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Insulate rim joists with closed cell, with rigid foam on exterior?

user-6259794 | Posted in Energy Efficiency and Durability on

I have a new construction home, zone 5. I wondered if you insulate the rims joists with closed cell foam, when we have 3″ of rigid foam on the exterior?

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  1. GBA Editor
    Martin Holladay | | #1

    In theory, you don't want to seal wood between two layers of vapor-impermeable foam. But researchers who have looked into the issue have discovered that the exterior rigid foam keeps the rim joists perfectly dry -- even when additional closed-cell spray foam is installed on the interior side of the rim joist. (That's the finding shared by Gary Parsons and Brian Lieburn of Dow Building Solutions at the recent EEBA conference in Frisco, Texas.)

    So go ahead -- install the interior spray foam. You don't have to worry.

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