By the EROS Project the potentials of a retrofitting process for a typical school building in West-Germany were to be demonstrated from an energetic point of view. Combining the renewal of the space heating system with the reconstruction of the building envelope yields synergetic effects. The project aims at minimizing the future energy consumption and optimizing the economy of the retrofitting at the same time. Thus, both operating costs and emissions are reduced. The goal is to improve the thermal insulation at least according to the standard of the German regulations on thermal insulation (1995) for new buildings.