Trip writes from West Hurley, New York: "If I add rigid foam to outside of kneewall behind our upstairs bedrooms, with existing fiberglass batt insulation with kraft paper facing in, will I create any moisture problems by trapping moisture behind the foam? Any other consideration I should account for?"

Answer: FOAMULAR® extruded foam insulation is not a vapor retarder. It is considered breathable. It has a closed-cell construction that does not let liquid water through but it will allow water vapor to pass through. If the back side of the knee wall is exposed to living space it would need to be covered with drywall. But it doesn't sound like it is so there are no other considerations.