Prevent Moisture Absorption of OSB Roof Panels with Spray Urethane

Originally published by: Journal of Light ConstructionApril 1, 2003

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Q: We build timber frame homes and wrap them with insulated panels made with skins of OSB on the outhouse and rigid foam insulation on the inside. We've been pleased with the results except for one thing: the swelling of the OSB at the edges. We've seen no damage to the roof structures, but the ridging is unsightly when the roofs are covered with asphalt shingles. So far, we haven't been able to find a solution, other than to make sure that the roofer covers the roof immediately with felt and uses good-quality shingle. Have you seen any instance where this swelling has contributed to roof failure?

A: Contributing editor Paul Fisette responds: This problem, caused by moisture adsorption, has been reported before with stressed-skin panels installed on roofs. The panel edges take on moisture more quickly than the rest of the panel and swell as a result -- one of the shortcomings of OSB. What can happen with roof panels is that warm, moist air from inside the house leaks through the seams. As the indoor air cools, the moisture condenses and wets the panel edges. The moisture may also collect on the underside of the impermeable asphalt roof felt and shingles, compounding the problem. The solution is to meticulously seal the seams of adjoining panels with a product like spray urethane. Inject foam-in-place urethane into the seams before assembly or drill holes and fill the seams after the panels have been installed. Be careful about using spray-in-place urethane to seal panels in cold weather. The necessary heat of reaction is wicked away by the cold, which can interfere with the curing process and leave an unprotected seam. Check with the spray-foam manufacturer for recommendations on cold weather installations.