One of the most discussed questions during the last few decades is the functionality of inverted roofs, with the thermal insulation of low water absorbing extruded polystyrene.
The main question is the one of heat loss caused by the cold rain water penetrating through the joints between the boards of thermal insulation itself and going towards the water outlet system beneath this insulation, respectively directly on the waterproofing. The snow melting period seems as the most serious from this point of view.
Two test sample frames were prepared for the purpose of investigation of this phenomena. One of them simulating the rain water outlet above the roof edge flashing and the second one for the simulation of outlet through the inner roof gully. The ratio of undergoing water in the various roof slopes in condition of artificial rain was measured. The ratio of undergoing water was actually high and that it decreased with the increasing slope, depending also on the scheme of water outlet. In case of installation of standard breathing membrane with the low equivalent diffusive thickness placed in the position of separation layer between the XPS insulation boards and the gravel ballast layer ratio of undergoing water became to be nearly inverted. This breather membrane took the function of certain rain water barrier in the structure.
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