# Rim Joist insulation options?



## cleveman (Dec 17, 2011)

I think you have a decent thermal barrier in place and should proceed with the batts.

I regularly have 1" on the exterior and 1" or 2" on the interior (depending on scraps), then I put in some batts.


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## jklingel (Dec 21, 2008)

Batts, esp fiberglass, are a poor choice here, as they would be exposed to air currents, killing their R value. Depending on where you are, more rigid or spray foam would be my suggestion. Too, you want to be careful sandwiching wood between layers of foam. If too thick, moisture is slow to leave the foam and can condense on the wood if it gets cold. Pick a side and insulate it; air seal, as you suggested you did on the outside.


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## Windows on Wash (Aug 30, 2011)

+1

Spray foam and seal all the gaps in the EPS board that is in there now.

Install a Roxul batt in there as it will be a much tighter fit and will not suffer convective losses.


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## Gary in WA (Mar 11, 2009)

Caulk/canned foam the joints at rim to floor sheathing/mudsill to stop air movement.* No rigid foam board inside*, unfaced insulation only, fig 3: http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/

Here it is again, fig. 10: http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements

This shows foam on both sides...as JK said, Fig.2; http://www.buildingscience.com/documents/information-sheets/critical-seal-spray-foam-at-rim-joist/

Because the wood rim (if solid wood) expands and contracts with the seasons, this cycling movement could break a shallow canned-foam seal around rigid foam board. Using spray foam is different because it would be 3" deep (about 3 times thicker than other), totally stopping any infiltrating/exfiltrating air. I would use 0.50# open-cell foam (SPF) because it "gives" more with movement with solid wood. If the outside foam board was perfectly applied (no air leaks; totally bonded), either foam would work.

Gary


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