Influence of Freeze–thaw ageing on moisture dynamics and mould growth in building envelope materials following accidental water exposure

Freeze–thaw (FT) cycling and moisture accumulation are critical degradation mechanisms that compromise the long-term durability of building envelope materials (BEMs), particularly in cold, humid climates such as Eastern and Central Canada and in Nordic region. This study investigates the influence of FT cycling on the moisture dynamics and mould resistance of BEMs, including gypsum board, oriented strand board (OSB), pine, plywood, and hempcrete. The FT procedure consisted of three immersion–freezing–drying cycles conducted in accordance with ISO 16987. The susceptibility of each material to mould growth was evaluated under both non-aged and FT-aged conditions following 24 h of partial water exposure. Mould growth was evaluated using the VTT model and was correlated with moisture dynamics measured by single-sided low-field proton nuclear magnetic resonance (¹H SS NMR) moisture profiling and relaxometry. The findings show that pine and OSB remained resistant against mould growth under 24-hour water exposure conditions due to their limited permeability. Gypsum board and plywood exhibited rapid mould growth due to high water uptake, whereas hempcrete demonstrated susceptibility despite moderate water content, attributed to its highly porous structure and nutrient-rich hemp shives. FT ageing caused material-specific changes, for example, pine developed hornification that reduced water uptake, plywood experienced adhesive degradation that smoothed moisture pathways, and OSB and gypsum board showed swelling and surface deterioration without significant changes in moisture profiles. In hempcrete, Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy (SEM) showed that FT cycles triggered a dual degradation pathway involving the leaching of soluble organic compounds from hemp shives and calcium-bearing phases from the binder. These processes increased pore connectivity, enhanced moisture retention, raised free water content by approximately 135% (from 74 to 174 kg/m³), and led to more severe mould development compared with the non-aged condition. Overall, FT-induced changes in moisture behaviour did not automatically increase mould risk across all materials; among those tested, only hempcrete exhibited a clear coupling between FT degradation, increased water uptake, and deteriorated mould performance.