Rehabilitation of historic settlements should be a main architectural field for the 21st century. In order to promote that issue, we need to asset evaluations of the energy performance of the towns and buildings of the past and especially of museums where the same antiquities are exposed. In this respect both quality and quantity of environmental improvements are crucial. As we say, historic buildings used to be sustainable but this feature is often neglected when conventional techniques for retrofit are employed. For example inadequate lighting, air-conditioning or acoustics abound in modern restorations[1]. But it is also important to question the efficacy of non-passive systems in the sense that the original atmosphere of the exhibits might be disturbed or even completely destroyed for the sake of commodity and ignorance of modern technicians and narrow-minded conservers. We could then arrive for instance to the absurd that, today, it may well be impossible to see an “inca” or “maya” statuette by the light of day, as if the “mayan” people had fluorescent lighting! We would present in this paper some European projects in which museums of painting and archaeology have been retrofitted according to an environmental point of view. The results are cost-effective but not only economically speaking because the climatic originality of the enclaves as been enhanced in the first place as a way to preserve and in the second place as a means to add value to the findings and exhibits that users and an increasingly tourism-based society expect to see.