Under specific hygrothermal (temperature and humidity) conditions, various surfaces in buildings can experience mold growth. In addition to obvious aesthetic and sanitary issues, mold growth is also a significant health risk. Mold spores are known to cause various illnesses in humans. To prevent mold growth it is important to develop methods that account for transient hygrothermal boundary conditions and inform designers if the environment would support mold growth. The most important factors are temperature, humidity, and substrate material.
A biohygrothermal model has been developed to assess mold growth under transient hygrothermal boundary conditions. The method is based on comparison of the measured or calculated transient boundary conditions with the growth conditions for typical molds found on building materials. More specifically, the moisture balance of the mold spores are modeled and compared with the critical water content at which spores would germinate. If germination would take place, the subsequent strength of the infestation can be estimated by comparing to previously determined growth curves. The critical climate conditions on any surface in question can be determined from measurements or from hygrothermal simulations with WUFI.
It is important to note that WUFI® Bio presents an assessment of the risk for mold growth and does not accurately simulate the actual growth processes. In particular, the model makes some conservative assumptions that result in over-prediction of mold growth. Moreover, the model is only suitable for interior surfaces. Outer surfaces have additional factors that cause mold growth (e.g., standing water, elevated humidity) as well as factors that mitigate mold growth (e.g., heating from sunlight, spore-kill by UV radiation, washing by rain). Applications in areas other than interior surfaces need to be considered on a case-by-case basis.
The basics of the coupled, biohygrothermal model (in German): K. Sedlbauer:
Vorhersage von Schimmelpilzbildung auf und in Bauteilen. (5.3 MB)
Dissertation Universität Stuttgart 2001.  K. Sedlbauer, M. Krus:
Schimmelpilze an Wohngebäuden – Altes Thema, neue Lösungen. (170 kB)
Beitrag zum 3. Dahlberg-Kolloquium “Mikroorganismen und Bauwerksinstandsetzung”, Wismar Sept. 2001