Towards the analysis and optimization of a coupled building energy sector, various component model libraries for hydraulic, ventilation, electrical, control, and building domains exist. However, no uniform open-source framework to couple these domains in a holistic building energy system simulation exists. Thus, we present BESMod, an open-source Modelica library, providing a modular approach towards domain-coupled building energy system simulations. BESMod relies on existing component specialized model libraries for the underlying physics. For the analysis of complex system simulations, user-friendly parameterization, consistent model interfaces, precalculated KPIs and debugging options are applied. The library is available at www.github.com/RWTH-EBC/BESMod. This paper motivates the library, lays out the interaction with existing model libraries and the general modular approach. An exemplary use case demonstrates the applicability of BESMod. Concluding, we motivate future development options.