Effects of Air and Moisture Flows on the Thermal Performance of Insulations in Ventilated Roofs and Walls

Abstract

This paper examines thermal and hygric aspects of the performance of ventilated air spaces in both walls and sloped roofs. A ventilated air space between roofing tiles and the insulated roof deck is required by most European construction codes and standards. The upper side of the roof deck is covered with a membrane (called here, a roof deck membrane). Since this membrane performs the same func tions as the roofing felt or the asphalt impregnated paper used behind the air gap in the frame wall systems of North American houses, one may call it a weather barrier. Results from experimental tests on different roof-deck membranes available on the French market revealed that migration of water vapor in the roof is mostly caused by convection through small slits in the structure. The main functional contribution of the ventilated air space is, therefore, to reduce the amount of condensed moisture that remains in the structure, whether it is a wall or a sloped roof. It is important to underline that similarly to a modern wall construction, the roof ing deck must also be constructed as the airtight structure. Both through the numeri cal modelling presented in this paper, as well as through the results of in-situ mea surements reported in the literature, it is shown that for an airtight roof deck structure and air velocities observed in ventilated cavities, the effect of air ingress into the insulation is insignificant. This study also shows that ventilation below the roof deck membrane is unnecessary when vapor permeable or capillary roof deck mem branes are used.