The Hygrothermal Performance of Ventilated Stucco Walls Using Spray Polyurethane Foam Insulations and a Smart Vapor Retarder in the Pacific Northwest ✓ Most Recent

Spray polyurethane foam insulations used in residential wood frame buildings have superior thermal performance compared with traditional fiberglass materials. Close-cell foam especially also has low water-vapor transmission property, and at the same time offers good airtightness to building envelope systems. Contrary to common building practice, the application of spray foam to the interior wall cavities requires careful moisture analysis and design to prevent potential moisture related damages in humid climates such as the US Pacific Northwest. This unique climate area has moderate temperatures and high moisture levels caused by precipitation and high relative humidity throughout the year. Recent research on wood framed wall systems with exterior stucco and cavity spray polyurethane foams is being conducted in this climate zone. Two ventilated stucco clad wall systems have been installed side by side in a natural exposure testing facility in the Seattle, Washington area. One wall is insulated with a hybrid insulation system comprised of two-inch spray polyurethane closed cell foam and unfaced fiberglass batts. The other wall is insulated with five and half inches of open cell foam and has a variable vapor resistance smart vapor retarder attached to the interior surfaces of wood studs. The previous study on these walls without ventilation between the stucco and the building paper showed high relative humidity and high moisture content in the exterior sheathing for a long period. The field test data from this study are discussed with respect to the previous results. This paper will describe the hygrothermal performance of these two walls and emphasize the need for clear design guidance for the application of spray polyurethane foam insulation in the wood frame wall systems in the Pacific Northwest.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings XII, December 2013