GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Is roxul or un-sealed XPS alone sufficient for rim joist insulation?

BrianM72 | Posted in General Questions on

I’m insulating the rim joists in my basement, and am about 2/3 done. I’ve used 2″ XPS, sealed around the edges with Great Stuff; a batt of Roxul over that for added R and fire protection; and a sheet of breathable fabric stapled over that to help keep the roxul in place.

The remaining 1/3 of my rim joist is very hard to get to. A couple of air ducts run along the top course of cinderblock, and even where that’s not the case the space between the first floor joist and the cinder block is only about 2″ (the space I have to reach through to do stuff).

Would just roxul, and/or XPS friction fitted and inserted against the rim joist — without the foam sealant — be adequate to increase the R without causing moisture issues? I can’t see how I will reach in there with sufficient accuracy to reliably seal all the seams with a foam gun.

Other alternatives or suggestions?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    I wouldn’t trust the rigid foam without the air seal of the canned foam. The air seal is important to avoid moisture problems unless you have sufficient EXTERIOR rigid foam already installed.

    For the confined space you’re talking about, one of the smaller two-part spray foam kits is probably your best option. This way you can just spray in enough foam so that the cavity is filled after the foam expands and you’re done, insulated and air sealed in one step.


  2. Expert Member
    RICHARD EVANS | | #2


    There might be some moisture issues as warm humid basement air sneaks around the XPS edges and condenses on the rim board. This risk may be elevated some depending upon how cold your climate is as well as how humid/warm your basement is. I suspect many, many homes around the country simply have fiberglass batts stuffed between the rim joists and yet are working just fine.

    The 2" of XPS will have some, but not much, vapor permeability. So if moisture does accumulate behind it, there may be some drying potential inwards. A safer option might be to use facelessType I EPS as it has higher vapor permeance for increased inward drying. You can use Rockwool alone too but I would add plywood or drywall to create an air barrier over it.

    If the tight spaces don't allow for that, consider purchasing a Froth-Pak and blasting in some 2# foam(although these things have disposal issues, among other things.)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |