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Highly reflective building envelope materials are widely identified as an effective design option to limit the peak surface temperatures of roofs in summer conditions, thus mitigating the urban microclimates and the energy demand for cooling. However, especially surfaces having high solar reflectance are subject to soiling (i.e. deposition of soot and other airborne particles), in addition to ageing, and biological growth. All these processes reduce the reflectance of bright surfaces (and increase the reflectance of surfaces having reflectance lower than roughly 0.20). As a result, a decrease in the solar reflectance of a cool roof yields to higher surface temperatures and a reduction in the energy savings expected thanks to high albedo roofing. Furthermore, the durability is also impacted.
To quantify this effects, we exposed in the urban environment in Milano and in Roma (Italy) 14 roofing membranes, including synthetic, factory applied coating on synthetic membranes, field applied coatings on modified bitumen, asphalt shingles. For each product class (e.g. synthetic membranes) we selected a high reflectance product and a mid-low reflectance one. We measured the UV-Vis-NIR spectral reflectance of three samples per product before the exposure – begun in April 2012 – and after 3, 6, and 12 months. Herein we present the results of the first year of natural exposure, reporting a remarkable loss in the solar reflectance, sometimes exceeding 15% of the initial value already after the first three months of natural exposure and in some cases by more than 30% after one year, depending on their initial value.
With the measured curves of solar reflectance over time as input data, we performed finite differences numerical modelling (by means of the software tool WUFI 5.2) of heat and moisture transport through typical roof assemblies. We analyzed the variation in the surface temperature and heat flux due to the change in solar reflectance, and we obtained relevant differences. For instance, in case of a highly reflective flat roof (with initial solar reflectance equal to 0.852, reduced to 0.624 after one year) over 14 cm of expanded polystyrene on a reinforced concrete slab (U-value of the roof assembly equal to 0.267 W m-2 K-1) we computed a significant increase in peak surface temperatures (during the first summer up to 7°C, and up to 14°C more at the end of the first year , assessed in the context of Milano, Italy). Knowledge about the soiling trends for different building envelope materials allows to better estimate the cooling energy demand of buildings, plan cleaning and maintenance operations (whether viable and sustainable), and to assess the service life and the life cycle, and eventually study the benefits of possible anti-soiling treatments.