In 2017, CU Boulder engineers turned glass microspheres into a 50-microliter film; 10 to 20 square meters of it could help cool a family-sized house

The film is about as thick as kitchen aluminum foil, slightly thicker, in fact. A layer of silver lies beneath it. That was all the researchers at the University of Colorado Boulder needed in 2017 to create a material that will cool any roof, or any surface covered by it, including when exposed to full sun rays, using no power or water. According to CU Boulder's announcement of the research, even 10 to 20 square meters of such film covering your roof can noticeably cool a whole family-size home in summertime.

The material is simpler than it may first appear

The researchers used minute glass spheres, measured in micrometers, and randomly distributed them throughout a polymer film. These tiny glass spheres scatter sunlight that falls on them, preventing the surface beneath from absorbing the light. Beneath the film is a silver layer that reflects any sunlight that passes through the glass layer. The resulting film is about 50 micrometers thick, roughly comparable to kitchen aluminum foil. That matters operationally because the film can be manufactured on rolls, much like cling film.


Why sunlight usually wins, and how this film beats it

All surfaces on Earth radiate heat as infrared energy, and some of that energy escapes into outer space. But during daytime, the incoming energy from the sun overpowers the natural tendency to radiate. The CU Boulder coating addresses both problems at once. It reflects almost all incoming solar energy while still allowing the surface to radiate heat upward into space. The peer-reviewed study, titled "Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling," published in Science and indexed in PubMed, reported a cooling capacity of 93 watts per square meter at noon under full sun.

What 10 to 20 square meters actually looks like on a roof

Lab results matter most when they translate into real-world building performance. Study co-author Gang Tan, of the University of Wyoming, noted that just 10 to 20 square meters of film on a rooftop could effectively cool a single-family house during the hot summer months. That roof area is roughly the size of a two-car garage and can do some of the cooling work normally handled by the compressor. For homeowners considering other efficiency upgrades, the passive film could be another option.

The same film could help power plants and solar panels too

The material's uses extend beyond houses. Radiative cooling systems can require substantial water and energy to maintain safe temperatures, and the CU Boulder scientists developed the film for that purpose. The Advanced Research Projects Agency-Energy has supported research on the film since 2015, specifically to explore radiative cooling for practical power plant use. Solar panels could also benefit from the film through a small efficiency gain. Because solar panels become less efficient as they heat up, the researchers say the film could recover one to two percent of that efficiency. In an industry where efficiency gains are often measured in tenths of a percent, even a small improvement can matter.

A breakthrough, not a finished product

At the time of the announcement, the technology was still at the patent-application stage. They were working with CU Boulder's technology transfer office to build a 200-square-meter outdoor test installation in Boulder. What was truly groundbreaking about this study was not its immediate impact on how American houses stayed cool, but the scientific evidence that a rollable piece of film costing pennies per square meter could achieve such an effect through light alone.