The impact of uncontrolled relative humidity on mould growth within timber-framed low-rise external wall systems in temperate climates

The shift towards more energy-efficient building envelope systems, with improved building airtightness, and occupant controlled passive ventilation has been identified as a key factor contributing to mould growth. Internationally, hygrothermal and bio-hygrothermal simulation tools are widely used to inform building envelope design and minimize moisture accumulation. However, the standards and guidelines governing these simulations have been primarily developed for the northern hemisphere cooler climates, where buildings typically require continuous conditioning for occupant health and comfort. In contrast, some countries like Australia have diverse climate zones with regulatory frameworks that mandate intermittent heating and cooling of selected room types, without a requirement for 24h conditioning. Historically, Australia’s buildings have not necessitated strict numeric-based indoor relative humidity requirements, leading to gaps in moisture management strategies. This study investigates the effects of uncontrolled interior relative humidity and interstitial mould growth within the most popular Australian low-rise timber-framed external wall system: clay masonry veneer. The research focuses on three temperate climate types in southeastern Australia, NatHERS climate zone 21, 64 and 66, where current regulations do not mandate RH control below the internationally recommended threshold of 70%. Ten-year transient one-dimensional hygrothermal and mould growth simulations were conducted. The results revealed that due to insufficient control, some room types of frequently experienced relative humidity levels exceeding 90%, increasing potential mould growth risks. The findings highlight the urgent need for specific interior relative humidity control measures in building regulations to mitigate surface and interstitial mould proliferation, ensuring improved indoor air quality and occupant health.