Heat flux and seasonal thermal performance of an extensive green roof

Green roofs influence the overall energy balance of buildings. In this study, the thermal properties of a green roof are determined using field data gathered from an extensive 0.56 ha green roof in Syracuse, NY. Sensors installed at five stations across the roof measure temperature at four depths within the roof layers. Data have been gathered from September 2013 to September 2015. Heat fluxes range from −5.76 W m−2 to 9.46 W m−2. Negative (downward) heat flux is found during summer and early fall, and positive (upward) heat flux dominates during the heating season. Solar radiation can heat the upper layers of the roof significantly above ambient air temperatures during the summer. Accumulated snow acts as an insulator during the winter months. Thermal resistance, R, is determined during a two-week period with significant snow accumulation, during which time heat flow through the roof reached a quasi-steady state. Thermal resistance for the overall roof is found to average 3.1 m2 K W−1. The largest individual thermal resistance is from the extruded polystyrene insulation layer (R = 2.6 m2 K W−1). Overall, the green roof dampens the extreme responses often observed on urban roofs. Vegetation and substrate layers may be used in addition to insulation, but are not recommended in lieu of insulation for a Central New York climate.