In this paper the temperature conditions on flat roofs are discussed considering the influence of long-wave radiation to the sky and the effect of a parapet as an important factor for the night-time roof temperature. The parapet leads to retention of cold air on the roof. Results of measurements on flat roofs and special meteorological data collected at the field test site in Holzkirchen are presented. For realistic assumptions of the boundary conditions a heat transfer coefficient depending on the ambient conditions including the temporary insulation effects of a cold surface air pond forming between the parapet walls is determined for flat roofs. The surface measurements on flat roofs with a parapet show a significant decrease of the temperature below the ambient air conditions during night time. These surface temperature recordings and meteorological data are used to validate hygrothermal simulations. Furthermore the possibility of interstitial condensation is investigated using an adapted hygrothermal simulation tool. The results show that the use of reflecting surfaces on flat roofs can lead to severe moisture accumulation in colder regions of Europe. Thus the color of the roofing membrane appears to be an important factor for the hygrothermal performance and moisture tolerance of certain constructions.