In the wake of World War II, Germany had to reconstruct its cities. The huge task was undertaken with the resources available at the time, and everywhere arose the so-called Schuettbeton, buildings raised with the rubble of brick constructions destroyed during the war. The debris was crushed and used as dry filling for concrete. The resulting facades were very porous facades, with poor thermal properties. They also aged badly. This project works on one of those postwar buildings with ambitious energy rehabilitation criteria, in such a way that the standard consumption for heating has dropped from 273 to 11 kWh/m2year. This impressive reduction has been possible thanks to the incorporation of a double layer of polycarbonate, where the inner gap is actually a passive greenhouse that functions in the winter months as a hot air cushion, and in the summer season as a space that is ventilated naturally to dissipate thermal loads.