To Beat the Heat, Cities Mandate Cool Roofs

Atlanta just passed a law that makes cool roof technology obligatory for all new construction

Summer in cities can feel like living inside an oven because roads, rooftops, and sidewalks soak up so much sun and trap heat. This is called the urban heat island effect. In fact, cities can be up to 10 °F (5.6 °C) hotter on average than nearby rural areas. On really hot days, roofs can be 27 °F (15.0 °C) hotter than the air around them, making the city even more sweltering, while the buildings below those roofs work their ACs in overtime, pumping even more hot air into streets to keep the indoor spaces cool.

Heat Island Effect temperature representation over Atlanta, GA. Photo: Pixabay

This crisis is well known to cities all around the world, as we've shown in our reports on temperature increases. In response, in Atlanta, the city government just passed a law that requires all new roofs to be “cool roofs,” as NPR reports. These roofs act like sunscreen on your house—they’re built or coated with light-reflecting materials that bounce sunlight away instead of soaking it in, much more efficiently than just painting your roof white.

If just 50% of the roofs were cool, Atlanta could cool down by 1.4 °F (0.8 °C); if 80% switch, it might get up to 2.4 °F (1.3 °C) cooler, and the hottest neighborhoods could cool by as much as 6.3 °F (3.5 °C).

Atlanta, GA, where the municipal government just mandated "cool roof" technology. Photo: Pixabay

Because rooftops get so warm, the air inside buildings gets hotter too, so air conditioners have to work extra hard—and that makes electricity bills go up. Cool roofs help save money: in air-conditioned homes they can reduce peak cooling demand by 11–27%, and in homes without AC they can lower indoor max temperatures by 2 to 10 °F (1 to 6 °C).

Savings by California region for cool roof technology applied to different types of roofs. Source: California State Government.

In California, cool roofs can save from $500 to $3000 per year in electricity costs, depending on the part of the state and its micro-climate. In places like New South Wales, Australia, dark-colored roofs can add around 38% to cooling costs, which in some cases is up to $700 more each year. Switching to cool rooftops would save hundreds if not thousands of dollars in cities where the cost of living is already extremely high.

Over the past 50 years, parts of the U.S. have seen big jumps in hot summer temperatures, especially at night. For example, Houston’s nighttime temperatures rose by about 5.8 °F (3.2 °C) over the last five decades, Dallas–Fort Worth's average night temperatures rose by 5 °F (2.8 °C), and Tyler by almost 7.8 °F (4.3 °C). That just means people—and buildings—don’t cool off at night anymore, and the heat of each day adds on to the previous.

Data source: ECMWF ERA5. Summer temperature data for heat islands in 65 cities was selected and analyzed with a reference average summer temperature from 1950-60, and then displayed with a 10-pt binomial smoothing. Analysis: Daniels Family Sustainable Energy Foundation

Cool-roof technology isn’t only smart—it’s vital for survival. In sub-Saharan Africa, equatorial regions, and other places where electricity and air conditioning are rare or too expensive, cool roofs are a lifesaver. They keep buildings cooler naturally, without needing power or air conditioning.