The Impact of Variability in Moisture Storage Properties of Wood Based Sheathing on Enclosure Durability Due to Temperature and Weather-Based Ageing

A key component of building performance with respect to durability and energy efficiency is at the meso-level, i.e., the wall assembly and components. Performance of these assemblies can be determined through in-situ experimental work (such as building and monitoring a test hut), or by hygrothermal modelling. Modelling is highly dependent on inputs including material data, thermodynamic equations, and weather data. This research examines material moisture storage, which is not well measured in comparison to thermal and vapour resistance. A parametric study was completed to determine the effects of variable moisture storage properties on mould growth risk based on commercial hygrothermal modelling software. These results demonstrated that variations in moisture storage via the sorption isotherm of as little as 10% can increase mould growth risk from a low to high category. Plywood and OSB samples were then measured during the experimental phase. Accelerated ageing was completed on some samples, and samples were measured under varying temperatures. The results of the measurements demonstrate that age may not have a significant effect on moisture storage, but temperature variations of approximately 15°C can result in sorption isotherm variation of, on average, 10%. Finally, the measured results were input into hygrothermal modelling software to compare mould growth risk using sorption isotherms that were adjusted for age and temperature. These results were also compared to in-situ test hut data from previous work. The use of sorption isotherms adjusted for age and temperature yielded lower mould index values of between 30% (exterior face of sheathing) and 60% (interior face of sheathing), demonstrating that using a single sorption isotherm in hygrothermal modelling of plywood and OSB wood frame walls over-predicted the mould growth risk.