This Coating Can Make Streets Cooler

A California engineering professor has jumped from reflective roofing to layered-on sealants.
Illustration: Inkee Wang for Bloomberg Businessweek

Dark asphalt soaks up rays from the sun, exacerbating the ­problems of a warming climate. Los Angeles is trying to mitigate that heat with CoolSeal, a sealant applied on top of set asphalt that reflects solar rays instead of absorbing them. George Ban-Weiss, a professor of civil and environmental engineering at the University of Southern California, says the coating, which is based in part on his research, should last five to seven years at a cost of $25,000 to $40,000 per mile. He says a test by city officials in L.A.’s Canoga Park neighborhood reduced street temperatures by about 10 degrees.

CoolSeal’s water base and lighter silver coloring help reflect 30 ­percent to 35 percent of light, compared with the 10 percent reflection rate of standard asphalt.

On the half-block of Canoga Park where it was tested, ­average pavement temperatures on hot L.A. days fell from 149F to 139F.

Emerson Unitarian Universalist Church’s parking lot with CoolSeal in Canoga Park.
Source: GuardTop

Ban-Weiss, whose previous research helped lead to more reflective roofing on California buildings, began working in 2015 to determine which streets could most benefit from reflective pavement. His sponsors include the National Science Foundation and the California Energy Commission.

The Los Angeles City Council has set aside $150,000 to lay patches of CoolSeal on 15 streets, one in each councilor’s district.

Small sealant maker GuardTop LLC is manufacturing CoolSeal. Ban-Weiss says cooler surfaces could be worthwhile anyplace with high dependence on air conditioning, but warns that their extreme reflectivity could be uncomfortable for pedestrians. Given the option on a hot day, he says, “it’s better to stand under a shady tree.”

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