Bill writes from Hollis, New Hampshire: "I want to add insulation to my cathedral ceilings. The space is made with post and beam, with tongue and groove planking on top of the beams. Above the interior planking (in order) is plastic vapor barrier, R-17 insulation in 2X4 cavity (unvented), wood sheathing, and then shingles. I obviously want to preserve the look of the interior, so doing improvements from the inside is a no go. I am replacing the roof, and was thinking about adding rigid foam insulation on top of the existing wood sheathing. Would this create two vapor barriers, and cause condensation issues between the two layers? If so, is there a rigid foam insulation product that "breathes" enough so that it doesn't cause another barrier? I'll also be adding venting on top of the rigid foam."

Answer: Having a second vapor retarder can be an issue but not in this case. This can be thought of as a wall with Fiberglas™ insulation in the cavity and FOAMULAR® sheathing for the exterior. FOAMULAR insulation up to 2" thick can have a higher perm value than OSB. Adding the FOAMULAR insulation to the exterior of the wall assembly will change the temperature of the cavity - making it warmer and therefore less likely to suffer from condensation.

Not having ventilation can cause moisture issues inside the living space and I recommend having a well-controlled humidity level within the house. Also, most shingle manufacturers reduce or remove warranty coverage for shingles applied to an insulated deck. We recommend 1-1/2 inchs to 2 inches of ventilation space along the roof.