French buildings highly contribute to the total national energy consumption. In order to inflect theincreasing tendency, significant efforts have been encouraged by public institutions.Accordingly, the GENHEPI concept, hereunder described, aims at methodically investigate retrofitoperations to ensure an effective renovation of existing buildings. Its first phase consists in preparingand elaborating projects development by a global energy approach. Modelling and sensitivity studiesof various technical solutions permit this analysis. Then, an incremental improvement of performances is allowed by monitoring and interpretation of data obtained on each project. A multi competent team is gathered around the projects: researchers, architects, engineering offices, firms, banks and public institutions. Working together stimulates creativity which proves reliable on the currently carried out actions.The main objectives are focused on a reduced consumption of primary energy and a considerabledecrease of greenhouse gas emissions, while keeping in mind the economic optimization. On the firstdemonstration operations, the objectives amount to a total value of 50 kWh/m/an of primary energyfor heating and cooling supply, and aim at a reduction factor of 4 on the CO2 emissions.The case study of the first GENHEPI building makes it possible to illustrate the concept. Development of the analysis and various simulations resulting from TRNSYS 16 model are exposed and discussed in this document. Results underline the necessity of a detailed work on the building envelope. Beside, intelligent regulation of the multi source energy system is an important criterion for energy savings.