Field study of hygrothermal performance of highly insulated wood-frame walls under simulated air leakage

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Climate change has led to governments mandating greater energy efficiency in building codes throughout Canada and the US. Increased energy efficiency requires a high level of insulation of the building envelopes, which may introduce unintended durability concerns. To investigate potential durability issues, six types of high thermal resistance (High RSI) wall assemblies were tested at a building envelope test facility. This study compares the hygrothermal performance of conventional 38 × 140 mm framing (i.e. baseline wall) with three types of deep cavity walls and three types of exterior insulated walls. The wall assemblies tested have an average installed thermal resistance of RSI 6.8 and they were installed at both north and south-facing orientations of the building envelope test facility to observe their relative performance. The hygrothermal conditions of test walls were monitored under three consecutive periods: baseline period without air leakage in fall/winter time, air leakage period with air injection in winter/spring time, and drying period without air leakage in spring time. The hygrothermal performance in terms of moisture content profile of OSB sheathing and mold growth risk at the interior surface of OSB sheathing was analysed with respect to wall types and orientation. The exterior insulated walls had lower moisture content than deep cavity walls and the baseline wall throughout the whole monitoring period, and there was no condensation or mold growth risk under simulated air leakage. Although the deep cavity walls had higher moisture content levels than the baseline wall, they were less responsive to air leakage than the baseline wall due to the hygroscopic characteristics of cellulose fiber insulation installed in the stud cavity. The deep cavity walls had higher risk of moisture problems than the baseline wall and the exterior insulated walls.