Laboratory Testing Of The Reflectance Properties Of Roofing Materials

Past simulation and test cell studies have shown potential reduction to building cooling loads from higher exterior surface solar reflectances (Givoni and Hoffman, 1968; Givoni, 1976; Chuan and Busching, 1983; Griggs and Shipp, 1988; Anderson, 1989; Akbari et al., 1990; Chandra and Moalla, 1991; Bansal et al., 1992). Furthermore, simulation analysis of macro-climatic effects have shown that light colored surfaces may reduce the magnitude of the urban heat island (Taha et al., 1988; Akbari et al, 1989; Akbari et al., 1992). Figure 1 shows a graphic illustration of the potential influence of surface reflectance on roof solar heat gain. The two photographs show three homes recently rebuilt in a South Florida neighborhood, taken both with a conventional and infrared camera. The roofs of the three homes are depicted in the infrared image at 4:53 PM on July 19th. The camera is looking west so that the roof sections seen are those facing east. The infrared color scale shows the surface temperatures of the captured image from 77°F (dark blue) - 113°F (white) in 3.6°F increments. All three homes have asphalt shingle roofs; the one nearest has dark gray shingles, the middle home has white shingles and the furthest house has cobalt blue shingles. The thermograph reveals the expected behavior: the dark gray roof has a surface temperature in excess of 113°F (white on IR scan) whereas the lighter colored white shingle roof is approximately 10°F cooler (red). The temperature of the blue shingle roof is in between (red-magenta). Recent monitoring experiments suggest that increasing roof solar reflectivity may significantly reduce cooling loads in occupied residential and commercial buildings (Boutwell et al., 1986; Akbari et al., 1992; Parker et al., 1993). Since roofs receive a significant concentration of incident solar radiation and are typically difficult to shade, they provide a natural focus to attempts to improve building solar reflectance. Other studies have identified solar reflectance as a primary property responsible for surface heat gain reductions (Anderson et al., 1991, Bretz et al., 1992). Several investigations have accumulated data on the solar reflectance of roofing materials (Wechsler and Glaser, 1996; Reagan and Acklam, 1979; Taha et al., 1992). However, specific data is still sparse and Rosenfeld et al. (1992) have identified the need for additional testing of roofing system types as well as uniform standards for testing of coatings and paints. Consequently, this report adds to the body of knowledge on the measured reflectance of roofing systems. Spectral reflectance properties were measured in the laboratory on 37 samples of differing roofing materials. A total of 21 of these were performed recently; the other 16 were performed last summer. Identical test procedures were used for all samples