Highly reflective roofs can decrease the energy required for building air conditioning, help mitigate the urban heat island effect, and slow global warming.  However, these benefits are diminished by soiling and weathering processes that reduce the solar reflectance of most roofing materials.  Soiling results from the deposition of atmospheric particulate matter and the growth of microorganisms, each of which absorb sunlight.  Weathering of materials occurs with exposure to water, sunlight, and high temperatures.  This study developed an accelerated aging method that incorporates features of soiling and weathering.  The method sprays a calibrated aqueous soiling mixture of dust minerals, black carbon, humic acid, and salts onto preconditioned coupons of roofing materials, then subjects the soiled coupons to cycles of ultraviolet radiation, heat and water in a commercial weatherometer.  Three soiling mixtures were optimized to reproduce the site-specific solar spectral reflectance features of roofing products exposed for 3 years in a hot and humid climate (Miami, Florida); a hot and dry climate (Phoenix, Arizona); and a polluted atmosphere in a temperate climate (Cleveland, Ohio).  A fourth mixture was designed to reproduce the three-site average values of solar reflectance and thermal emittance attained after 3 years of natural exposure, which the Cool Roof Rating Council (CRRC) uses to rate roofing products sold in the US.  This accelerated aging method was applied to 25 products–single ply membranes, factory and field applied coatings, tiles, modified bitumen cap sheets, and asphalt shingles–and reproduced in 3 days the CRRC’s 3-year aged values of solar reflectance.  This accelerated aging method can be used to speed the evaluation and rating of new cool roofing materials.

Haley E. Gilbert (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Sarah Quelen (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Lea Marlot (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Chelsea V. Preble (Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA, USA), Sharon Chen (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Amandine Montalbano (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Olivier Rosseler (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Hashem Akbari (Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Canada), Ronnen Levinson (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA), Hugo Destaillats (Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA)