A side by side comparison of the cooling effect of building blocks with retro-reflective and diffuse-reflective walls

Solar irradiance incident to an urban canyon with traditional diffuse reflective (DR) surfaces is subjected to multiple reflections, which increase the solar absorption of urban buildings and consequently contribute to the development of the unban heat island. Retroreflective (RR) materials direct the incident radiation back to its source and thus have the potential to reduce the heat intake of the urban surfaces. This study experimentally investigates the temperatures of the inner and outer walls of two small-scale building blocks with RR and DR surfaces, respectively. Two blocks stood side by side and were set at an experimental site such that both blocks were sunlit for a half day and were shaded for the other half day. It was found that the RR wall can keep the building blocks cooler than the DR wall during a sunny weather, and that the cooling effect is very small during a cloudy weather. The RR surface loses retro-reflectivity when the sunlight incident angle on the surface exceeds a certain angle (about 40°).