A performance-based service life design format based on climatic exposure on one hand and “resistance” of wood against mould growth on the other hand, is presented in this paper. A limit state for onset of mould growth is defined as the occurrence of traces of mould observed by microscopy. A dose-response model is proposed by which onset of mould growth can be predicted for an arbitrary climate history of combined relative humidity φ and temperature T. The model is calibrated and verified against a comprehensive set of experimental data published by Viitanen (1996) describing mould development on spruce and pine sapwood as a function of climatic exposure. The model is applied to predict time to onset of mould growth under natural outdoor climate (under shelter) as well as mould development in building attics and in crawl space foundations. The predicted response shows reasonable agreement with experimental observations and proven experience, although biological processes of this type display great variability. The results indicate clearly that a generally applicable, quantitative model can be used as a powerful tool for moisture design in practice. The model is designed to facilitate continuous improvement of prediction capability by further laboratory testing of various materials under specified climate conditions. It is concluded that the model in combination with currently available building physics software is suitable for moisture safe design of wood-based components in the building envelope. A further step should be to develop methods to manage uncertainties by introducing reliability and risk concepts into the service life design process.

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