Wood sheathing durability from moisture sorption isotherm variability due to age and temperature

Abstract

Hygrothermal modelling can be superior to experimental work in exploring durability of building components and assemblies, however, these models require accurate material property inputs. The results of hygrothermal models can be used to calculate mould growth risk, an indicator of building durability. Mould growth is particularly important in organic building materials such as wood. Wood framed buildings are extremely common in North American low-rise residential construction, and part of these systems is sheathing board. OSB and plywood are the most commonly used types of sheathing board. Material properties of OSB and plywood sheathing include moisture storage, represented by the sorption isotherm – however, some common hygrothermal modelling programs do not account for varying sorption isotherms. This study examines the variability of sorption isotherms in OSB and plywood sheathing after accelerated ageing and under different temperature conditions. The measured data was then input into hygrothermal models, and the results were post-processed to determine mould growth risk via the mould index, a scale for mould growth. Accounting for age and temperature resulted in differences of up to 60% in mould index, with the adjusted models generally displaying lower mould index values. Therefore, the use of a single static isotherm instead of one that accounts for age and temperature can result in the over-prediction of mould growth risk of up to 60% in low-rise residential wood framed walls using OSB or plywood sheathing board.