Buildings account for 40% of the worldwide CO2 emissions. These emissions are directly related to their energy consumption. 80% of the design decisions impacting energy consumption are made during the first 20 % of the design process and therefore address the architect. Necessary decisions do not only concern building geometry but also materialization and building service systems. Choices in either of these fields significantly influence the future energy consumption of the building. Therefore it is necessary to support the architectsi decision-making. From the first sketch on, the evaluation of energy performance needs to be incorporated into the design process. This paper shows a method and results which where produced in a case study at the ETH Zurich by using a special tool in early design phases for energy and exergy analyses.