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Junk Science and the Heat-Island Effect

According to a new report, the use of reflective pavement to mitigate the urban heat island effect has unintended consequences

A “heat island” is an urban area that's hotter than surrounding rural regions because of the predominance of dark, heat-absorbing materials on rooftops and roadways. Reflective roofing is a common strategy for lowering urban temperatures, but a new report says that cool pavements create unintended problems.
Image Credit: Benoit Brouillette / CC BY 2.0 / Flickr

Among the most interesting exhibitors at the GreenBuild International Conference and Expo, an event held in early October in Los Angeles, may have been the Asphalt Pavement Alliance, a group that challenged what we thought we knew about the urban heat-island effect with peer-reviewed research from Arizona State University (ASU).

A “heat island” is an urban area that is hotter than nearby rural areas. The research from ASU calls into question many common assumptions about the ability of reflective pavement to mitigate the problem.

Reflective surfaces redirect solar energy. For this reason, high albedo, reflective, or “cool” roofs have been suggested as an important tool for lowering the urban heat-island effect. However, efforts to apply the same principle to non-roof hardscapes, including pavement, 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, was authored by Jiachuan Yang, Zhihua Wang, and Kamil E. Kaloush of the ASU National Center of Excellence for SMART Innovations. It pulls together research from around the world, including previously unpublished data from the team’s field research, that demonstrates the limits and side effects of relying upon reflectivity to reduce the urban heat island effect.

Reflective pavement can increase cooling loads

Some of the undesirable side effects associated with the use of reflective pavements 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 over UV radiation and visual glare.

“Unfortunately, efforts to promote reflective pavements have moved more quickly than the scientific and engineering research,” the report notes. “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, Director of Sustainable Engineering for the National Asphalt Association.

Two respected research teams are currently performing separate pavement research projects, with reports expected within the next year. The first is a study funded by the California Department of Transportation at Lawrence Berkeley National Laboratory, in conjunction with the University of California, Davis. It looks at the impact of pavement albedo as a heat-island mitigation strategy. The second team is the National Center for Asphalt Technology at Auburn University, working in conjunction with Iowa State University National Concrete Pavement Technology Center. Those researchers are conducting a pavement albedo aging study funded by the Federal Highway Administration.

In its adoption process for the upcoming version of Green Globes, the Green Building Council Consensus Committee is proposing the removal of urban heat-island requirements for hardscape.

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 for the drafters of other green building standards, rating systems, and codes to reevaluate the science and be prepared to eliminate provisions, including credits for urban heat-island effect mitigation, based solely upon a pavement’s reflectivity.

Stuart Kaplow is an environmental attorney and past chairman of U.S. Green Building Council Maryland. This post originally appeared at his website, Green Building Law Update.

One Comment

  1. Peter L | | #1

    The Heat Island Known As Phoenix, AZ
    Phoenix, AZ is about to shatter all time records for high-temps in October. This October 27th the Phoenix area might see close to 100F. On October 26th marked the 22nd day in a row at 90 or hotter and by the end of the week Phoenix will break the record long stretch of 90-degree days for October set back in 1952. Summer nighttime averages are climbing due to the heat island and expansion of housing deep into the desert areas.

    Deserts typically cool off at night but that is no longer true due to the concrete, asphalt and millions of tile roofs. The heat-island is alive and well in Phoenix.

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