Purdue University created paint so white that a coated surface stayed 8°F below the surrounding air at noon

On a scorching summer afternoon, your roof can quietly become one of the biggest sources of heat inside your home. As it absorbs sunlight, it radiates that warmth into the rooms below, making your air conditioner work harder just to keep up. But what if a coat of paint could do the opposite? A team of researchers at Purdue University, led by mechanical engineering professor Xiulin Ruan, developed a white paint that keeps coated surfaces cooler than the air around them, something ordinary paint does not do. In outdoor tests, a coated surface measured 8 degrees Fahrenheit cooler than the surrounding air during peak sunlight at noon, and as much as 19 degrees Fahrenheit cooler at night. According to this Purdue University report, the formulation reflects up to 98.1 percent of sunlight, making it the whitest paint on record at the time of its release.

Most commercial white paints reflect only 80 to 90 percent of sunlight and still absorb enough heat to warm up rather than cool down. Purdue's paint sends heat away from a surface faster than the surface absorbs it, which is why it can drop below ambient temperature instead of merely resisting a rise in temperature.

Why this matters for your electricity bill
Space cooling accounts for a significant share of residential electricity use across the US, particularly in warmer states during summer. Ruan's team calculated that covering roughly 1,000 square feet of roof with this paint could produce a cooling effect of about 10 kilowatts, which they say is more than the output of a typical home's central air conditioner.


For homeowners managing rent and rising utility costs, that calculation may be worth noting. A coating that lowers a roof's surface temperature before the air conditioner even switches on points toward lower cooling loads, based on the researchers' own figures.

The chemistry behind the whiteness
Two design choices give this paint its extreme reflectivity. The first is a very high concentration of barium sulfate, a compound already used to whiten photo paper and cosmetics. The second is particle size variation. Because particles of different sizes each scatter a different part of the light spectrum, mixing sizes lets the paint scatter more sunlight overall rather than missing gaps in the spectrum.

Xiangyu Li, who worked on the project as a Purdue PhD student before becoming a postdoctoral researcher at MIT, explained that barium sulfate stood out after the team screened numerous white commercial products for reflectivity. Joseph Peoples, another Purdue researcher on the project, noted that a high concentration paired with a broad particle size range is what pushes reflectance to its ceiling. Researchers did test the limits of that concentration too. Push it too far, and the paint becomes structurally weaker and prone to peeling, so there is a practical ceiling on just how white the coating can go.

The 8-degree claim, supported by a published study
The 8-degree Fahrenheit drop at noon comes from the team's full study, published as the cover research in the journal ACS Applied Materials & Interfaces, in a paper titled "Ultra-White BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling." The paper confirms a solar reflectance of 98.1 percent for the paint form and reports a standard figure of merit of 0.77, a rating the authors describe as among the highest recorded for radiative cooling materials.

The researchers compared the barium-sulfate paint with a matching BaSO4 film and note that the paint used a 60 percent volume concentration to improve outdoor reliability. In field tests, they report the film stayed more than 4.5 °C below ambient at times, while the paint delivered similar cooling performance with added durability, underscoring the trade-off between durability and peak subambient cooling.

The research builds on six years of lab work at Purdue, itself following decades of broader attempts, dating back to the 1970s, to develop radiative cooling coatings as an alternative to mechanical air conditioning. The cooling effect was not limited to hot weather either. During a test at an outdoor ambient temperature of 43 degrees Fahrenheit, the paint still cooled the sample surface by 18 degrees Fahrenheit.

What comes next
Patent applications for the formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization, and the underlying work was supported by Purdue's Cooling Technologies Research Center along with the Air Force Office of Scientific Research. That means the technology is now moving through the licensing process that typically precedes commercial rollout, though a specific consumer product timeline has not been published.

It is worth being precise about what this paint can and cannot do right now. It is a peer-reviewed, field-tested result, not a product sitting on a hardware store shelf yet. The temperature figures cited throughout this article come from controlled outdoor testing using thermocouples, a standard instrument for accurate surface temperature readings, rather than from simulations or lab estimates alone.

Still, for a country where cooling demand keeps climbing, and summers keep getting hotter, a coating that can pull a rooftop below ambient temperature using nothing but reflected light is the rare kind of lab breakthrough that could eventually show up somewhere as ordinary as your own roof.