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Old 01-01-2018, 10:30 AM   #1
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Adding insulation to rim joist


I have been searching, but not found my situation here.

I have 16" truss floor joists with the rim joists being 1" xps foil insulation with osb on the outside. The insulation was installed on the outside and caulked well. There is no air leaking in that I can find. I have about half on 2x6 knee wall and the rest 10" concrete walls (T-mass with 2" of foam in the middle.

I am thinking that I should add some insulation there as I am getting ready to finish the basement. When the house was built my insulator tried to talk me into spray foaming there and I know that is the best solution, but my inspector required it to be fire-proofed and that would have been impossible so I didn't do it.

I only have 9 1/2" x 16" between the 2x4 trusses and 2x4 blocking (was required on each side of joist by inspector). I'm thinking about putting a block of rock wool in between the joists. With no air movement that would work well and still allow any water to evaporate if it condenses. If I put in foam and caulked the edges moisture could still get in between the blocking and joists anyway.

Looking for some insight before I dive into doing anything.

Thanks
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Old 01-01-2018, 12:40 PM   #2
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Re: Adding insulation to rim joist


Sounds like you've got a pretty good handle on it. The foam on the outside is a good thermal break, and if it's as well air sealed as it seems, then you're good to use just about whatever insulation you like there.
Comparing your setup to most homes, you're gold plated!
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Old 01-01-2018, 12:47 PM   #3
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Re: Adding insulation to rim joist


https://buildingscience.com/document...in-crawlspaces

https://buildingscience.com/document...ace-insulation
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Old 01-01-2018, 02:21 PM   #4
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Re: Adding insulation to rim joist


Thanks for the replies. I built this house 11 years ago and tried to do things right as I plan to spend the rest of my life in it.

Sounds like rock wool would work good. I have been going back and forth between that or 1 1/2" xps which would be neater, but the rock wool would breathe better.
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