Effects of Plant and Substrate Selection on Thermal Performance of Green Roofs during the Summer

Publication date: Available online 12 March 2014
Source:Building and Environment
Author(s): Mingjie Zhao , Paulo Cesar Tabares-Velasco , Jelena Srebric , Sridhar Komarneni , Robert Berghage
Green roof assemblies influence the total roof surface energy balance for a building. The energy balance for a green roof depends mostly on the selection of plants and substrates suitable for the building’s location. This study measured thermal properties of common green roof materials and selected two types of plants and substrates to simulate transient thermal performance of different green roof assemblies. The selected plants and substrates have the highest and lowest reflectivity values to establish upper and lower bounds of thermal performance. The simulations use a previously developed green roof model including weather data for four cities representing different climate zones in the U.S. Based on the simulations, substrate heat fluxes and net radiation fluxes are compared for five days in July of the typical meteorological year. The results show that green roof assemblies receive net radiation fluxes that differ by 20%, and peak net radiation fluxes that differ by 16%, due to their different spectral reflectivity values. However, the substrate heat fluxes are similar for different green roof assemblies, as a roof insulation layer diminished this flux. Overall, the material selection of green roof assemblies is more important for buildings located in climate zone 4 or 5 than buildings located in climate zone 2 or 3, where limited water availability for evapotranspiration during hot, dry summers results in little thermal performance variability. Independent of the climate zones, simulation results show that the plant type has an important effect on the net radiation.