Estimation of mould growth levels on rendered façades based on surface relative humidity and surface temperature measurements

Many façades made with thin rendering on thermal insulation have problems with biological growth. In this study, surface temperature and surface relative humidity were monitored over a 20-month period on test house façades with different constructions (thermal inertia), surface colour and compass directions. This data were used to test three theoretical indices of biological growth with the aim of indicating the potential of mould growth on different types of rendered façades. The results show that thin renderings on thermal insulation have significantly higher surface humidities compared to façade constructions with higher thermal inertia, and therefore have a higher potential for mould growth. The colour is the most important factor for the surface humidity levels on south-facing façades (in the northern hemisphere) as darker surfaces absorb more solar radiation and therefore have a higher average temperature. On a north-facing façade the heat storage capability of the façade and its effect on the surface temperature is most important.