By Audrey McGarrell and Stacey Weister Cool Roof Rating Council

Heat is the leading weather-related cause of human mortality, surpassing hurricanes, tornadoes, lightning, and blizzards. The impacts of heat are intensified by the urban heat island (UHI) effect, which occurs when cities are hotter than surrounding rural and suburban areas. Architectural coatings can play an important role in UHI mitigation by reflecting solar radiation rather than absorbing it. By efficiently reflecting sunlight and emitting absorbed heat, cool exterior walls maintain a lower surface temperature, improve occupant comfort and safety, and reduce the demand for air conditioning, which decreases the building’s contribution to peak demand, greenhouse gas emissions, and the UHI effect. This article describes methods for evaluating the “coolness” of architectural coatings, including laboratory testing and natural weathering to determine the durability of the radiative properties. It also summarizes current technologies for increasing coating solar reflectance; discusses the latest research on the impacts of cool exterior walls; describes U.S. building codes, standards, and voluntary programs that require or promote their use; and explores strategies for discussing cool coatings with customers.


The urban heat island (UHI) effect is a growing concern among communities in the United States and across the globe. Architectural coatings that highly reflect solar radiation and emit absorbed heat are an effective tool to combat urban heat and make communities safer and more comfortable. This article outlines the issue of urban heat and offers “cool” coatings as one solution that is gaining traction. We explore how cool exterior walls work; how the cooling properties of products are evaluated; examples of current technologies; adoption in codes, standards, and programs; and how they fit into sustainability efforts and marketing in the architectural coatings industry.

The Urban Heat Island Effect

Heat is the greatest weather-related cause of human mortality—surpassing hurricanes, tornadoes, lightning, and blizzards.1 Extreme heat can also cause heat illness and increased respiratory and cardiovascular problems, which can strain health services, as well as create disruptions to key infrastructure such as power grids and water supplies.2 The negative impacts of extreme heat are intensified by the urban heat island (UHI) effect, a phenomenon in which cities are hotter than surrounding rural and suburban areas. According to the U.S. Environmental Protection Agency (EPA), daytime temperatures in urban areas are about 0.6 °C to 3.9 °C (1 °F to 7 °F) higher than temperatures in outlying areas, with nighttime temperatures about 1.1 °C to 2.8 °C (2 °F to 5 °F) higher.3 This temperature difference occurs because heat from the sun is retained in areas with a high concentration of buildings, parking lots and roads, and a lack of trees and green space (see Figure 1). Tall buildings that block or slow air movement, along with waste heat released by vehicles and air-conditioning units, contribute to the formation of UHIs. Smaller, more intense heat islands also exist within cities, and they can disproportionately affect low-income neighborhoods and communities of color.4,5,6

FIGURE 1 Explanation of the urban heat island effect. Image Credit: Cool Roof Rating Council


Although air-conditioning can help keep buildings safe and comfortable, increased air-conditioning use also releases waste heat into the environment, exacerbating already hot temperatures. Furthermore, not all homes and businesses are equipped with air conditioning. For example, the New York Times reported that, in the heat waves that occurred in Oregon and Washington state in June 2021, an estimated 600 more people died than would normally be expected during that time period.7 Fewer than two-thirds of homes in Oregon have air conditioning, and only 44% of households in the Seattle metropolitan area have it.8 In Multnomah County, OR, where Portland is located, none of the people who passed away during the 2021 heat waves had central air-conditioning.7

In addition to the immediate threats of extreme heat, hot temperatures also have indirect consequences, such as increased formation of ground-level ozone, a key ingredient in smog and a dangerous pollutant, especially for individuals with existing respiratory conditions.9 The increased energy used to power air conditioners during hot times can also lead to greater use of as-needed power plants, commonly known as “peaker” plants, which are large emitters of air pollution and contribute to blackouts and brownouts due to electrical grid strain.

Heat waves across the United States continue to be more frequent, longer, and more intense,10 and scientists predict that cities across the globe will be approximately 4 °C (7.2 °F) hotter by 2100 due to climate change.11 As such, solutions to keep buildings and communities cooler are increasingly important.

Continue reading in the March-April digital issue of CoatingsTech