The relative role of solar reflectance and thermal emittance for passive daytime radiative cooling technologies applied to rooftops

Building roof surfaces can be 30–50 °C hotter than the surrounding air in summer, in turn, warming the air through convective heat flux. Advances in material science have enabled rooftop coatings with solar reflectance as high as 0.96 and emissivity approaching 0.97. We use building energy simulations to isolate how improvements in each rooftop radiative property impacts surface temperatures and heat fluxes. The analysis is conducted for two U.S. cities: Phoenix (a hot and arid city), and Atlanta (a hot humid city). Results show that use of rooftop materials with solar reflectance above 0.9 results in surface temperatures that are always below ambient air temperatures, even when the materials have conventional emissivity values. Specifically, increasing rooftop solar reflectance from 0.2 to 0.96, while fixing emissivity at 0.9, results in a mean reduction in the rooftop temperature of about 10 °C. Furthermore, the high reflectance roof results in a cooling of more than 30 W/m² during summer for both cities. On the other hand, increasing emissivity from 0.9 to 0.97 had little impact, suggesting that the focus of development efforts should be maximizing solar reflectance, provided thermal emittance values can be maintained at or above 0.9.