So far, inverted roofs could only be insulated with a double layer—and therefore more energy efficiently—with individual approval. The Deutsche Institut für Bautechnik (DIBt, or German Institute for Building Technology) has now approved the double layer installation in gravel inverted roofs. Samplings and long-term studies of existing inverted roofs in Germany and Austria illustrate that Styrodur maintains its mechanical and physical properties over a very long period of time with virtually no variations. The accumulation of a permanent water film between the panel layers is largely prevented by the use of a water-draining and vapor-permeable separation layer. There is therefore no risk of excessive moisture accumulation in the lower panel layer, which could lead to a reduction of thermal insulation efficiency.