Effect of ageing on solar spectral reflectance of roofing membranes: Natural exposure in Roma and Milano and the impact on the energy needs of commercial buildings

Highly reflective roofs, widely known as cool roofs, can reduce peak surface temperatures and the energyrequired to cool buildings, mitigate urban microclimates, and offset CO2. However, weathering, soiling, and biological growth affect their solar reflectance. In this study, the solar spectral reflectances of 12 roofing membranes were measured before the exposure and after 3, 6, 12, 18, and 24 months of natural ageing in Roma and Milano, Italy. The membranes with an initial solar reflectance greater than 0.80, forexample, decreased in reflectance by 0.14 in Roma and 0.22 in Milano after two years. Then, for a typical highly insulated commercial building, the annual cooling load savings were calculated to be reduced by 4.1-7.1 MJ m-2y-1 per 0.1 loss in reflectance. When the buildings are non-insulated, the savings reductionis 58-71 MJ m-2 y-1 in Milano and 70-84 MJ m-2y-1 in Roma. Ageing yielded a reduction of the cooling load savings that could be achieved with a new white membrane of 14-23% in Roma and of 20-34% in Milano. Moreover, in Milano, an aged, white, highly insulated roof, which has a solar reflectance of 0.56, may reach a surface temperature 16°C higher than a new roof, which has a solar reflectance of 0.8