Moisture Risk in Spray Polyurethane Foam Unvented Attics Due to Air Leakage Paths

IBACOS completed a high-level analysis of the moisture damage potential—caused by airflow leakage paths (cracks) to the outdoors—in unvented cathedralized attics insulated with closed-cell spray polyurethane foam (SPF) in cold climates. IBACOS began this preliminary analysis by collecting limited field data from three existing houses retrofitted from vented to unvented attics, focusing on a 2,050-ft2 house in Minnesota that is insulated with closed-cell SPF at the roof deck. IBACOS then used computational fluid dynamics (CFD) analysis to quantify the airflow rates through individual leakage paths, used CONTAM software to simulate hourly flow rates through the leakage paths, correlated the CONTAM flow rates with indoor humidity ratios from Building Energy Optimization (BEopt) software, and finally used Wӓrme und Feuchte instationӓr Pro (WUFI) two-dimensional (2D) hygrothermal analysis modeling software to determine the moisture content of the building materials surrounding the cracks. The results indicate that localized damage from the high moisture content of the roof sheathing is possible under very low airflow rates. Although the study used existing houses, the results apply to new construction with similar leakage rates. Reducing the number of assumptions and approximations used in this project would produce more accurate understanding of the real-world moisture damage potential in unvented attics. This would include collection of more field data to better define the leak types, as well as laboratory measurements to characterize the flow and pressure relationships at attic leakage pathways.

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