Prestazioni termiche estive di tetti ventilati a manto discontinuo

This study investigates the thermal behaviour, during the summer season, of a light-structure pitched-roof building in which varies the air-permeability between the elements of the waterproof layer (tiles), compared to a concrete flat roof building. The survey applies a finite elements numerical model where the fluid-dynamic problem was coupled with the heat transfer problem, then be solved in unsteady state in the summer condition set. This condition provides an hourly variation of solar radiation and wind temperature. Wind speed was assumed with a variable profile according to the altitude. By regulating the cooling power in order to reach the indoor set point (26°C), was evaluated the different performances of the pitched roof varying both the air-permeability between the waterproof elements and the functional layers of the roof. In the summer conditions, the air convective action in the ventilation layer of the pitched roof determines a high degree of temperature of the discontinuous tiled roof compared to the flat roof. Relatively to the adopted simplifications, the air ventilation performance is equally determined by the airpermeability between the tiles and the opening of the gutter section. With indirect evaluation, the impact of Buoyancy driven forces is drastically reduced also with of a moderate wind speed. Moreover, the available under-roof volume of a pitched roof allow to control the heat wave transfer towards the air-conditioned environment, balancing the lower thermal inertia of the light-structure pitched-roof compared to the higher mass of the concrete flat roof. This all leads to a reduction of energy requirement and a better indoor comfort thank to the lower radiation temperatures.