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....
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