Rim joist insulation on house with exterior insulation
Hi everyone,
I am curious about the best method for insulating a rim joist on a house that will have 6” of exterior rockwool. I currently have fibreglass batt insulation installed in the rim joist cavity, but I’ve noticed there is slight ice build up on the interior of the rim.
I am located in climate zone 7a, and my interior vapour barrier transitions from the interior to the exterior to go around the rim joist before coming back to the interior. I would be nervous to put EPS pieces that are spray foamed in place, possibly creating a vapour barrier sandwich.
In a perfect world I would have the exterior insulation on, which would keep the rim joist nice and warm. Given that is not the case I’m open to suggestions.
I suppose my main concern is it being able to successfully dry to the inside once drywall is complete. The rim is from the top of level 1 to level 2, the house is a slab on grade.
re-reading Martin’s FHB article I still believe this is a appropriate method, but it seems like I didn’t give much thought to the sequence of insulation. The -30 temperature made the decision for me!
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Replies
Northof60,
What is your vapour barrier? Poly?
Yes 6mm poly.
Northof60,
Even if the poly is also acting as your main air-barrier, I would cut it away at the rim-joist, leaving 1" at the top and bottom, and tape it to the exterior. That inside, around, and back in detail is designed for permeable materials like house-wrap. Leaving the poly there precludes drying. You are then safe to put the foam board on the inside without moisture worries.
It sounds like this is NOT a rim joist on top of a foundation wall, it's between the first and second floors?
Putting EPS tight to the inside face of the rim joist and sealing the panels in place with canned foam should be safe. The EPS will greatly limit the ability of any moisture to reach the rim joist, but it will allow for a small amount of inward drying. With mineral wool on the exterior, you will have lots of outward drying potential so you should have a pretty safe assembly.
Note that I've had issues with canned foam expanding and sealing/curing properly in below freezing temperatures. I find it to be more bubbly and "crispy" or "brittle" when used when it's very cold. If you have any way to warm up that surface while you're working, even temporarily, that's probably a good ided to try to do.
Bill