# which caulk to use to seal rim joist?



## chandler48 (Jun 5, 2017)

If you are insulating the rim joist area, which you should be doing, you would usually use minimally expanding foam around the perimeter, the apply your insulation. I recommend Roxul for a good tight fit. Waterproof, vermin proof, mold proof and fire proof.

If all you are doing is caulking, I would use an elastomeric caulk which stretches and contracts with the building.


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## carpdad (Oct 11, 2010)

I think alexplus? which I think is the cheapest. Caulk these days all stay somewhat flexible but even if cures hard, you're still air sealing. Even left over grout will work. After applying, push the caulk in and flat with a flat stick or scraper.
Is this from outside or inside? If from inside, floor joist bays also? If yes, 1" foam board cut 1/4" less than the bay dimensions and fill the gaps with canned foam. Works well even without further insulation and easiest to do. Applying just enough foam is like holding a cracked egg without totally cracking it.


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## jackallis (May 4, 2019)

chandler48 said:


> If you are insulating the rim joist area, which you should be doing, you would usually use minimally expanding foam around the perimeter, the apply your insulation. I recommend Roxul for a good tight fit. Waterproof, vermin proof, mold proof and fire proof.
> 
> If all you are doing is caulking, I would use an elastomeric caulk which stretches and contracts with the building.



plan is to do both. 

caulk first and then foam board and spray foam.


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## Windows on Wash (Aug 30, 2011)

Whatever is cheapest and decent. Don't overspend here.


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## Kemper100 (Sep 16, 2017)

I just used the cheapest caulk and then insulated over it with rigid foam board.


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