The international building physics toolbox (IBPT) is a software library specially constructed for HAM system analysis in building physics. The toolbox is constructed as a modular structure of the standard building elements using the graphical programming language Simulink. Two research groups have participated in this project. In order to enable the development of the toolbox, a common modelling platform was defined: a set of unique communication signals, material database and documentation protocol. The IBPT is open source and publicly available on the Internet. Any researcher and student can use, expand, and develop the contents of the toolbox. This paper presents the structure and the backbone of the library. Three examples are given to visualize the possibilities of the toolbox.