Moisture and biodeterioration risk of building materials and structures

There are several biological processes causing aging and damage to buildings. Partly this is due to natural ageing of materials, and partly it is caused by excess moisture. The demands on durability, energy balance and health of houses are continually rising. For mould development, the minimum (critical) ambient humidity requirement is shown to be between RH 80 and 95 % depending on other factors like ambient temperature, exposure time, the type and surface conditions of building materials. For decay development, the critical humidity is above RH 95 %. Mould typically affects the quality of the adjacent air space with volatile compounds and spores. Decay development forms a serious risk for structural strength depending on moisture content, materials, temperature and time. The worst decay damage cases in Finland are found in the floors and lower parts of walls, where water accumulates due to different reasons. Modelling of mould growth and decay development based on humidity, temperature, exposure time and material will give new tools for the evaluation of durability of different building materials and structures. The models make it possible to analyse the critical conditions needed for the start of biological growth, but it is also a tool to measure the progress of mould and decay development under different conditions. Numerical simulation makes it possible to evaluate the risk and development of mould growth on the structure surfaces. Thus the moisture capacity and moisture transport properties in the material and at the surface layer have to be taken into account in the simulations. In practice there are even more parameters affecting mould growth, e.g. thickness of the material layers combined with the local surface heat and mass transfer coefficients. Therefore, the outcome of the simulations and in-situ observations of biological deterioration may not agree. In the present paper, results on mould growth in different materials and wall assemblies will be shown and models on the risk of mould growth and decay development will be evaluated