The role of emissivity of the window surface inside and outside the atmospheric window in the radiative cooling effect

The emissivity design of building surfaces is crucial for radiative cooling (RC) heat dissipation. This work explores regularities of the influence of the emissivity inside and outside the atmospheric window (AW, 8-13 μm) on the window RC effect using a verified transient building heat transfer model. Results show that the RC effect brought by the radiant heat transfer of the window in non-atmospheric window bands (NAW) may exhibit a reversal phenomenon of first improvement and then weakening with the increase of the outer surface emissivity ε‾NAW. The occurrence of this reversal phenomenon is influenced by multiple factors such as meteorological parameters like ambient temperature and solar radiation, as well as architectural features including window orientation and window-to-wall ratio. In addition, as the window-to-wall ratio increases, when no reversal phenomenon occurs, the average RC regulation ability gap between the emissivity inside and outside the AW decreases from 7–40 times, which is observed when the reversal phenomenon occurs, to about 2–5 times, and the relative importance of ε‾NAW greatly increases. Regularities revealed provide more reliable guidance for the performance research and material design of RC windows.