A.Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: Unless you are located in a cooling-dominated climate, I would steer clear of installing a synthetic roofing underlayment in the manner you describe.

With a couple of exceptions, polyethylene- and polypropylene-based roofing underlayments are vapor-impermeable and would function as an exterior vapor retarder. (See "Synthetic Roofing Underlayments," 5/06.)

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