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I am insulating my rim joists with rigid foam board in a manner similar to that described on the familyhandyman's website.

I am considering adding a second layer of foam board to get closer to the neighborhood of R20, which I've seen recommended for rim joists. I am wondering where would be best to caulk - after the first layer, second layer, or both?

Given that the caulk's purpose is to stop air flow, it would seem unnecessary to have two layers of it, but I am including "both" as an option in case you want to argue for it.

I can't really think of a difference between applying after the first layer of insulation or the second...can you?

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I would caulk on both of them. The purpose of the caulk it to prevent air leaks and convection, and not sealing both will reduce the effectiveness of the insulation a bit.

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