State of the art on the development of cool coatings for buildings and cities

Urban systems, from their early origins, were acknowledged to be responsible for both benefits and penalties due to anthropogenic actions affecting human wellbeing. In this view, local overheating exacerbated by urban heat island phenomenon has been identified as a result of anthropogenic actions responsible for citizens' health issues and other serious socio-economic consequences in urban areas, where almost the 70% of the world population is expected to live in thirty years. This fact imposes to respond to an urgent research question concerning the development and the real-world application of effective mitigation strategies against urban climate change phenomena, for a better population resilience. Among the variety of these strategies, the implementation of “cool” coatings over urban surfaces exposed to the solar radiation, i.e. cool roofs and cool pavements, represents a proved solution to counteract such overheating effect and its negative consequences on the population living in urban context. In this view, the present work reviews the state of the art about the development of new materials and their main applications as cool roofing and paving systems for passive cooling purpose of buildings and cities, which have been published in more than 260 papers in the last decades. Both indoor and outdoor passive cooling benefits were clearly demonstrated and quantified with varying climate conditions, material characteristics and the built environment context. Despite that, the investigation around this issue is still active worldwide, from chemistry, material science and engineering fields. Additionally, new triggers were highlighted as possible starting points for future scientific focus. In particular, still active discussions give rise to promising research findings expected to clarify (i) the effect of cool coatings on pedestrians' glare, possibly mitigated by directionally-reflective materials, (ii) the role of cool coatings for HVAC optimization, (iii) the combined benefits of cool coatings and thermal-energy storage techniques for UHI mitigation.