Frank-Eric Ngamassi, Zondits guest, 7/28/2022
As cities across the world are experiencing escalating temperatures, cool pavement approaches are being considered to reduce what is known as the urban heat island effect. Nearly 40% of American metropolitan towns are covered with pavement,1 most of which are made of impermeable materials that absorb and retain solar heat causing the pavement temperature to rise. This rise in pavement temperature subsequently results in a rise in surrounding air temperature, contributing to environmental and health issues. As metropolitan areas expand, cities are searching for ways to reduce this heat island effect, cool pavement offers one potential solution. However, studies conducted on cool pavement strategies show conflicting results.
Cool pavement products work by treating the surface of the pavement with a coating that reflects a higher proportion of incident light and solar radiation, thus preventing heat gain and keeping the pavement cooler. Multiple studies show that the implementation of cool pavement in hot regions result in a measurable reduction in surface temperatures. According to one study by Arizona State University and the City of Phoenix, the reflectivity of pavement treated with a cool pavement product along 36 miles of roads decreased the temperature surfaces by as much as 10 to 12 degrees Fahrenheit at noon compared with untreated streets.2 These measurements were done using vehicles measuring air temperature distribution and helicopters taking surface-temperature photos.
One concern with cool pavement is that the increased surface reflectivity actually redirects the solar radiation onto surrounding building structures, potentially causing air conditioning loads to rise and increasing energy consumption during peak cooling periods. In one study done in Phoenix3 a 40% increase in pavement reflectivity caused a 5.3-10.9% increase in peak cooling demand of nearby buildings. In another study, Hessam AzariJafari1 noted that “by reflecting light—called incident radiation—onto nearby buildings, cool pavements can warm structures up, increasing AC usage in the summer and lowering heating demand in the winter.”
Arizona State and UCLA also measured how pedestrians experience heat around surfaces treated with cool pavement. They used a mobile sensor station to measure mean radiant temperature or the heat load people experience from different directions on city streets. While the cool pavement was up to 6 degrees cooler than regular roads, the mean radiant temperature was 4 degrees warmer on the reflective streets.4
Based on these findings, cool pavements do have lower surface temperatures when measured during hot daytime periods. However, it’s clear that heat mitigation in hot climates is complex and requires detailed investigation to fully understand how cool pavement applications in a given city impact buildings, homes and people.
References:
- phys.org: Countering climate change with cool pavements
- ASU News: Trying to cool off neighborhoods with a new kind of road surface
- Urban Climate: Effect of reflective pavements on building energy use
- AZCentral: ‘Cool pavement’ experiments help urban planners find ways to ease rising temperatures
Other interesting links:
- EPA.gov: Heat Island Compendium
- CoolCalifornia.org: How Cool Pavements Work