As efficient control of HVAC systems becomes a focusin research on building energy efficiency, the limits ofbuilding simulation software in this respect are reachedquite soon. Co-Simulation of building envelope, HVACsystems and controls in the respective tools of the differentfields allows for easy collaboration between expertsfrom the different domains, and therefore is a promisingapproach. In this paper, the coupling of TRNSYS andSIMULINK using a co-simulation framework based onthe Functional Mock-up Interface (FMI) specification isdemonstrated and discussed with the help of an examplefrom the IEA SHC Task 44.