A normalization procedure to compare retro-reflective and traditional diffusive materials in terms of UHI mitigation potential

Nowadays cities constitute the major source of pollution and energy consumption worldwide: the continuous rise in population and the unstoppable expansion of urban boundaries cause the lack of green and open areas and the spread of built surfaces with low solar reflectance capability. This fact leads to multiple effects, e.g. the increase of anthropogenic heat and pollutant concentration, the reduction of evapotranspiration and therefore the growth of urban heat canyon configurations, which causes the trapping of the solar radiation within the urban environment. As a consequence, urban areas are typically characterised by higher temperatures with respect to the neighbouring rural ones; this phenomenon is known as the Urban Heat Island (UHI) effect and it has a strong impact in human health and energy use, mainly related to thermal discomfort and energy consumption increase, especially for cooling in summer. Several UHI mitigation strategies are already widely known, such as green roofs and vegetation, cool roofs, cool pavements and thermochromics materials. Traditional cool materials are characterised by high albedo properties and they have a diffusive behaviour with regard solar reflectance, so they reflect the incoming solar radiation following the Lambert law. For that reason, the application of diffusive cool materials for walls and building facades leads to the formation of multiple reflections due to the urban heat canyon configuration and inter-building phenomena. In this context, retroreflective (RR) materials could represent an effective solution for the mitigation of UHI phenomenon especially in urban heat canyon configurations, as they reflect the incoming solar radiation mainly towards the incident direction. However, it is difficult to assess benefits provided by RR materials compared to those obtained with the application of diffusive high reflective ones, because RR materials’ global reflectance is usually lower than that of diffusive surfaces. This paper is aimed at assessing a method to compare traditional diffusive and RR coatings: a procedure was proposed to determine a corrective parameter to be applied to RR materials with different global reflectance with respect to diffusive ones; this methodology allows to establish the effective improvements given by the directional reflectivity of an RR surface compared to the related diffusive one. For this purpose, an experimental campaign has been carried out by investigating the solar reflectance of a white diffusive, a beige diffusive and a RR material. Furthermore, also the RR reflection coefficient at backward direction and diffusive material reflection coefficient at normal direction have been measured for different incident angles with respect to the perpendicular direction. Finally, a corrective parameter has been calculated, in order to better compare the UHI mitigation potential of RR materials with respect to diffusive coatings, especially related to the major energy dissipation beyond the urban heat canyon.