Design options for thermal performance of composite ventilated flat roof structures

Abstract

This study explores the potential for passive cooling by manipulating the traditionally monolithic flat roof structure in Malta. This is achieved by introducing a horizontal air cavity within the roof thickness, thus isolating the cast in situ structural concrete slab form the topping screed, typically laid to falls. The behaviour of this roof type was studied for the cases when the air cavity was both ventilated and non-ventilated. A test rig was set up under laboratory conditions and the thermal performance of these cases was tested for both the summer and winter conditions. A specimen representing the default roof construction system in the Maltese Islands was also tested so that comparison between the two systems was possible. Results suggest that the most effective use of a horizontal cavity would be in the hot season with the cavity ventilated. The dissipates the excess heat form the intense solar radiation, reducing indoor surface temperatures and alleviating discomfort conditions in upper floors, thus reducing the demand for cooling. Conversely, in the cold seasons, it is best to partly block the ventilation so that the air in the cavity helps in insulating the roof. Some trickle ventilation is still likely to be required to reduce condensation within the cavity.