The WUFI® Mold Index VTT add-on was developed in collaboration between the Finnish research institute VTT and Fraunhofer IBP. Hannu Viitanen and Tuomo Ojanen have been dealing many years with the growth conditions of mould on wood and other building materials. The mathematic-empirical model predicts mould growth as a function of substrate material, temperature and relative humidity. For evaluation the so-called Mould Index (MI) was developed, which indicates the intensity of growth using an easy-to-understand six-point scale. It is also taken into account that growth can decrease during longer dry periods. The assessment of the risk of mold growth according to this approach has also been included in the North American ASHRAE Standard 160 “Criteria for Moisture Control Design Analysis in Buildings”.
The add-on allows an easy application of the Viitanen model [1, 2, 3] using the default setting for many different building materials. Based on measurements or hygrothermal simulation results the material surface can be assessed with just a few simple settings. The evaluation is supported by a traffic light scheme  which classifies the results considering the user exposure of the surface from uncritical (green) to inacceptable (red). In the yellow range, the user has to decide whether the growth risk for the specific component is still acceptable or already too high.
Literature A. Hukka and H. A. Viitanen, “A mathematical model of mould growth on wooden material”, Wood Science and Technology, vol. 33, no. 6, pp. 475–485, 1999. doi: 10.1007/s002260050131.  H. A. Viitanen, A. Hanhijärvi, A. Hukka, and K. Koskela, “Modelling mould growth and decay damages”, in Proceedings of Healthy Buildings, vol. 3, 2000, pp. 341–346.  H. A. Viitanen, Factors affecting the development of mould and brown rot decay in wooden material and wooden structures, English. Uppsala: Swedish University of Agricultural Sciences, Dept. of Forest Products,1996.  Viitanen, H.; Krus, M.; Ojanen, T.; Eitner, V.; Zirkelbach, D.: Mold risk classification based on comparative evaluation of two established growth models. In: Energy Procedia 78 (2015), pp. 1425-1430. (http://dx.doi.org/10.1016/j.egypro.2015.11.165).