Hygrothermal models are increasingly used as research tools by building scientists and and design tools by designers to simulate heat, air, vapour, and water movements within and through building envelope and even to predict the consequences in terms of mould/decay growth and metal corrosion. This work was designed to generate data on decay initiation under marginal moisture conditions to support the development of decay prediction functions for hygrothermal modelling. Following the first phase of the study under constant moisture conditions, Oriented Strand Board (OSB), Canadian softwood plywood and spruce heartwood were exposed to fluctuating relative humidity (RH) conditions to evaluate decay initiation and progression. After exposure to RHs fluctuating between 90% and 98%, no basidiomycete growth was found on solid spruce or plywood after 92 weeks, and only a few were attacked by decay fungi after over two years’ exposure. Basidiomycetes from natural spore infection were first found to grow in OSB after 39 weeks, and about 23% of the OSB specimens remaining after periodic bending tests were infected by basidiomycete during the two years’ exposure. However, decay was in general very minor on all the infected specimens, and the bending tests did not show appreciable reduction in stiffness or failure load, even for the samples with visible basidiomycete growth. Based on the data from the first phase of the study, the calculation of the RHT(95), developed by the National Research Council of Canada as a surrogate for a damage function, showed similar values at the time when decay initiated in both near-saturated RH condition and the 40% MC condition. This indicates that there may be an appreciable level of validity behind the RHT(95) index.

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