Exterior Vapor-Impermeable Foil-Faced Foam Board / Interior XPS Foam
We’re in the middle of a lower level (Bi-Level/Split Foyer home) bathroom remodel. Down to the bare studs. The old fiberglass insulation was removed and 1/2″ dual foil faced foam board (Polyiso) is behind the 2×6 exterior wall and also behind the band joist above it. Outside the foam board is 1″ air gap and then 3.5″ brick exterior “siding.” Both are above grade. The stud wall sits on a 2×6 plate that sits on a 2×4 pressure treated plate on top of 6.5″ thick concrete block basement wall. The concrete wall is not insulated on the interior, and because I can’t guarantee 100% dry in this section of block wall, I plan on leaving it as is. Heavy AC use (ducted heat pump) in the summer keeps the lower level and upper level of home at 50% RH. Shoulder seasons mean open windows and winter equals heat pump + portable dehumidifier run in the lower level. Climate zone is mixed humid- 4a and the winter design temperature is 9 degree F.
The internet is full of sites that say to foam the band joists. But almost all of these sites do not assume a vapor impermeable foil faced foam board outside of the band joist, or also outside the lower level walls, as in our scenario. Can I use one or two layers of extruded polystyrene insulation (XPS), caulked around the edges, in this scenario for both the band joist and the lower level bathroom wall without moisture issues in wood? Should I use a more vapor permeable insulation behind the drywall instead? I have old 1.5″ XPS sheets available. It will all be encased behind new fiberglass shower and/or drywall. Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part