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Q.The specs for spray-applied urethane foam insulation say that it does not absorb water (hydrophobic). So why is it never used on the outside of foundations?

A.Insulation contractor Patrick Dundon, of Windsor, N.Y., responds: Spray-applied polyurethane foam is in fact frequently used as an exterior foundation insulation. Foam Enterprises of Minneapolis, Minn. (800/888-3342), has been supplying insulation contractors with this system for several years.

Urethane will not absorb water, but if there is sufficient force (a high water table, for example, or improperly directed runoff), water will penetrate the foam. In addition, urethanes do not stand up to sunlight well, which means a protective coating is necessary to prevent UV degradation. An exterior foam insulation job requires coating the below-grade portions with a cold-applied asphalt and those portions above grade with an acrylic roof coating or a cementitious EIFS-style parging.