A performance-based service life design format based on climatic exposure on one hand and “biological wood resistance” on the other hand is presented in this paper. Important concepts are the introduction of limit states as well as reliable performance models for the biological processes, by which onset of deterioration can be predicted for arbitrary climate histories. Examples of applications are assessment of risk for onset of mould growth in the building envelope and decay of wood commodities exposed to outdoor climate above ground. A method for service life design related to risk for mould growth on wood in the building envelope is demonstrated in more detail. In the building envelope, which separates exterior climate from the indoor environment, the microclimate will vary significantly with time. Whether microbial growth will occur or not, depends on humidity, temperature, duration of exposure and type of material (substrate) on which the growth may take place. A mould resistance design (MRD) model is used by which onset of growth can be predicted for an arbitrary climate history of combined relative humidity  and temperature T. The model is calibrated and verified against a comprehensive set of experimental data describing mould development on wood specimens (spruce and pine) as a function of exposure of relative humidity and temperature. The predicted response shows reasonable agreement with experimental observations, although biological processes of this type display great variability. Practical application is demonstrated by assessment of mould risk based on results from simulations of an external wall design with hygro-thermal computer software (WUFI). The results show that a generally applicable, quantitative model can be used as a powerful tool for moisture safe design of the building envelope, in combination with currently available building physics software. It is discussed how reliability and risk concepts can be implemented into the service life design process for wood. At the present state of knowledge, verification of service life design methods in this area need to be performed through reality calibration against well known and widespread solutions in existing buildings, where acceptable performance has been proven by use in practice over long time. Design methods can also be verified against documented performance in specific cases where the performance is known from reality. Both of these strategies are illustrated in the paper.

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