Among the most interesting exhibitors at the recent Greenbuild International Conference and Expo in Philadelphia may have been the Asphalt Pavement Alliance challenging what we thought we knew about urban heat island effect with new research from Arizona State University.
The term “heat island” describes urban areas that are hotter than nearby rural areas. The new research from ASU calls into question many common assumptions about the ability of reflective pavements to mitigate urban heat island effect.
Reflective surfaces redirect solar energy and for this reason high albedo, reflective, or “cool” roofs have been suggested as an important tool for urban heat island effect mitigation. However, efforts to apply the same principle to non roof hardscapes, including pavements, overlook the complexities of urban geography and how ground level reflections interact with pedestrians, vehicles, and the built environment.
The report, "Unintended Consequences: A Research Synthesis Examining the Use of Reflective Pavements to Mitigate the Urban Heat Island Effect," authored by Jiachuan Yang, Zhihua Wang, Ph.D., and Kamil E. Kaloush, Ph.D., P.E., of the ASU National Center of Excellence for SMART Innovations, pulls together research from around the world, including previously unpublished data from the team’s field research, that demonstrates the limits and side effects of relying upon reflectivity to reduce urban heat island effect.
Specific areas of concern identified include increased cooling loads (and energy costs) for buildings subjected to solar reflections, increased light pollution from illumination at nighttime, increased wintertime snow and ice buildup even with additional deicing salts, and even human health concerns from UV radiation to visual glare.
"Unfortunately, efforts to promote reflective pavements have moved more quickly than the scientific and engineering research. As this report indicates, reflective pavements may cool a pavement’s surface but there can also be negative environmental and social impacts on the areas adjacent to the pavement," said Heather Dylla, Ph.D., Director of Sustainable Engineering for the National Asphalt Association.
The International Green Construction Code mandates heat island mitigation for not less than 50% of site hardscape, including that the hardscape materials be light colored with a Solar Reflectance Index of at least 29. LEED credit SSc7.1 Heat Island Effect – Non Roof contains substantially the same requirement for 1 credit. And Green Globes offers 1 credit for at least 25% of a site hardscape having an SRI of 25.
Given the growing body of evidence of unintended consequences associated with reflective pavements and the potential negative impact they may have on energy usage, it is time the drafters of green building standards, rating systems and codes reevaluate the science and be prepared to eliminate provisions, including credits for urban heat island effect mitigation based solely upon a pavement’s reflectivity.
The report can be downloaded at http://ncesmart.asu.edu/news/unintended-consequences.