Modern approaches, as demonstrated throughout California supermarket roofs To Japan Pavilion at EXPO 2025.go even further. Typically, once the sun is up and radiating heat, surfaces cannot become colder than the ambient temperature. But back in 2014, Raman and his colleagues achieved daytime radiative cooling. They tuned photonic films to absorb and then emit heat in the infrared. eight to 13 micrometers— a range of electromagnetic wavelengths called the “atmospheric window” because this radiation escapes into space rather than being absorbed. These films can dissipate heat even in bright sun, cooling the building's interior to up to 9°F below ambient temperature, without the need for AC or power.
This was a proof of concept; Today, Raman says, the industry has largely moved from advanced photonics that exploit the atmospheric window effect to simpler materials that scatter sunlight. Ceramic cool roofs, nanostructured coatings and reflective polymers all offer the potential to reflect more sunlight at all wavelengths in a more durable and scalable manner.
Now the race continues. Startups like SkyCool, Plank Energies, Space coolAnd i2Cool compete to commercially produce and sell coatings that reflect at least 94% of sunlight in most climates and more than 97% in humid tropical regions. Pilot projects already has provided significant cooling for residential buildings, in some cases reducing AC power requirements by 15–20%.
This idea could go far beyond reflective roofs and roads. Researchers are developing reflective textiles that can be worn by people most at risk from heat exposure. “This is personal thermal management,” says Gan. “We can implement passive cooling in T-shirts, sportswear and apparel.”
COURTESY OF SKYCOOL Systems
Of course, these technologies and materials have limits. Like solar power grids, they are vulnerable to weather. Clouds prevent reflected sunlight from reaching space. Dust and air pollution darken bright surfaces of materials. Many coatings lose their reflectivity after a few years. And the cheapest and strongest materials used in radiation cooling typically consist of Teflon and other fluoropolymers, “forever chemicals” that do not biodegrade, posing an environmental hazard. “These are the best class of foods that tend to survive outdoors,” says Raman. “So for long-term scaling, can you do it without materials like these fluoropolymers and still maintain durability and achieve that low cost?”
As with any solution to climate change, one solution will not fit all. “We can't be too optimistic and say that radiative cooling can meet all our future needs,” says Gan. “We still need more effective active conditioning.” A shiny roof isn't a panacea, but it's still cool.
Becky Ferreira is a science reporter based in upstate New York and author First Contact: The Story of Our Alien Obsession.
