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

Alternatives to spray foam and rigid foam board – rim joist insulation

kieran973 | Posted in General Questions on

I live in southern CT and would like to add rim joist insulation to an unfinished field stone foundation basement. I don’t want to create a moisture/wood rot issue by putting down fiberglass batts directly against the joists, but I also would like to avoid using any kind of spray foam or spray foam derived rigid foam board – no polyiso, no EPS or XPS, not even any canned spray foam from the box stores. What are my options? Do I have any? Are there non-toxic rigid board insulators that can be sealed with caulk for example? Thanks.

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. the74impala | | #1

    Detail tape with Tescon Vana Tape, then fill cavity with rockwool

    Spray or brush on Viscon/Viscon Fiber to air seal, then rockwool that.

    Caulk all joints with Contiga Hf, then fill with mineral wool.

    Go to for those products.

    Personally, I just finished about 10 bays with 2" EPS and can foam. Will put in mineral wool before closing things up for about R23, same as the wall above. Not sure about how "toxic" the EPS is in this application, but pretty happy with the results so far. Seems pretty air tight and much warmer than before. Not too hard compared to the options above.

  2. Expert Member
    BILL WICHERS | | #2

    Rigid foam is not "derived from spray foam". Polyiso is the closest chemically to spray foam, but it's made differently and in a controlled manufacturing enviornment, not in the field the way spray foam is applied to buildings (and it's usually the "in the field" part where problems come from in terms of the mix being off -- factories can control EVERYTHING about their manufacturing processes to avoid this issue).

    XPS and EPS are ENTIRELY different products. XPS is basically made by bubbling a blowing agent through the polystyrene resin (I'm simplifying a bit). EPS is made by expanding (hence the "E" in "EPS) polystyrene beads with steam. They are not polyurethane based, and they are made with an entirely different process. EPS doesn't even offgas any blowing agents over it's lifetime, which is the reason why the manufacturers can advertise that the R value doesn't degrade.

    That all said, rigid foam is generally the safest option where you need to prevent moisture migration. I would recommend EPS in your situation, since it allows some drying (it's the most vapor open option), but won't allow bulk moisture to pass through in the way that any of the batt-type fiber-based insulation materials will. EPS isn't toxic (neither are the others, really, unless you burn them), and is your best bet if you want something that isn't going to offgas anything.

    Note that the common "canned foam" is one-part spray foam and is VERY different from the two-part spray foams used for larger insulating projects. If your primary concern is avoiding the potential issues with spray foam (smell, incomplete curing, etc.), canned foam doesn't have those issues. In many ways, canned foam isn't really any different than caulk in terms of the offgassing issue.

    As far as I know, there really aren't any other insulating materials that are really suitable here unless you get into the really exotic stuff that would be the most expensive part of your home (vacuum panels and aerogels). Those are both still more of labratory things than production items at the moment.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |