Measurements of water vapour permeability of oriented strand boards

Abstract

Oriented strand boards (OSB) are one of the most used elements in modern timber-based buildings. An important property is their permeability to water vapour. It is commonly characterized by the diffusion resistance factor (μ-value). It is not a constant, but a moisture-dependent function that is most often determined using the cup method. However, results found in the literature have large variance. This paper presents the results of carefully performed laboratory measurements using the cup method. Measurements were made at different sorption moisture contents of OSB. The measured μ-values ranged from 40 for samples with high moisture content to 160 for samples with low moisture content. The measured values were used to derive moisture-dependent functions that cover the entire hygroscopic range. They can be used in hygrothermal modelling of building envelopes containing OSB.