This paper attempts to outline the current state-of-the-art in the Czech Republic regarding the use of integrated building performance simulation as a design tool. Integrated performance simulation for reducing the environmental impact of buildings is illustrated by means of three recent HVAC selection and design studies for the new "Indonesian Jungle" pavilion in the Prague Zoo, for conversion of a historical building (water mill) into a museum, and for a city center office with thermal comfort complaints. The paper elaborates the modeling and simulation work that was carried out to support the design teams of the above projects. This includes a discussion on how the simulation results were transformed in relevant design information. The paper finishes by indicating directions for future work that is needed to be better equipped in order to address design problems such as the one indicated above