How high to seal vapor retarder in open ceiling basement
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 &quot;least painful&quot; 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.