Not all energy-reduction measures involve retrofits. Indeed, in some cases, so called passive strategies can produce excellent results. Recent studies have shown, for example, that exploiting a building's thermal storage capacity can reduce air- conditioning, costs from 30 to 70 per- cent. The principle is simple: a building acts as a heat sink, which means that it stores heat and so(like a large cathedral) takes a long lime to reach the same temperature as the surrounding air. Thus, in summer it is more efficient to let the inside temperatures gradually rise over the course of the afternoon: at the end of the day(when the building is no longer occupied)