Hygrothermal impact of water resistive barriers on mould growth in wood-frame wall assemblies

• High-precision testing and empirical characterization of three WRBs • Spatial modeling of WRBs with quantified moisture transport properties • Wall configuration and climate effects on the severity of WRB performance outcomes • System-level mould index comparison for three wall types and four climates • Seasonal mould-index peaks analyzed across the wall types and climates . Water-resistive barriers (WRBs) are essential in building envelope systems, especially in wood-frame structures where moisture-related decay is a major concern. This study delivers a detailed experimental characterization of three WRB membranes, spun-bonded polyolefin (SBPO), a self-adhesive film, and building paper, quantifying key transport properties such as vapor permeability, liquid diffusivity, moisture storage, and saturation content using high-precision gravimetric methods. The moisture-dependent functions were integrated into DELPHIN hygrothermal simulations, and mould growth risk was assessed using the VTT model across varying wall configurations and climates. The results show that although material properties like vapor permeability and liquid diffusivity strongly influence wall moisture behavior, they are not sufficient on their own to ensure effective moisture control. Wall design and climate conditions also play a critical role in determining overall performance. Results show that in cold-wet conditions, wall types I and III incorporating the high vapor-resistance SBPO membrane exceeded the ASHRAE 160 mould index threshold of 3.0 at the outer OSB surface, indicating visible mould potential. In the warm-humid climate, Wall Type II showed a marked increase in mould index at the inner OSB surface, as the drying direction reverses relative to cold climates. Under these conditions, it behaved oppositely to Types I and III, with greater inward moisture accumulation. These results first show that mould severity is higher on the outer surface of OSB; second, show that WRB vapor resistance influences the magnitude of mould index in a climate-dependent manner, higher resistance increased mould severity in cold climates by limiting outward drying, while in warm-humid climates higher liquid water transport governed mould accumulation due to inward drying. Moreover, the timing of mould peaks was controlled by climate and wall configuration rather than membrane selection. These results, derived from precise laboratory measurements and system-level simulations, form the core contribution of this research and support a data-driven framework linking WRB properties to wall durability while guiding membrane selection across different climates and wall designs. .