Millstein and Menon were the first

In cities, people experience intense thermal discomfort during hot days in outdoor or non-air-conditioned indoor environments. With its ability to reduce surface and air temperature, reflective materials undoubtedly modify the thermal comfort in a built environment. Human thermal comfort is a rather subjective measure that is related to relevant environmental, physiological and other aspects that affect the comfort level of human bodies. Environmental factors that contribute to human thermal comfort include environmental temperature (though quite vaguely defined), humidity, wind speed, radiative exposure, ambient evaporative and sensible fluxes, etc.; a latest survey on this topic can be found in [15]. The use of reflective materials mainly modifies the environmental temperature and radiative exposure, which in turn lead to the change of thermal comfort level. In non-air-conditioned indoor environment, reflective roofs reduce indoor temperature by reflecting solar MSDC-0160 and preventing the transfer of heat into the buildings. Synnefa et al. [97] evaluated the effect of reflective roofs in 27 cities around the world with TRNSYS thermal simulation software. They assessed the impact on indoor thermal comfort by using threshold temperature from the ASHRAE standard 55 [118]. Results showed that increasing roof albedo by 0.4 could reduce hours of discomfort by as much as 75% during the summer for a threshold value of 27 °C. For a threshold temperature of 29 °C, reductions in hours of discomfort are 5–97% and 9–100% when albedo is increased by 0.4 and 0.65, respectively. Using the same software, study of a cool roof application on a 700 m2 roof in Sicily found that hours with temperature exceeding 27 °C could reduce by up to more than 40% in studied rooms under non-insulated conditions [54]. After accounting for building insulations, the reduction was about 20%. Enhanced indoor thermal comfort by reflective roofs is also reported in Barcelona, Palermo, and Cairo [119].