Mould resistance design (MRD) model for evaluation of risk for microbial growth under varying climate conditions

Abstract

The risk for microbial growth depends on the microclimatic conditions at material surfaces. In the building envelope 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). A limit state for onset of mould growth is here defined as a prescribed level observed by microscopy (40×) in laboratory tests. In this paper, a mould resistance design (MRD) model is proposed by which onset of 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 new experimental data describing mould development on wood specimens (spruce and pine) as a function of exposure of relative humidity and temperature and material and surface characteristics. The exposure in the tests comprised both steady and time-variable conditions.Application of the MRD-model 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 together with building physics software can be used as a powerful tool for moisture safety design in practice.The model is designed to facilitate continuous improvement by further laboratory testing of various materials under specified climate conditions. The MRD-model is controlled by a basic parameter in the form of a critical dose Dcrit, which depends on the substrate or material surface on which growth may take place.