Bright is the new black—multi-year performance of high-albedo roofs in an urban climate

Abstract

High-albedo white and cool roofing membranes are recognized as a fundamental strategy that dense urban areas can deploy on a large scale, at low cost, to mitigate the urban heat island effect. We are monitoring three generic white membranes within New York City that represent a cross section of the dominant white membrane options for US flat roofs: (1) an ethylene-propylene-diene monomer (EPDM) rubber membrane; (2) a thermoplastic polyolefin (TPO) membrane; and (3) an asphaltic multi-ply built-up membrane coated with white elastomeric acrylic paint. The paint product is being used by New York City's government for the first major urban albedo enhancement program in its history. We report on the temperature and related albedo performance of these three membranes at three different sites over a multi-year period. The results indicate that the professionally installed white membranes are maintaining their temperature control effectively and are meeting the Energy Star Cool Roofing performance standards requiring a three-year aged albedo above 0.50. The EPDM membrane shows evidence of low emissivity; however this had the interesting effect of avoiding any 'winter heat penalty' for this building. The painted asphaltic surface shows high emissivity but lost about half of its initial albedo within two years of installation. Given that the acrylic approach is such an important 'do-it-yourself', low-cost, retrofit technique, and, as such, offers the most rapid technique for increasing urban albedo, further product performance research is recommended to identify conditions that optimize its long-term albedo control. Even so, its current multi-year performance still represents a significant albedo enhancement for urban heat island mitigation.

Land Surface Albedo Estimation from Chinese HJ Satellite Data Based on the Direct Estimation Approach

“…Surface shortwave albedo, defined as the ratio of the reflected to the incident solar shortwave radiation at the Earth's surface, is one of the key parameters in surface energy budget studies [1][2][3].…”

Urban vulnerability and resilience within the context of climate change

“…The use of clear material (roofs and facades) would allow us to catch less heat which would be reflected towards the space and limit heating the city. So, the attenuation of UHI could be possible by increasing albedo of buildings (Akbari et al, 2001(Akbari et al, , 2009Gaffin, 2012). Despite this solution for cooling a city, this cannot avoid extreme hot events as predicted in climate scenarios and lack of comfort in public space.…”

Appendix II: NPCC 2015 Technical Details

“…During peak sunlight times, membranes can easily reach surface temperatures of 170°F (77°C) (Gaffin et al, 2012). And, such peak temperatures do not require high summertime air temperatures, but are generally much more strongly dependent on incident sunlight conditions; it is sometimes the case that the surface temperatures are even higher during spring rather than summer, when less hazy urban air prevails.…”

“…Nocturnal temperatures as low as −11°F (−24°C) have been observed (Gaffin et al, 2012). The explanation for how temperatures can drop this low has to do with the fact that the rooftop is experiencing net negative longwave radiation imbalance, in that downward longwave radiation is emanating from high tropospheric altitudes with correspondingly low blackbody radiation temperatures.…”