MEASUREMENT OF SOLAR REFLECTANCE OF ROOFS: EFFECT OF PAINT AGING AND A DISCUSSION ON ASTM E1918 STANDARD

Abstract This paper reports a study on the variation in the solar reflectance of fiber-cement roof tiles used in buildings at the Federal University of Santa Catarina (UFSC) applying the technical standard ASTM E1918. Laboratory tests were employed using the ASTM E903 procedures for comparison. The solar reflectance was evaluated for newly painted roofs and also for old roofs with different ages to compare their degradation over the years. In addition, to provide possible changes in ASTM E1918 limits, tests were performed to determine the influence of clouds, extended hours of measurement and the distance between the sensor (pyranometer) and the surface on the solar reflectance values. The results obtained for newly painted roofs revealed a considerable improvement in solar reflectance after the first paint layer. The mean solar reflectance value for the white roofs (painted and measurement taken in 2016) was 0.70. For the same roofs (samples), the mean solar reflectance value determined in the laboratory was 0.76. Results obtained in 2016 in the field for roofs painted in 2014 were in the range of 0.49-0.64 and for roofs painted in 2013 they were 0.59-0.64. The variation in the values obtained for the solar reflectance indicates that factors such as the skill of the painter, condition of the substrate, nature of the paint and the weather during the painting process can influence the final coating quality. The study also revealed that, in January (summer in the southern hemisphere), the results obtained in field tests on overcast or cloudy days differ from those obtained when the sky is clear by ± 0.02. Small changes ( ± 15 cm) in the sensor height did not appear to affect the solar reflectance values obtained. On applying extended times in the field tests (around 5 h), the differences in the measurements did not exceed 0.02.