I'm currently finishing off an attic space above an unheated garage and planning to install a mini-split system for heating and cooling. My rafters are 2x12s with a bunch of nails sticking through the roof, so instead of using those plastic baffles to maintain airflow to the ridge vent, I've opted to cut 1" thick XPS foamular board into strips to fill the gaps and help with air circulation. Once that's done, I'm considering putting either fiberglass or rockwool insulation on top. Since I'm going to drywall the interior of the attic, I've heard it's best to use faced insulation, but given that I'm installing foamular boards, should I stick with unfaced insulation instead to prevent trapping moisture? Or will the XPS boards not impact moisture as much as I think?
1 Answer
You definitely want to avoid having two vapor barriers since that can create moisture issues down the line. It’s also about how well you sealed the XPS; those foam baffles aren’t really airtight and are designed for ventilation, not vapor control. So, I'd suggest going for unfaced fiberglass batts or rockwool to play it safe. That way, you minimize the risk of moisture getting trapped.
What if I moved the 1" strips inward a bit? That might leave some space for the insulation to air out against the sheathing, right?