In 2023, U.S. scientists created “cooling glass” that reflects 99% of solar energy and radiates building heat toward space

Imagine applying paint on your roof that helps you keep your house cooler without adjusting the thermostat. It's not just a future idea anymore; the science behind it is proven and working in the lab, even if it hasn't reached hardware store shelves yet. In November 2023, University of Maryland engineers led by professor Liangbing Hu and first author Xinpeng Zhao published a paper titled ‘A solution-processed radiative cooling glass’ in the journal Science describing a new coating they call “cooling glass.” Researchers discovered the coating can cool whatever surface it’s applied to by about 3.5 degrees Celsius at noon without using a single drop of electricity.

It might seem small, but for the generation watching longer summers and higher power bills, this kind of quiet fix counts.

How glass can throw heat into space


The coating works in two ways. To start with, it reflects most of the solar energy that falls on it. According to the research, the glass can reflect 99% of solar energy, meaning that the surface itself absorbs very little heat from the sun, though a building's overall temperature still depends on many other factors like insulation, windows, and internal heat sources.

Secondly, and this is the clever part, it gets rid of the heat that does build up by releasing it as infrared radiation straight up into the sky. This is possible thanks to what scientists refer to as the “atmospheric transparency window,” a narrow band of the electromagnetic spectrum that penetrates the Earth’s atmosphere without heating it. Any heat given out in this band will not be reflected back down. It goes out into space where the temperature is about minus 270 degrees Celsius, a few degrees above absolute zero.

This idea is not new. In a 2020 study, ‘Terrestrial radiative cooling: Using the cold universe as a renewable and sustainable energy source,’ in Science, researchers Xiaobo Yin, Ronggui Yang, Gang Tan and Shanhui Fan explain how the cold universe itself can be a renewable energy source, extracting heat from the surface of the Earth as a clear night sky cools the ground. The UMD team has come up with a coating that does this trick better than previous materials, and one that can survive real weather.

A material built for rooftops, not just labs

Many good lab materials often have problems once they leave the controlled environment. This material is an exception. According to the paper, the coating can withstand challenges such as rain, UV radiation, pollution, and even fire, maintaining its efficiency in temperatures as high as 1,000 degrees Celsius. Moreover, the material can be applied to various surfaces, such as ceramic tiles, brick, or metal, thus making it feasible for use on real buildings.

The team achieved this by using finely ground glass particles instead of the plastic-based binders that older cooling coatings relied on, since polymers break down faster outdoors. The particle size was also carefully tuned to be small enough to release infrared heat efficiently, but still able to scatter and reflect sunlight.

Why this matters for your electricity bill

Air conditioning is quietly becoming one of the biggest drains on the planet’s power grid. According to estimates from the International Energy Agency's Future of Cooling report, ACs and fans already account for almost one-fifth of total electricity use in buildings worldwide, and about one-tenth of total global electricity use. That figure is likely to rise dramatically in the next few decades as incomes rise and summers become hotter.

A passive coating that cuts how much a building has to run its AC from the start could take some of the heat off without asking people to change habits or buy new appliances. The UMD research team estimates that if their coating were applied to a mid-rise apartment building, it could reduce its annual carbon emissions by roughly 10 percent, although this number is based on the team’s modelling and not an independent field trial.

What happens next

But the researchers haven’t stopped at the lab bench. They’ve launched a company to test and scale up the coating for broader use, and are continuing to study how it performs in various climates and building types. It’s worth noting that “cooling glass” is not something you can yet go down to the hardware store and buy. It takes years of further testing, safety checks, and cost reductions to go from a peer-reviewed paper to a common construction product.

However, such an approach is quite promising. Given that global temperatures continue to set records every year and that we increasingly rely on cooling even for normal daily activities, technologies of this kind bring some hope: it means that there is a possibility to cool down without producing more carbon emissions.

So far, the science looks solid: the laboratory tests have been peer-reviewed, and the next step is turning the coating into something that can be used on real rooftops.