Solar gain mitigation in ventilated tiled roofs by using phase change materials

International Energy Agency has recently warned about the fast growing of space cooling in buildings, especially in the hottest parts of the world. In this context, different passive cooling techniques, such as the use of phase change materials (PCM) in building envelope and Above Sheathing Ventilation (ASV) systems, have received an increased attention over the last years. PCMs have the capability to store solar radiation as latent heat and to increase thermal inertia of the building. Different experimental and numerical studies have demonstrated the ability of ASV in roofs to reduce solar heat gain. The efficiency of ASV systems, linked to external wind conditions, can be further improved by enhancing air permeability of tiles. Very few studies that combine PCM layers on the roof of the building together with ASV systems have been carried out, and none of them on ventilated roofs with increased air permeability. Thus, taken a model of an ASV system installed in an experimental mock-up (Ferrara, Italy) as a benchmark case, this work analyses the effect of including a PCM layer in two different configurations: first, laid on the roof deck (PCM1 case study) and secondly, suspended in the middle of the ASV channel (PCM2 case study). A 2D CFD (Computational Fluid Dynamics) model was considered to simulate airflow and heat transfer around and through the building envelope, under three days of extreme hot conditions in summer with high temperatures and low wind speed. Results showed slight differences in terms of mean temperatures at the different roof layers, although temperature fluctuations at deck in PCM1 case were smaller than half of those estimated for the benchmark case. PCM2 configuration achieved a reduction of about 10 Wh/m² (18%) in building energy load with respect to BM case over a 24h period. PCM1 got only 4%. Then, in this preliminary study authors have shown that, in terms of energy performance, a suspended PCM layer in the middle of the ASV channel would be a better measure than laid on the deck surface, although this last option would significantly decrease thermal stress of the insulation layer.