Illinois researchers tested a passive cooling film in Houston; treated metal roofs measured more than 40°F cooler in direct sunlight

American summers aren't merely hot anymore; they're a major public health concern. According to a 2021 NOAA report on data-driven insights and projectionsfor extreme heat hazards, heat is the leading cause of weather-related fatalities in the country, while the CDC predicts that more than 1,220 Americans lose their lives to heat each year, which is higher than the combined annual toll from flooding, tornadoes, hurricanes, and lightning.

Researchers in professor Lili Cai's lab at the University of Illinois Urbana-Champaign have developed a coating capable of keeping metal roofs more than 40°F cooler than untreated metal in direct sunlight, according to a press release from the university's Mechanical Science & Engineering (MechSE) department. The technology was field-tested this summer on public infrastructure in Houston ahead of the FIFA World Cup through a deployment by SolarMantle, which is a startup Cai co-founded in 2025, in partnership with the FIFA Sustainability Team and the City of Houston.

What actually happened in Houston


Aman Mehta, a MechSE alumnus of Illinois and currently the principal engineer at SolarMantle, initiated the project of applying the film on the roof during his undergraduate research in Cai's lab. The film requires no electrical power, no batteries, and nothing rigorous as such. All it does is reflect the sunlight to the atmosphere and emit heat absorbed by the roof that otherwise would be transferred to the building under it. According to the University of Illinois article, Mehta stated, “It was incredibly rewarding to stand on that rooftop in Houston and realize that research we worked on as students at Illinois was now solving a real problem for a major city. That’s what engineering should do — take ideas from the lab and create tangible impact in the world.”

How does a coated surface beat the sun?

It is a combination of optics and thermodynamics, meticulously designed. The subcategory of “passive daytime radiative cooling” technology was also mentioned in a research study conducted at Columbia University in 2018. The Science paper titled “Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling,” published by Jyotirmoy Mandal et al., found that a porous polymer film that can be applied to virtually any surface reflects more than 96% of incoming solar light and simultaneously releases energy in the form of long-wavelength infrared radiation straight into outer space, cooling surfaces even lower than the surrounding air temperature without using any energy.

Illinois has been working along those lines for many years. A 2023 study titled “Hierarchically Patterned Self-Cleaning Polymer Composites for Daytime Radiative Cooling,” published in the journal Nano Letters by a group led by Cai herself, notes that surface contamination from dust and bacterial buildup limits the practical use of these coatings, which is precisely the kind of subtlety that defines the difference between an effective laboratory demonstration and something that can actually be used on top of buildings in Houston. The self-cleaning composite the group developed is designed to resist exactly that kind of buildup.

What the 40-degree number also means and why this matters

The reduction of 40 degrees in surface temperature of the roof in a global sporting event is quite an accomplishment, but this does not mean that the material is affordable, durable, or sold at a hardware store. Radiative cooling films do have practical challenges related to weathering effects, scalability to city block size, and price per square foot against a gallon of white roof paint. However, the technology points in a promising direction. Air conditioning uses substantial energy and can add heat to urban environments. A roof coating that cools without electricity or moving parts is the kind of practical technology that deserves more attention. If SolarMantle turns the project into city contracts, it could become a notable product from a university engineering lab.