Basement – Rim Joist Insulation without a Sill Plate


I've seen a few similar questions but the answers don't add up…

I would like to use some rigid foam insulation to insulate my rim joists. Here is the problem: everything I have seen shows putting the foam on top of the sill plate like this (via Family Handyman).

Insulated Rim Joist

In my situation, my joists run into sloped concrete block:

My Joists

If I stuck some rigid foam in those gaps it would be floating above the wall and sticking out 2". So what is the proper way for this type of joist?

My idea is to attach a 1×3 under the joists and set the foam on top of that. But I know a big part of this insulation method is sealing the rigid foam with caulk or spray foam. So would this work? Or will I just need to add some extra caulk to seal the small gap between the 1×3 and the wall?

Best Answer

That's fine. The thing you want to make sure is that you seal off any air paths from the inside of the basement to the rim joist, the reason being that if you insulate it but let humid air hit it, that air can condense on it and cause rot. That's why they tell you to use rigid foam--which is an air barrier--instead of something like air-permeable fiberglass or mineral wool batts. So as long as you make the rim joists airtight, you're fine. At that point, it's just an issue of aesthetics. Your idea of using a 1x3 there will work fine if, again, you make absolutely sure that interior air can't touch the rim joists, using caulk or expanding spray foam.