Green roofs: determinants of thermal performance and impact on the local urban microclimate

With rapid worldwide urbanization, green roofs and more recently green walls are increasingly presented and adopted as efficient tools to improve the urban thermal environment. Although the effectiveness of green roofs in reducing sensible heat flux from urban zones and improving the urban microclimate has been analyzed and reported in many studies, their performance in temperate regions, and the meteorological and hydrological determinants of this performance, still require further investigation. We analyzed experimental results of green and reference conventional roofs in Beijing China, and Princeton, New Jersey, to assess and contrast their performance. Our findings indicate that the average surface temperature differences between green and reference roofs in the summertime are about –4.0 ºC in Beijing and –2.6ºC in Princeton, respectively, while in wintertime the green roofs were beneficially warmer than conventional roofs by +7.0 ºC in Beijing and and +18.9 ºC in Princeton. The collected data was then used to further develop and validate the Princeton Urban Canopy Model, by including a Richard's equation based model for the green roofs. The sensitivity of the thermal performance efficiency of green roofs to various hydrometeorological parameters was then analyzed using the model and the experimental data sets.