Indoors, a cool roof can help lower temperatures by several degrees. The impact is greatest on the floor directly below the roof. In homes, this can be the difference between life and death during heat waves.

A number of programs have demonstrated that reflective surfaces can reduce indoor air temperatures. In Philadelphia, the Energy Coordinating Agency upgraded rowhomes with a white roof coating and taught residents the proper use of window fans. They found air temperature reductions in the upstairs rooms of 5ºF (2.7ºC). The results are available in more detail here. In the developing world where informal dwellings are often too hot to enter during the day, the application of reflective roofs and walls made the homes livable for the first time. A video from !Kheis municipality in South Africa shows the impact of cooler surfaces on its residents.

Field studies of brick dwellings outside of Ahmedabad, India found that homes with light colored coatings applied experienced reduced indoor air temperatures and the temperature of residents’ skin by 5.5ºF (3°C).

Outdoors, reflective roofs reduce urban air temperatures during the day, particularly when paired with green infrastructure. Studies show they cool even more on very hot days.

A comprehensive review of UHI mitigation techniques deployed at a city scale found that, on average, a 0.1 increase in overall reflectivity resulted in a 0.5ºF decrease in average air temperatures and a 1.6°F decrease in peak air temperatures. As a point of reference, a 0.1 increase in overall reflectivity would be achievable by switching around 25% of a city’s roofs from dark to light color.

One particularly interesting study compared weather station data in the Almeria region of Southern Spain to similar surrounding regions. Almeria has a tradition of whitewashing its greenhouse that is unique to the region. As a result, Almeria reflects much more sunlight than its neighbors. Over the 20 years in the study, researchers found that Almeria has cooled 1.5°F (0.8°C) compared to the surrounding regions.

Newer research is indicating that impact reflective surfaces have on urban cooling actually increases during periods of extreme heat. Researchers in Guangzhou, China found that cool roofs reduced city air temperatures in the afternoon during heat waves by 2.2°F, compared to 1.4°F during typical summer afternoons.

Li and Bou Zeid 2013 find that “not only do heat waves increase the ambient temperatures, but they also intensify the difference between urban and rural temperatures. As a result, the added heat stress in cities will be even higher than the sum of the background urban heat island effect and the heat wave effect. Results presented here also attribute this added impact of heat waves on urban areas to the lack of surface moisture in urban areas and the low wind speed associated with heat waves. Given that heat waves are projected to become more frequent and that urban populations are substantially increasing, these findings underline the serious heat-related health risks facing urban residents in the twenty-first century.”