Evaluation of Urban Green Rooftops on Temperature Reduction and Energy Saving Potential: Case Study of Seoul National University Civil and Environmental Engineering Building Rooftop

Abstract

Urban green infrastructure (UGI), such as green roofs, are forms of innovative approach to land development that have been widely implemented as they are expected to reduce buildings energy consumption, as well as to minimize the impact of urban heat island through roof surface temperature reduction. In this study, an investigation was conducted to evaluate the temperature dynamics and energy saving potential based on the result of monitoring four different rooftop types, including concrete, conventional/convex green roofing, innovative design of concave green roofing, and pond of the Seoul National University (SNU) Civil and Environmental Engineering Department building for one year, from January 2017 to December 2017. The innovative design of concave green roof was developed in 2012. It has a 320 mm retention wall surrounding the vegetation, and a 65 mm retention board underneath the soil. Most convex green roofs do not have a retention wall and their drainage boards typically have a maximum depth of 25 mm. Furthermore, pond, a foundation that is able to withhold an ample amount of rainwater, which is also a promising UGI, is often neglected as a method to reduce the roof surface temperature in the summer and stabilize it in the winter.

In the summer, the concave green roof reduced the temperature to a greater extent than the concrete surface, with a significant difference of 17.5 ℃, followed by pond with 17 ℃. In the winter, the concave green roof had a higher temperature than the concrete surface due to its lower albedo, with a maximum difference of 5.6 ℃. These temperature results revealed that the indoor space under the concave green roof had the lowest average of heat gain and heat loss with 2.63 kWh/day and 3.9 kWh/day, respectively, compared to the concrete rooftop with 11.1 kWh/day and 8.9 kWh/day, respectively. In conjunction with those results, to reach the standard comfortable indoor temperature, the concave green roof had the lowest cooling and heating hours of approximately 266 hours and 2,533 hours, respectively, which greatly reduced the consumption of cooling and heating energy. Likewise, the concave green roof excelled in many aspects, including technical, economic, and social, compared to the conventional solution for energy saving, the solar panel. Through this research, it is evident that the concave green roof is the most suitable approach to overcome the limitations of a normal rooftop surface or the convex green roof, which is the staple of UGI and often used to stabilize building rooftops temperature fluctuation. Securing the roof surface temperature in order to maintain a comfortable condition for the indoor space is proven to be compulsory, as it also leads to the improvement of long-term energy saving performance of a building.