# Insulating Rim Joists



## Bud9051 (Nov 11, 2015)

Hi MGK,
I'll throw this out for consideration. Since that floor joist is not resting on the foundation and since the load is being supported by the rim joist, how important is it? Currently it looks like a 2x10, but if it were removed, nothing would fall down. If it became a 2x8, zero effect on the performance of the floor above.

There are different way to seal the rim joist to the foundation other than can foam. Caulking or rope caulk could be applied through a few limited access points. If you can deal with the mess, an angled brush and some mastic. After being sealed, then the some rigid could be pieced in plus something like mineral wool or just fiberglass.

Food for thought,
Bud


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## Windows on Wash (Aug 30, 2011)

If you want to drill holes (small and not near the ends) in the rim you can inject them with foam. 

After that, the 2" XPS should be good. Caulking works well and tends to stay flexible long term. 

I prefer to cover the foam with some fibrous insulation or another non-combustible.


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