Ventilated flat roofs: A simplified model to assess their hygrothermal behaviour

The hygrothermal behaviour of the opaque building envelope (facades and roofs) is a topic worthy of investigation for the scientific community, especially in consideration of changes in construction techniques and lifestyle habits. For the presence of waterproofing layers, placed in the outer part of a structure, hygrothermal behaviour of flat roofs is of particular interest. In this paper, an original analytical model for evaluation of the hygrothermal behaviour of ventilated flat roofs is proposed. In particular, the authors provide a method for assessing the ventilation air flow rates necessary to avoid interstitial condensation in flat roofs. Dependency of the necessary ventilation air flow rates on geometrical, thermophysical, and climate parameters is investigated and discussed. After a simplified analytical treatment applicable to any type of ventilated flat roof, some examples are shown, referred to study cases of flat roofs that are very widespread in Italy. The results obtained by the authors and discussed in the paper can be useful for architects and building engineers during the early design stage of buildings.