Study of thermal performance of extensive rooftop greenery systems in the tropical climate

Abstract

Singapore is a Garden City. On the other hand, with increasing population and limited territory, the government had to adopt a high-density and high-rise residential strategy. The high concentration of buildings in cities has resulted in many environmental issues, such as the Urban Heat Island effect. The rising ecological concerns for building design attempt to create harmony between buildings and their surroundings through mitigating their negative impact on the environment. The greening of buildings is essentially one of the ecological measures. The National Parks Board (NParks) launched a skyrise greenery approach/programme recently to promote the greening of buildings. To explore the thermal benefits, a before and after measurement and an experiment were carried out. The green roof tends to experience lower surface temperature than the original exposed roof surface, especially in areas well covered by vegetation. A maximum temperature difference of 18 °C was observed. However, the substrate temperature measured can exceed the surface temperature of the original exposed roof when the substrate is very dry. In areas that tend to be sparsely covered by vegetation, the peak temperature recorded was up to 73.4 °C during the day time. Ambient air temperature, correspondingly, at 300 mm above the substrate surface can reach 40 °C. This is worsened by the lower wind speed recorded after the measurements. The resulting substrate moisture, when the green roof systems are well covered by vegetation, will tend to keep substrate temperature lower than the original exposed bare roof. The heat flux through the roof structure was greatly reduced due to the installation of extensive systems. Maximally, over 60% of heat gain was stopped by the system. The impact of different types of vegetation may vary as well. Those with relatively extensive greenery coverage led to better thermal performance.