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 1x3 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 1x3 and the wall?

  • what is on the other side?
    – DMoore
    Jan 29, 2015 at 19:36
  • The outside air... The basement wall goes up about 2-3 feet above the ground. Jan 29, 2015 at 19:41
  • Are you planning on finishing the basement?
    – DMoore
    Jan 29, 2015 at 19:52
  • It's possible... But not likely. Jan 29, 2015 at 20:06

4 Answers 4


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.


You'll want to look at how deep that sloped concrete block is to the outside, and what's on the other side. Also, investigate how the home is insulated above this.

I try to keep the insulation barrier continuous between floors to eliminate paths between the insulation to the inside of the home. A rim joist, like exterior sheathing, is a poor insulator by itself, so you typically insulate it to maintain the same insulation being placed in the walls above and below. One advantage to the foam as opposed to the fiberglass is that it's less likely to be dislodged when running wiring through the walls.

But you don't have that. Personally, I'd extend any insulation you plan to place on the walls of the room below up into the joists. Unless that slope is dramatic, there's little advantage to insulating that joist and doing nothing to the wall below. Conversely if you only insulated the wall below, those joist bays would be a significant gap if uninsulated.


I have a similar issue in my own basement. After doing a ton of research it sounds like we are just out of luck. We simply cannot insulate that space because it will trap moisture between the insulating material (whether spray or rigid) and the floor joist. Reason? because the joist itself will conduct heat down to where it meets the concrete, causing condensation.

I had started to seal up mine before stopping to research. Just removed the 3 bays I had done and already (in 4 weeks) had slight dampness in the wood.

Best thing is to seal the air gaps with caulk (which allows vapor through). You could also insulate the face of the concrete leaving an inch gap to the floor joists. I believe that cuts your effective insulation by 50% though. Also, sadly, the caulk will need to be periodically re-done.

Here's a link to latest research I could find:


  • Wouldn't that also affect the situation in the photo from Family Handyman? Jan 29, 2015 at 19:43
  • No because the joists are not in contact with the concrete. Jan 29, 2015 at 21:31
  • SOL? Would you expand that abbreviation, please? Or use something else if the expanded language doesn't fit this site's audience?
    – alt
    Jan 30, 2015 at 19:07

Although I wouldn't normally recommend XPS for home basement applications, I don't think you have much of a choice.

Basically you have two options:

  1. Frame basement, put crosses running a foot below ground level, and fill up everything north of the cross with insulation (roxul would be fine). That is the norm in basement construction in my area.
  2. If not framing or finishing basement... I suggest foil-covered poly sheets. The link is not an endorsement but one I used a few years ago. You would simply cut out the joist slivers in the sheets and install them on the wall, fill in any gaps with spray foam (Great Stuff is fine), and then aluminum tape everything. Your sheets do not need to go to ground level and in fact I would suggest cutting them a foot short or more. This is probably a bit more expensive than option 1 but could easily be checked off your to-do list in one day.

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