5TH International Congress on Technology - Engineering & Science - Kuala Lumpur - Malaysia (2018-02-01)

Can Leca Be Used To Improve The Thermal Performance Of An Extensive Green Roof?

It is widely accepted that green roof systems exhibit varying thermal performance such as reduction in temperature and heat amplitude compared with the traditional roofs. Because the structural load of the majority of existing old buildings in Taiwan has frequently only met its minimum requirement of building weight load, hence the development of lightweight green roof system is urgent. Moreover, Huang, Chen and Liu (2017) found that the thermal performance in reducing the temperature of bare rooftop is contributed largely by the growth medium layer rather than the plant layer, hence lightweight expanded clay (LECA) was chosen to mix with the common garden soil to explore the possibility of developing a lighter growth medium with superior insulation capability. LECA is a lightweight aggregate, made from heated clay, with a hard ceramic shall and a porous core. Sailor and Hagos (2011) found that growing media incorporating slate as their aggregate had thermal conductivities that were two to three times those of media that used a porous silica-based aggregate. Field experiment of three stages was conducted from May to September on the rooftop of a multistory student dormitory in one university located in Taichung metropolitan area, the largest city in Taiwan located in a warm oceanic climate/humid subtropical climate (Cfa) zone. One control group (the 50 x 50 x 10-cm cement board capped with ceramic tile and with 50 x 50 x 4-cm Styrofoam board placed at the bottom) and four experiment groups (the 50 x 50 x 10-cm cement board capped with ceramic tile and with a 50 x 50 x 30-cm glass tank at the top, and with 50 x 50 x 4-cm Styrofoam board placed at the bottom) were prepared, and the following parameters were recorded using high-precision equipment: ambient temperature, relative humidity, solar radiation, surface temperature of the simulated bare rooftop (control group), and temperatures at the bottom of the growth medium (four experiment groups). The first stage involved comparing the thermal performance of extensive roof with four depths of traditional growth medium (pure garden soil): 10cm, 15cm, 20cm, and 25cm. The results suggests that a decline in marginal temperature reduction with increasing soil depth indicating that the pure-soil roof with 10 cm depth of growth medium was most efficient in reducing the surface temperature of bare rooftop, 16.24°C in maximum. The second stage involved investigating the temperature reduction of bare rooftop by the extensive roof with different proportion of LECA laid at the bottom. Because of the insulation, absorption, and evaporation effects of the pure soil layer, and additional insulation, absorption, and evaporation effects of the LECA layer, the 10% LECA and 40% LECA laid-at-the-bottom roofs, 15.87°C and 15.21°C, respectively, were shown to be lighter and superior than 0% LECA (pure-soil) roof (14.70°C) in temperature reduction of the bare rooftop. The third stage involved investigating the temperature reduction of bare rooftop by the extensive roof with 10% and 40% LECA laid at the bottom or mixed with soil without plant. The results suggest that the extensive roofs with LECA laid at the bottom provide superior effects in temperature reduction of bare rooftop than the extensive roof with LECA mixed with soil. In sum, 10% LECA laid at the bottom of the extensive roof is lighter and superior in temperature reduction of bare rooftop than the traditional extensive roof with common garden soil.
Yi-Yu Huang, Tian-Ri Ma, Yu-shi Wang