GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Exterior Vapor-Impermeable Foil-Faced Foam Board / Interior XPS Foam

ElectricHoosier | Posted in General Questions on

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 Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    I have exactly this same situation, with foil faced polyiso on the outside of my rim joist. Since I wanted a little bit of drying ability, I used 2" EPS on the inside, sealed in place with canned foam. EPS is a little bit vapor open, so this allows a little bit of drying.

    I don't have a pressure treated sill plate, but I added a capillary break. In your case, you have a pressure treated sill plate so you have less to worry about, but a capillary break is still a good idea -- and you might already have one (see if you have sill sealer or some other kind of "masonry to sill plate" gasket material in place).

    Concrete walls do NOT need to dry. You can insulate them without worry, just use insulation that won't be a problem with moisture (that means no batts here). Many people use polyiso, but you can use EPS or XPS too (XPS being the least green option). The only real concern you need to have is the potential for moisture wicking up to the sill plate, but if you have a capillary break that's no longer an issue.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |