Optimizing inner and outer surface radiative properties of exterior walls to minimize building energy use and urban heat release

The demand for building cooling needs is increasing every year, driven in part by the large number of existing buildings with uninsulated exterior walls. Retrofitting these walls to improve thermal resistance poses several challenges, including limitations in available cavity space, potential reductions in interior space, and higher labor intensity compared to roof retrofitting. This study investigates the impact of modifying the radiative properties of both the inner and outer surfaces of exterior walls on minimizing air conditioning thermal loads in two hot-climate cities during both summer and winter periods. A validated one-dimensional heat transfer model is used for the numerical analysis. Phoenix, representing a hot arid climate, and Kolkata, representing a tropical climate, are selected as case study locations. The results show that applying a highly solar-reflective coating to the exterior wall surface, combined with a low-emissivity coating on the interior wall surface, leads to the maximum energy savings across both seasons. This combination not only reduces peak cooling demand but also contributes to mitigating the urban heat island effect. The added effective thermal resistance achieved by these surface treatments is estimated at 0.28 m²·K/W for Phoenix and 0.32 m²·K/W for Kolkata. Correspondingly, the reduction in thermal load through the exterior walls reaches up to 31 W/m² in Phoenix and 28 W/m² in Kolkata. The impact of the outer surface’s thermal emissivity is found to vary depending on the local climate and building characteristics.