What kind of foam board should I use to insulate rim joists in Wisconsin?
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What kind of foam board should I use to insulate rim joists in Wisconsin?
Matt Kime
Sept. 12, 2021Last updated: Sept. 21, 2021
My home was built in 1927 and at some point fiberglass was used to insulate the rim joists. I've started to remove it because its become a home to mice in some places, and just plain not working in others. But what to replace it with?
I'm leaning toward 4" of polyiso, with a <25 fire spread. It won't work as well at low temps and will lose some R value over time but I think I'm properly compensating by installing 4". (2x 3" layers) It seems that XPS would be the alternative but I'm concerned with fire code rating. While XPS has a lower fire spread rating its not suitable for exposed inside walls. It would need a layer of sheet rock. Thoughts?
Polyiso would work fine, and I wouldn’t worry too much about the fact that the R value fall off a bit overtime. It is a small portion of your building envelope I think 4 inches would serve you well. For fire protection, what if you used EPS and just glue pieces of drywall to it maybe? Here is our page all about different types of rigid foam insulation, have a look there and we can discuss more if you like.
The difference between Polyiso, EPS & XPS Foam Insulation & Styrofoam