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

• We exposed for two years 12 roofing membranes in Roma and Milano (Italy).
• Membranes with initial albedo 0.80 lost 0.14 in Roma and 0.22 in Milano.
• We computed the impact of albedo loss on the energy needs of commercial buildings.
• Ageing reduces the cool roof cooling load savings of 14–23% in Roma or 20–34% in Milano.
• An aged roof (albedo = 0.56) may be even 16 °C hotter than when new (albedo = 0.80).

Highly reflective roofs, widely known as cool roofs, can reduce peak surface temperatures and the energy required 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, for example, 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−2 y−1 per 0.1 loss in reflectance. When the buildings are non-insulated, the savings reduction is 58–71 MJ m−2 y−1 in Milano and 70–84 MJ m−2 y−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 0.80.