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Community and Q&A

Insulating basement rim joist

cfowens | Posted in Energy Efficiency and Durability on

I am currently working to insulate the rim joists in my home’s basement. So far I have gone through and cut 2 inch rigid foam board to fit the cavities, tacked in place with liquid nails adhesive, and then used Great Stuff spray foam around the edges to air seal. The 2 inches of rigid foam board gets me R10. 

To boost my R-value a bit more, I thought of cutting some faced R19 fiberglass batt insulation I have laying around and putting that in the cavities as well, in addition to the 2 inch rigid foam board that’s already there. I would keep the faced side facing inward toward the basement, of course.

Does this present any concerns?

I understand that using fiberglass insulation alone in rim joists is not recommended, due to moisture/condensation concerns. But i’m thinking that throwing the fiberglass in there on top of the 2 inch rigid foam board I already have in place might be alright, since I’m already air sealed.

Appreciate any insight anyone might be able to offer as to the acceptability of this approach.

Thanks in advance!

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  1. Expert Member
    Zephyr7 | | #1

    The foam should protect you from any moisture issues as long as you stick to the usual R value ratios used in exterior walls.

    I personally like to use the Safe n' Sound mineral wool batts for this purpose. They are 3" thick, and is around R12.5ish. Mineral wool also provides the required fire protection for the rigid foam, and it stays in place much better than fiberglass so you don't usually need any additional supports when using it.


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