insulation, by what ever means, you'll be good.
    – bcworkz
    Apr 15, 2014 at 18:08

1 Answer 1


You can always spray foam over top edge of the wall layer of XPS, this ensures that there's no air leakage from behind the XPS on the wall going up. I'd also make sure you caulk/tape all seams between XPS boards, and spray the bottoms/corners so it gets a real good seal from air movement.

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