Optimized radiative parameters of building roof surfaces for energy efficiency: Case studies in China

To improve the thermal performance of the roof and building energy efficiency, this study investigates the optimized combination of the shortwave absorptivity and longwave emissivity for exterior roof surfaces. A simplified 3-day field measurement was first performed for model validation. Next, simulations were performed in 12 representative Chinese cities with both radiative parameters varying between 0.1 and 0.9 for the exterior surface of un-insulated and insulated roofs, focusing on the thermal load of the building. Experiment confirmed that WUFI Plus could reliably process the radiative boundary condition, with the daily surface temperature difference between the measurement and simulation being approximately 1 °C. Furthermore, simulation showed distinct requirements for the radiative parameters of the building roof surface, with regional and seasonal differences being significant. In general, for cold areas and seasons, a combination of high shortwave absorptivity and low longwave emissivity is preferred. Conversely, for hot conditions, a low shortwave absorptivity combined with a high longwave emissivity is optimal. For scenarios with balanced heating and cooling loads, optimization can only be achieved through case-by-case analysis. Finally, a map of optimized radiative parameter combinations for building roofs is provided, accounting for annual building energy efficiency.