Hi all

I’m looking to insulate my basement rim joists in the next few weeks, getting rid of the stuffed in fiberglass batts. I’ve done some research here and other places, and I’m kind of stuck trying to decide on material to use. Based on cost and material availability, I’ve narrowed it down to using 2″ unfaced XPS or 2″ foil-faced polyiso.

My home is in Maryland, climate zone 4. The rim joists are solid wood; on the exterior of the rim joists, there is just vinyl siding and housewrap. I plan to caulk and seal all along the perimeter of the rim joist bays, sticking the foam in, and caulking/spray foaming the edges of the foam.

My initial thought was to use XPS, since I planned on using it for the mostly below-grade poured-concrete walls; I would use scraps of leftovers on the rim-joists. But I’ve read that there’s a risk of condensation forming in the XPS at the interface of the foam and rim joist, particularly in humid summer months with moisture driving in from the exterior. The concern being that the moisture may take awhile to dry out of the XPS and potentially rotting the joist.

Foil-faced polyiso might be an answer, and I’ve seen others mention using it on rim joists; the foil-facing should prevent any vapor transfer. But is it possible for condensation to still occur at the foil/rim joist interface? Granted, it’s a very small amount of space between the two. But with the foil-face, if any condensation does occur there’s no way for that moisture to dry to the interior like there is with the XPS.

So I’ve gotten myself wrapped-up a bit thinking about this, and I’m hoping someone with more experience could point me in the right direction.

Thanks for any help!