Cool materials; solar reflectance; soiling; ageing; natural exposure

Highly reflective building envelope materials are widely identified as an effective design option to limit the peak surface temperatures of roofs in summer conditions, thus mitigating the urban microclimates and the energy demand for cooling. However, especially surfaces having high solar reflectance are subject to soiling (i.e. deposition of soot and other airborne particles), in addition to ageing, and biological growth. All these processes reduce the reflectance of bright surfaces (and increase the reflectance of surfaces having reflectance lower than roughly 0.20).