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

Several passive cooling design techniques are known for reducing solar heat gain through building envelope in summer season. These include the use of phase change materials (PCM), which has received an increased attention over the last years, and the strategy of increasing the above-sheathing ventilation (ASV) in ventilated roofs. However, few studies combine both technologies to maximise the building resilience in hot season. The effect of including a PCM layer into a ventilated roof is numerically analysed here in two different configurations: firstly, laid on the roof deck (PCM1 case) and, secondly, suspended in the middle of the ASV channel (PCM2 case). A computational fluid dynamics model was implemented to simulate airflow and heat transfer around and through the building envelope, under 3 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 the PCM1 case were smaller than half of those estimated for the benchmark case. However, PCM2 configuration achieved a daily reduction of about 10 Wh/m2 (18%) in building energy load with respect to the benchmark case, whilst PCM1 got only 4% due to the lower ventilation at night time. Therefore, a suspended PCM layer in the ASV channel would be a better measure in terms of energy performance than laid on the deck surface, although this last option significantly decreases thermal stress of the insulation layer.