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Rim joist insulation question

ben_riegel | Posted in General Questions on

Hello, I live in climate zone 6a (coastal Maine).

I’m in the process of installing insulation in the rim joist in my crawl space. I have cut and attached pieces of 2in polyiso board insulation in each rim joist cavity and carefully air sealed around the perimeter of each board. I would also like to put pieces of 3.5in rockwool insulation (R-15) on the interior side of the polyiso in each cavity (similar to flash and batt but with rigid insulation instead of spray foam) to increase the r-value.

If I did this, the rockwool would be in contact with the floor joists on each side, the subfloor on the top, and the mudsill on the bottom. This is probably a stupid question but … would I have to worry about condensation on the mudsill in this situation? The mudsill can get decently cold during the winter because there is not much on the exterior of the house covering it (you can look under the siding and see it). The rockwool would keep the mudsill colder and thus more likely to get below the dew point. Is this a legitimate worry here or am I worried about nothing?

Thanks for your help!

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  1. Expert Member


    The illustrations in this article should assuage your fears.

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