As an aside, I can't for the life of me figure out what to call the last floor joist that runs parallel to the basement foundation. Does it have a name?

In my house, there is a little cramped gap an inch or two wide for me to access this gap. I'm wondering what the best way to go about insulating this space is.

After reviewing DIY kits for spray foam, I've decided based on friends' experience that hiring a contractor to spray the rim joists will likely save time and money and result in more insulation. I'm thinking that they will be able to fill this space, but I just wanted to confirm since most guides and conversation I've come across is mostly about the perpendicular joist spaces.

  • 1
    I'd imagine foam. Cramming R in there is pointless. I imagine the only concern would be over foaming and creating pressure on the joist from the foam expansion. Commented Jan 22, 2013 at 3:07

1 Answer 1


Since you have a gap, spray foam is probably your best option. During framing, there are corners and other locations that get insulated when the walls are being raised because they are inaccessible later on. At that stage, some rigid foam insulation is useful since it won't mold like fiberglass will and the structure isn't weather tight yet. If you didn't have a gap, drilling a small hole and using blown insulation would have been the best option.

For reference, a "rim joist" is the last joist on the row and wraps around the outside of the floor structure.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.