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I'm finishing my basement with an open ceiling am trying to figure out how high the vapor retarder needs to go. The wall will be insulated with mineral wool and the rim joists with 2" XPS + 3.5" mineral wool. A vapor retarder will be used for the wall, but I'm unsure about the space between rim joists. Does it need vapor retarder? If so, what is the best way to do so? Doing each cavity separately seems like a pain.

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Having the ends of your joists rot out is also a pain. If you don't seal the spaces from the warm, moist air inside, it will infiltrate, cool, and dump water in the cavity and insulation - that's why a vapor barrier is used.

Probably the "least painful" approach (assuming cold climate) would be to insulate the rim joist spaces with the mineral wool outside and the XPS inside, and use closed-cell spray foam to seal the XPS in each cavity, since XPS is a vapor barrier itself. You'll need some sort of fire barrier (such as sheetrock) over the XPS, though.

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  • Thanks! I'm trying to do the right thing and generally having structural components rotting is not the right thing :-) when you say mineral wool outside, do you mean against the concrete exterior? If so, I thought the XPS should go against concrete. Oct 1, 2020 at 14:28
  • If using the XPS as the vapor barrier, it has to go inside. You could use all XPS, or you can approach the vapor barrier some other way, with the rockwool on the inside.
    – Ecnerwal
    Oct 1, 2020 at 14:44
  • Thanks. I'm not keen on fighting vapor barrier, so all XPS might be easiest. Oct 1, 2020 at 15:32

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