A large experimental study on 8 different building materials is going on in collaboration with VTT and TTY, Finland. These materials are tested as pure materials and as a part of building envelope constructions both in laboratory and outdoors. The laboratory tests are made partly under constant, but different conditions, and partly under varying conditions to simulate the effect of fluctuated wet, dry and frost periods. In the field exposures outdoors, the materials and constructions are tested under real conditions. The main aim of this experimental programme is to use the measured results in developing of an improved model to predict mould growth. This improved model is based on an existing mathematical model developed on wood at VTT during 1990's. The focus in this new model is to extend the use for other materials, too, and also for a greater variety of conditions. This paper presents how the existing model is improved and also discusses the whole problem of modelling such a phenomenon. A sensitivity analysis of modelled mould growth and comparison with measured mould growth are also presented. The experimental set-ups and the results since 2005 from these extensive measurements on many different materials are presented more detailed in another paper to this conference.