This study investigates the risk of mould growth on sheathing boards in brick masonry walls in four Canadian cities: Vancouver, Ottawa, Calgary, and Saskatoon. The hygrothermal simulation tool WUFI® Pro 6.8 (1D) and the VTT Mold Index were used to conduct this investigation. The impact of moisture penetration through brick veneer cladding on the potential for mould growth in Oriented Strand Board (OSB), Fiberboard (FB), and Plywood (Ply) sheathing was assessed.
For hygrothermal simulations, a severe weather year was selected based on a 31-year historical weather dataset (1986-2016) using the severity index (Isev) method prescribed in the ASHRAE Standard 160-2021. The calculation period was set for seven years, and two wall orientations were considered: (i) direction with the least solar radiation and (ii) maximum wind-driven rain direction. For each orientation, three rain penetration cases (1%, 2% and 3% of wind-driven rain) were considered, and two Air Change Rates (ACH 0 and ACH 15) were considered in the drainage cavity for each of the three rain penetration cases. As per ASHRAE 160-2021, the rain penetration was deposited on the outer layer of the water-resistive barrier (WRB).
The results showed that for the 1% rain penetration and no ventilation, the mould growth index (MGI) for all three sheathing boards remained at zero (i.e., No mould growth) for Vancouver’s north-oriented wall (least solar radiation); however, the southeast-facing wall (maximum wind-driven rain) experienced a higher MGI (up to 5.3). For the same case (i.e. 1% rain penetration), the remaining simulated cities experienced MGI>5 (i.e., 50% visually covered surface). In the case of increased rain penetration and no ventilation, each sheathing board’s mould growth performance significantly decreased (MGI>5) for all four cities in both orientations; however, an air change rate of 15/hour (ACH 15) in the drainage cavity reduced the mould growth (MGI<1, local growth microscopic level) in Calgary, Ottawa and Saskatoon. In contrast, ACH 15 was insufficient to reduce the MGI < 3 (i.e., visuals of mould <10% surface coverage) for the Vancouver location, except for the 1% rain penetration case.