This paper focuses on the environmental impacts assessment module recently implemented in an energy balance tool. It describes the data and the methodology used to evaluate these impacts. It is based on a life cycle approach and includes not only the energy consumption during the operation phase, but also the impacts of the contruction materials, from manufacturing to waste disposal. This tool has been used within the framework of a national projet to perform a life cycle assessment of 20 different buildings located in Switzerland. One of these case studies is presented in this paper.