Dennis writes from Bellingham, Massachusetts: "I have 3.5" Owens Corning fiberglass w/Kraft vapor barrier between 2x4 studs in my home. I plan to strip the existing exterior siding down to the plywood sheathing, and install vinyl siding. I'd like to add 3/4" or 1" foam insul board to exterior before applying siding. Will this cause problems with moisture being trapped within walls and cause more trouble than the R-increase is worth?"

Answer: Extruded foam insulation will not act a vapor retarder but will definitely add R-value. I don't see any problem with what you plan to do. If you tape the seams, the foam insulation will qualify as a weather resistant barrier and you will not need housewrap. Be aware of the fact that adding thickness will change the appearance and fit of the walls around windows and doors.