A lot of current buildings are operated energy inefficient and offer a great potential to reduce the overall energy consumption and CO2 emission. Detecting these inefficiencies is a complicated task and needs domain experts that are able to identify them. Most approaches try to support detection by focussing on monitoring the building's operation and visualizing data. Instead our approach focuses on using techniques taken from the cyber-physical systems' modeling domain. We create a model of the building and show how we constrain the model by OCL-like rules to support a sound specification which can be matched against monitoring results afterwards. The paper presents our domain-specific language for modeling buildings and technical facilities that is implemented in a software-based tool used by domain experts and thus hopefully providing a suitable contribution to modeling the cyber-physical world.