Mineral wool question for garage under living area
Hi folks -- I have a flat ranch with a walk-out garage under my living room and kitchen, directly off the basement. I have a medium-term project going on right now involving framing repair and rewiring, so the original 1968 drywall ceiling, vapor barrier, and blown-in cellulose insulation were removed to access the framing. But I'm in upstate NY, and it's December so that garage is cold. I'd planned to insulate it with mineral wool anyway so I am bodging in mineral wool batts sized for 2x6 framing right now -- but I have 8" joists, so there's a gap of ~2" between the bottom of the joists and the bottom of the batts.
When the wiring work is done and this is ready to button back up, I'm thinking 2" rigid EPS foam (foil faced; same as I'm going to use when I air seal and insulate the rim joists after wiring is done) will fill that gap pretty well before drywall goes on. Is this reasonable? If so, foil towards the living space (up) or towards the garage (down)?
I also assume I'll want to put a 6mil vapor barrier back up before 5/8 drywall since this is a garage -- will the foil faced foam be a problem behind a poly vapor barrier?
Nothing is closed up and everything except the temporary mineral wool is theoretical so there's plenty of time to course correct if I am way off down the wrong path.
Posted Dec 9, 2025, 9:54 AM UTC