What kind of foam board should I use to insulate rim joists in Wisconsin?

Matt Kime
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?

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