I'm working on my unfinished basement in northeast PA (zone 5), where it can get really cold—like last January when it hit -16°F! I'm planning to install rigid foam board insulation on the rim joists. Since I'll also be using batts throughout the joist cavities, would using 2" foam board sealed around the edges be enough? It seems like that's the thickest board available locally without having to special order.
2 Answers
You're on the right track with insulation for your rim joist. The ideal R-value for above-ground walls is R21, which would require about 4" of foam. If that's too thick for your space, using 2" foam and then supplementing with mineral wool might work well too. Just make sure everything is sealed properly!
For a zone 5 area like yours, you typically want an R-value of around R20 for the rim joists. A 2" foam board usually provides an R-value of about R10. If you really just have the 2" board, consider doubling it up or adding 2" of spray foam on top to meet that R20 recommendation. That way, you'll have a solid insulation layer!
Thanks for the tips! I might give that mineral wool a shot alongside the 2" foam.