This article introduces recent advances in the development of a new multi-zone simulation model prototype for coupled heat, air, moisture, and pollutants transport in buildings. The enhanced capabilities of the model will be demonstrated by application to a reference test case that deals with mold risk inside a library building. Selected rooms of the library have been monitored by measurements and visual observations. Mold was found even on historical books. With the aid of numerical simulation, the complex phenomena that have led to this critical situation were investigated and can be better understood.