Effects of convection heat transfer on Sunagoke moss green roof: A laboratory study

The application of green roof has been encouraged to mitigate urban heat island occurrence and to provide ecosystem services in urban areas since a few decades ago. Because very few literature on the usage of moss in green roof implementation is available, this paper introduces the thermal performance of Sunagoke (Racomitrium Canescens) moss green roof in addressing urban heat island effect. In addition, the insights of evaporation cooling on Sunagoke moss are also discussed. The experiments were performed inside the Artificial Climate Chamber with the ambient temperature and humidity set to 30 C and 70% RH, respectively, to simulate a typical summer environment in Japan. The effects of convection heat transfer by means of wind velocity and irradiance were tested on a dry Sunagoke model house, a moist Sunagoke model house, and a control model house. By altering the wind velocity from 0 to 3 m/s, the convection heat was found to dominate the entire heat transfer in dry roof surfaces that lack evaporation (dry Sunagoke and control model house). In contrast, for the whole heat transfer process, the latent heat of moist Sunagoke green roof governed and diverted 70% by no-wind convection and 91% by wind-induced convection. The latent and convection heat of moist Sunagoke green roof were clarified to have inverse correlation to each other, and the diversion rose upon reaching wind-induced convection state. In terms of normalised temperature differences, the influences of the roof and atmosphere were balanced on dry Sunagoke moss in wind-induced convection, as the temperature ratios were close to 1. However, on moist Sunagoke moss, the evaporation consumed most of the heat, hence, the temperature ratio was lower than the other two model houses. The ratio further diminished near 0 in wind-induced convection. Nevertheless, the effects of wind velocity above 2 m/s were clarified identical, since no significant changes were found in the convection heat transfer coefficient, surface temperature, conduction heat flux, and interior temperature afterwards. (C) 2017 Elsevier B.V. All rights reserved.