Modeling the outdoor cooling impact of highly radiative “super cool” materials applied on roofs

Highly reflective “cool materials” are commonly used to reduce temperatures in the urban environment. Recently developed “super cool” materials feature an even higher albedo and emissivity (both above 0.95) than traditional cool materials. To examine the impacts of super cool roofing materials on outdoor air temperature compared to traditional cool roofs and green roofs, we conduct a sensitivity study with the microclimate model ENVI-met. Simulated surface temperature of super cool roofs remained around 6 K below ambient air temperature during high solar irradiation, which is consistent with observations. Super cool roofs – with an averaged street-level air temperature cooling of around 0.85 K in times of high solar radiation – provided 0.1 to 0.15 K more cooling than commonly used cool roofs and green roofs. Results also showed that super cool roofs could lower pedestrian-level air temperatures in some areas by up to 2.4 K. However, spatial analysis demonstrated that cooler air from roof level blocks the vertical air exchange of street canyons. Finally, the street-level cooling performance of all three roof types is predicted to decrease non-linearly with increasing building height by around 0.003 K per meter and to cease at building heights of around 100 m.