The Significance of Using Temperature-Dependent Material Properties in Hygrothermal Performance Assessment of Exterior Wall Systems

The reliability of hygrothermal models depends on the quality of their inputs. Conventionally, thermal and moisture properties are treated as temperature-independent, yet previous studies have shown that many of these properties are temperature-dependent. This paper investigates the impact of using temperature-dependent material properties on hygrothermal simulation results compared to standard temperature-independent properties at the building envelope level. A representative exterior wood-frame wall assembly is modelled with constant material properties, including water vapour permeability, sorption isotherm, and water absorption coefficient corresponding to values measured at 3 °C, 21 °C, and 45 °C, as well as a case in which the properties vary with temperature. The variation in hygrothermal response is evaluated under several scenarios, including different climates (Toronto and Vancouver, Canada), cladding types (fibre cement and stucco), sheathing materials (oriented strand board (OSB) and plywood), and with and without rain-penetration load. Results indicate that the variation attributed to temperature-dependent material properties was greater for Vancouver than for Toronto and increased with rain penetration. In particular, the choice of cladding seemed to have a greater impact than the choice of sheathing material, with stucco showing greater differences than fibre cement. Overall, however, the temperature at which material properties are defined has a minimal impact on hygrothermal simulation results and wall performance assessments, with maximum hourly differences in sheathing moisture content (MC) differences ranging from 1.18 to 4.32 wt% without rain penetration. These findings demonstrate that the use of temperature-dependent material properties does not have a significant impact on the hygrothermal simulation results or the performance assessment of exterior wood-frame wall assemblies.