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Q.I am reroofing a flat roof framed with 2x10s on a house in Tucson. The foil-faced batt insulation was installed more than 25 years ago and seems very spotty, so I’d like to replace it while the roof is stripped. Winters are mild here, but the summer heat is brutal. I use both a/c and evaporative cooling depending on the outside humidity level, so humidity inside varies. What’s the best way to insulate the ceiling joists to provide ventilation? And how should I handle the vapor barrier? Since I don’t want to remove the drywall ceiling, is there some way to coat the back of the drywall? Or can a synthetic roof coating be used as a spray-on vapor barrier?

A.Paul Fisette, director of the Building Materials and Wood Technology program at the University of Massachusetts in Amherst, responds:Vapor transmission by diffusion isn’t a major concern in your dry climate, so unless you generate excessive indoor moisture, don’t lose too much sleep over the vapor barrier. If your local code requires a vapor barrier on the inside, use an oil-based paint or a paint like Glidden Insul-aid on the ceiling to satisfy the inspector. But remember, the vapor barrier should go on the warm side, which is the outside in your climate. So don’t waste time applying anything to the back of the ceiling drywall; instead, put it on the outermost side (top) of the insulation if you decide to use one at all. (Asphalt roofing provides a good exterior vapor barrier.)

I would fill the roof joist cavities with cellulose insulation and cover the top of the cavities with a foil reflective barrier. Then leave an air space between the top of the reflective barrier and the underside of the roof sheathing. Or you can use a white/reflective roof color in place of the foil, since your biggest concern is overheating. When choosing your roofing, look for the solar reflectance rating. Be careful: Conventional white asphalt roof coatings may have a reflectance of about 20% as compared with 80% for some specially designed reflective coatings. The high reflectance materials will do a good job of keeping the heat out and can greatly reduce your cooling load.