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Community and Q&A

Insulating Rim Joists Without Foam

BuildingNewb | Posted in Energy Efficiency and Durability on

Hey guys, I had previously air sealed my rim joists with a combination of spray foam and rigid foam board.

However, after a few water leaks (that have been subsequently repaired), I have grown increasingly paranoid about the ability for spray foam and rigid foam to conceal water and/or pest damage.

Therefore I was wondering if you guys could recommend another method of insulating the rim joist that would still allow access in the event it was needed. I know fiberglass insulation isn’t best practice but would it be ok to use provided that the perimeter of the rim joist is adequately sealed with spray foam? Would Roxul or cellulose be a better option over fiberglass?

I just generally don’t feel comfortable doing anything that would limit access in the future. When our front door leaked I noticed some water dripping. When I cut out the rigid foam blocks, there was plenty of wet wood behind the foam. Another form of insulation could’ve allowed it to dry out quicker while also help reveal the problem sooner.

Thank you very much for your time and help everyone!

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  1. sean_web | | #1

    No answers for you but I feel your pain. Recently demo’d old plaster ceiling in the basement of my century home and there’s certainly a lot of value in being able to see how the structure is doing, as well as keep an eye on potential water and rodent issues as you mention. Plus I plan to update electrical and having that rim joist accessible will make running wires up walls a lot easier.

  2. johngfc | | #2

    It surely depends on your climate - were are you? Here in a dry Colorado climate zone 5, I have three houses with fiberglass batts stuffed into the rim joists (no foam or other moisture sealing) in crawl spaces. All three houses are at least 30 yrs old with furnaces and ducts in the marginally insulated crawl spaces, and all are fine with no evidence of dry rot or moisture problems. If you're in a similar climate I suspect you'd have no issues with condensation, and that for bulk water mineral wool batts would be preferred since they don't get soggy like fiberglass.

  3. BuildingNewb | | #3

    We are in climate zone 5 but right on the border of climate zone 6. We had fiberglass stuffed in there as well without issue but the basement also hasn't been heated yet.

  4. Expert Member
    BILL WICHERS | | #4

    The issue with fiberglass, and any other fibrous insulating material, is that moisture can migrate through and condense on the cold surface of the rim joist. You need a way to limit moisture migration to prevent moisture and rot problems with the rim joist. Spray foam works by adhering to the rim joist and removing anywhere for moisture to accumulate, combined with spray foam's ability to limit moisture migration (especially closed cell spray foam). Rigid foam works too.

    I used EPS on my own rim joists, because I have foil faced polyiso on the exterior and EPS allows some limited drying to the interior as a bit of insurance against moisture issues. There is zero drying potential through the foil faced polyiso. I "foam in" EPS panels using canned foam.

    Even if you spray foam the perimeter, the face of the rim joist will still get cold enough to condense water, which is why you don't want to just stuff batts in. If you are worried about access, I would foam in rigid foam panels. EPS especially is pretty easy to remove with basic tools (even a drywall knife will work), since it's the least strong of the rigid foams -- especially if you use Type I EPS, which is what I used.


  5. Expert Member


    Fiberglass has been used to insulate rim joists here for decades. To do it successfully you need to seal the perimeter of each bay as an air-barrier, fill the void with batts, then install an effective vapour-barrier on the interior side. Here they use a square of poly bedded in acoustical sealant. It's incredibly labour intensive, hard to do well, and still means the rim joist is inaccessible.

    If you want to avoid foam, keep the area open for inspection, and allow the joists to dry after any bulk water intrusion, the best bet is to plan your framing so you can insulate the outside of the rim joist (or the whole exterior) with mineral wool.

  6. GBA Editor
    Kiley Jacques | | #6

    Insulating a rim joist with anything other than rigid or spray foam is certainly atypical (for a reason). However, in warmer climates 2 in. of rigid foam may be enough. In this article, Insulating Rim Joists, Martin Holladay suggests installing 2 in. of rigid foam followed by a layer of fiberglass insulation as a money-saving approach. He warns: “Fiberglass batts should never be installed near a rim joist unless the rim joist is first insulated with at least 2 in. of rigid foam or spray foam.”

  7. plumb_bob | | #7

    What about spray foaming most of them, and then having several that are tightly fitted rigid foam and caulked around the perimeter? these would be easy to remove and inspect.

    1. Expert Member
      BILL WICHERS | | #8

      I don't see a problem using rigid foam panels like this. I would recommend a faced rigid foam here since it will be more durable, and easier to reuse. kraft or fiberglass faced polyiso would probably be ideal for this, since it's pretty durable but still relatively vapor open.


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