The paints, coatings, and chemicals making the world a cooler place

Modern approaches, as demonstrated everywhere from California supermarket rooftops to Japan’s Expo 2025 pavilion, go even further. Normally, if the sun is up and pumping in heat, surfaces can’t get cooler than the ambient temperature. But back in 2014, Raman and his colleagues achieved radiative cooling in the daytime. They customized photonic films to absorb and then radiate heat at infrared wavelengths between eight and 13 micrometers—a range of electromagnetic wavelengths called an “atmospheric window,” because that radiation escapes to space rather than getting absorbed. Those films could dissipate heat even under full sun, cooling the inside of a building to 9 °F below ambient temperatures, with no AC or energy source required.

That was proof of concept; today, Raman says, the industry has mostly shifted away from advanced photonics that use the atmospheric-window effect to simpler sunlight-scattering materials. Ceramic cool roofs, nanostructure coatings, and reflective polymers all offer the possibility of diverting more sunlight across all wavelengths, and they’re more durable and scalable.

Now the race is on. Startups such as SkyCool, Planck Energies, Spacecool, and i2Cool are competing to commercially manufacture and sell coatings that reflect at least 94% of sunlight in most climates, and above 97% in humid tropical ones. Pilot projects have already provided significant cooling to residential buildings, reducing AC energy needs by 15% to 20% in some cases. 

This idea could go way beyond reflective rooftops and roads. Researchers are developing reflective textiles that can be worn by people most at risk of heat exposure. “This is personal thermal management,” says Gan. “We can realize passive cooling in T-shirts, sportswear, and garments.” 

COURTESY OF SKYCOOL SYSTEMS

Of course, these technologies and materials have limits. Like solar power grids, they’re vulnerable to weather. Clouds prevent reflected sunlight from bouncing into space. Dust and air pollution dim materials’ bright surfaces. Lots of coatings lose their reflectivity after a few years. And the cheapest and toughest materials used in radiative cooling tend to rely on Teflon and other fluoropolymers, “forever chemicals” that don’t biodegrade, posing an environmental risk. “They are the best class of products that tend to survive outdoors,” says Raman. “So for long-term scale-up, can you do it without materials like those fluoropolymers and still maintain the durability and hit this low cost point?”

As with any other solution to the problems of climate change, one size won’t fit all. “We cannot be overoptimistic and say that radiative cooling can address all our future needs,” Gan says. “We still need more efficient active air-conditioning.” A shiny roof isn’t a panacea, but it’s still pretty cool. 

Becky Ferreira is a science reporter based in upstate New York and author of First Contact: The Story of Our Obsession with Aliens.