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Community and Q&A

Foam board

Aron_Robinson | Posted in General Questions on

We’ve installed foam sheathing on the outside of our house down to the level even with the rim joist. Can we still install foam board on the inside of the house in the rim joist in place of fiberglass?

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  1. GBA Editor
    Martin Holladay | | #1

    You asked me that question before, on February 11. (You posted your question on this thread.)

    My answer is the same as the one I gave you the last time you asked:

    "If you have an adequately thick layer of rigid foam on the exterior side of your rim joist, you don't have to add additional thermal insulation on the interior. If you are worried about air leakage, you can always address cracks with caulk.

    "However, if you want to take a belt-and-suspenders approach, you can install spray foam on the interior of your rim joist. Open-cell foam might be better than closed-cell foam in your case, since you have an exterior layer which is fairly impermeable to water vapor.

    "If you insist on using closed-cell spray foam, I advise (1) doing it on a day when the rim joist has a very low humidity level, and (2) using only a thin layer -- perhaps 1 or 2 inches."

  2. Aron_Robinson | | #2

    .Thanks martin.. I wasn't sure...inulsators are there today. I am insulating the rim joist myself this weekend. My foam sheathing stops right at the top of the concrete basement wall. I have half inch foamular on the outside. I have 2 options for insulation in the rim joist. Fiberglass batts or foamular that is sealed with canned foam. Wich would you recommend? Btw...I can make the foam as thick or thin as I want...because I'm using leftover half inch board andcan use multiple layers in half inch increments. I'm in zone 4 kentucky...gonna be drywalling over thi next week and want to get it right.

  3. GBA Editor
    Martin Holladay | | #3

    If all you have is 1/2 inch of XPS on the exterior of your rim joist, that isn't much. It has an R-value of about R-2.5 and is somewhat permeable to water vapor, with a permeance of over 2 perms. So I wouldn't worry too much about trapping moisture.

    As long as your rim joist is dry, go ahead and insulate the interior of the rim joist with rigid foam. The more, the better. Pay attention to air sealing at the perimeter of the foam.

    Don't install fiberglass batts on the interior side of a rim joist. Fiberglass is air-permeable and vapor-permeable; it can allow the warm, moist interior air to contact the (relatively) cold rim joist, and you don't want that to happen.

  4. Aron_Robinson | | #4

    Thanks martin.

  5. Aron_Robinson | | #5

    One other question and I'll make my decision. The insulator said they can froth pack the rim joist with open cell spray foam for $650. The other option is I can do it for free with foamular foam board that I have on hand already. Is it worth the 650 or am I better sticking with the foam board?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Is it worth the $650?"

    A. The answer depends on how you value your own time -- and how much money you have in your bank account.

  7. Aron_Robinson | | #7

    Martin...I just spoke with the insulator. They were actually going to do a flash of 1 inch and then install a batt of insulation. What are your thoughts on this?

  8. GBA Editor
    Martin Holladay | | #8

    It's probably OK, but it has disadvantages.

    The fiberglass provides an ignition barrier that protects the spray foam. However, think about durability. Will the batts be exposed? It's illegal to leave kraft facing exposed, because kraft facing is flammable. So you'll be looking at unfaced fiberglass... the the fiberglass is likely to fall out eventually... or be discovered by mice.

  9. user-945061 | | #9

    It might be overkill for new construction, but on retrofits I generally spray a glycol-borate solution to rim joists before applying any sort of foam. PRG sells a product called Bora-Care.

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