Recommendation For Insulating a Rim Joist


Old 09-19-19, 03:43 PM
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Recommendation For Insulating a Rim Joist

I have finished basement that is a walk out. There is a suspended ceiling through out. The walls are wood framed 2x6 construction with fiberglass insulation, vapor barrier and sheetrock
The rim joist above the suspended ceiling was never insulated and needs to be. The two perimeter walls that are parallel to all the floor joists look to have a double rim joist as the cavity where insulation would go is only 2 1/2 deep, not much room for insulation to sit on.. The other 2 perimeter walls have a 4 deep bays. Looking at options, I would prefer not use spray foam as the rooms are already finished and there is a suspended ceiling in place and I see this as a potential to be very messy. Fiberglass bats as well as Rockwool would seem to allow air filtration which in turn would potentially allow warm air to hit the cold rim joist and start mold and rot issues. Rigid foam board seems to be the closest to ideal but requires a fire barrier.
The best solution seems for me seems to be Dow Thermax polyisosynurate rigid boards as it appears to have the required fire barrier on its surface built in. The problem is no one seems to sell this locally or large quantities are required to be purchased on a special order basis.Why is this product so difficult to find? Does anyone have an idea where this can be purchased? This is in the southern New Hampshire area.
Does any one have any other suggestions?

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Old 09-19-19, 11:29 PM
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Couple of froth packs and you should be good, they are not too messy as long as you have a drip cloth and go slow!
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