Impact of weathering and soiling on the solar spectral reflectance of clay roofing tiles: two years of natural exposure in Roma and Milano

Weathering and soiling affect the optical and radiative response of building envelope materials to solar and infrared radiation. Because of the ageing processes, very bright materials lose their solar reflectance over time, while for dark materials may show a slight increase. The maximum losses in reflectance usually occur in the visible region of the solar spectrum, because of UV ageing, and especially because the solar absorbance of black carbon decreases with wavelength. Thus, considering materials with the same initial solar reflectance, those with higher near infrared reflectance should provide a better potential of retaining their initial optical performance. Natural clay tiles are widespread on Italian roofs, and they provide high reflectance in the near infrared, since they contain iron oxides. Herein we report the results of two years of natural exposure in the urban environments of Roma and Milano of two types of clay roofing tiles. The two products, selected from the market, are a traditional terracotta tile, with initial solar reflectance equal to 0.47, and a version of the same tile with a white rutile titanium dioxide based paint on top, with initial solar reflectance equal to 0.73. Their solar spectral reflectance was measured at 3, 6, 12, 18, and 24 months of exposure. The solar reflectance loss after 2 years is then equal to 0.15 for the white tiles, and equal to 0.11 for the terracotta ones.