A process for evaluating the benefits of near-infrared reflective roof coatings used on asphalt shingle roofs
Powers, Catherine N.
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Reflective roof coatings keep the roof cooler by minimizing solar absorption and maximizing thermal emission. Keeping the surface of the roof cooler allows less heat to be conducted into the interior of the building which reduces the cooling load in air-conditioned buildings and improve comfort conditions in non-air conditioned buildings. A number of cool white materials, compatible with most roofing products, are available on the market. To appeal to homeowners, special cool “color” products have been developed to match the dark colors of conventional residential roofs but are highly reflective in the invisible near-infrared (NIR) spectrum. Although many studies highlight the benefits of cool white coatings on roof membranes of low-slope roofs, knowledge of NIR reflective coatings on asphalt shingles of steep slope roofs remains limited. The intent of this exploratory study is to present a process that can be used to evaluate the perceived and actual benefits of NIR coatings field-applied to asphalt shingles on single-family houses. The proposed process can be applied to a large sample of homes and occupants in a future study. A questionnaire was designed to attempt to evaluate occupants’ perceived benefits in regards to their indoor environment and occupant satisfaction following applications of NIR coatings. Along with subjective data collection, a field-experiment was developed to objectively compare the thermal performance of an NIR reflective field-coated asphalt shingle roof system with that of a conventional asphalt shingle roof system. Questionnaire results indicated that occupants did not perceive any significant changes to their indoor environment but were satisfied overall with the application and appearance of the roof coating. Additionally, 50% of occupants stated that their monthly energy costs somewhat decreased after the application. Interestingly, 63% of respondents experienced some form of roof leak following the coating application. Among those who experienced roof leaks, 100% of the roofs were 10 years or older. Field results showed that the coated roof surface was 2 to 5℉ cooler than the uncoated roof surface at midafternoon. Statistical testing for correlation between coated roof surface temperature and external conditions revealed that relative humidity was negatively correlated with coated roof temperature, while solar altitude angle was positively correlated with coated roof temperature. Multiple linear regression analysis was used to develop a model for predicting the surface temperature of the coated asphalt shingle roofs from the ambient temperature, sky conditions, dew point temperature, relative humidity, solar altitude and azimuth angle.