Mould resistance design for external wood frame wall systems : Simulation and evaluation of wall structures under varying conditions of exposure using the MRD model

Abstract: Moisture induced damages to building envelopes can result in microbial growth possibly affecting the health and wellbeing of occupants. Recent failing structures and damaged buildings indicate a lack of tools to estimate risk of mould growth and moisture damage. In this work a so-called mould resistance design (MRD) model has been applied for mapping the risk for mould growth on a number of wood-containing wall structures. The MRD model introduces an engineering approach to moisture safety design in a similar way as for structural design, where load and resistance is compared. The MRD model introduces and quantifies the concepts of climatic exposure and material resistance and compares them through an MRD index. This MRD index incorporates a limit state, which gives the critical dose of exposure for a given resistance to initiate onset of mould growth.   Three conceptual wall structures were evaluated and analyzed in terms of MRD index: two wall systems with an air gap and one wall system without. A parametric study investigating the effect of parameter variation on MRD index was conducted. Evaluated parameters were: climate (geographic location), orientation, air changes per hour in the air gap, driving rain penetrating the facade layer, exterior plaster properties and wood type. The simulations were performed using the hygrothermal calculation software WUFI. The results indicate that the wall systems with a ventilated air gap performs better in terms of MRD index i.e. suggests a lower risk of initiation of mould growth than the wall system without air gap. The results of orientation variation show that wall systems perform differently dependent on layering structure. The inherent water sorption properties of the exterior plaster are shown to have a large effect on the results. In addition, uncertainties were found on how to accurately include hydrophobicity as a parameter in the model. The report concludes that geographical location and its specific climate is the most important parameter to consider when designing for moisture safety. The MRD model is recommended to be used in combination with traditional moisture safety evaluation.