Influence of soil water content on thermal performance of green roofs

Abstract

Covering bare roofs with growing medium creates a bionetwork that has ecological benefits. This study investigates the effect of soil water content (SWC) on the thermal performance of green roofs. The objective is to determine whether green roofs are effective under various climatic conditions. Simulation of a direct application extensive green roof was done on the roof of the Hillman building at the University of the Witwatersrand in Johannesburg, South Africa. Measurements of SWC (measured as volumetric water content - VWC) and temperature were taken from December 2011 to January 2012. Laboratory soil bulk density tests were done once every two weeks. In order to observe the effect of SWC on thermal performance, average temperature (Tave) and amplitude (A) in temperature variation were plotted as a function of VWC. These parameters increased slightly with SWC in either vegetation or soil. SWC levels were higher in vegetation compared to soil. Heat capacity (Cv) and thermal conductivity (Kq) were also plotted as a function of VWC so as to see the effect on thermal properties. The relationship between Cv and VWC was linear. High SWC increased Cv, hence a reduction in both Tave and A was shown with increasing SWC. The trend for (Kq) was not clear due to inconsistences in the method used to approximate thermal diffusivity (Dq). Research on other methods for computing Dq is recommended. Kq appeared constant with increasing SWC. On the other hand, with evapotranspiration, as SWC increased, there is a tendency for VWC Vegetation to initially decrease. This trend may point out evapotranspiration as the dominant factor compared to the thermal properties. The study needs to be repeated in winter so that a comparison is done between summer and winter. Data should also be collected over a longer period of time.