The paint that cools buildings by sending heat into space
A reflective roof coating can reduce the amount of heat entering the building below. But the technology does not have to work directly on the building. California-based SkyCool Systems uses rooftop panels covered with a specialised optical film (image: SkyCool Systems)
In an outdoor experiment at Purdue University, researchers placed a sample of white paint in direct sunlight. An infrared camera showed something that seemed impossible: the painted surface was colder than the air around it.
It was not simply less hot than conventional paint. At midday, the surface remained as much as 4.5°C below the ambient temperature. At night, the difference reached approximately 10.5°C.
The paint had no refrigerant, compressor or electrical connection. It cooled itself by reflecting 98.1 per cent of incoming sunlight and releasing heat at wavelengths that could pass through the atmosphere.
This process is known as passive daytime radiative cooling. Scientists have proved that a surface can cool itself beneath the midday sun. The challenge now is turning that neat piece of physics into a building material that can be produced cheaply, applied easily and trusted to remain effective for years.
The World Economic Forum included passive radiative cooling among its top emerging technologies of 2026. Its possible uses extend well beyond white roofs. Films, panels and specialised coatings are being developed for supermarkets, factories, shipping containers and overhead power lines.
How passive radiative cooling works
Everything with a temperature above absolute zero emits electromagnetic radiation. People do it, buildings do it and so does the Earth itself. At everyday temperatures, most of that energy is released as infrared radiation, which is invisible to human eyes.
Earth’s atmosphere behaves a little like a blanket with a narrow gap in it. Greenhouse gases absorb much of the infrared energy rising from the surface and radiate some of it back down. But between wavelengths of approximately 8 and 13 micrometres, the atmosphere is comparatively transparent, particularly when the sky is clear and dry.
Heat emitted within this so-called atmospheric window can pass through the atmosphere towards the cold of space.
Radiative-cooling materials are designed to release heat particularly strongly within this range. But that solves only half the problem. A surface sitting in full sunlight can receive around 1,000 watts of solar energy per square metre. Unless most of that radiation is reflected, the incoming heat will overwhelm the cooling effect.
An effective surface must therefore perform two jobs simultaneously: keep solar energy out and allow its own heat to escape. Under favourable conditions, it can lose more energy than it absorbs and become colder than the surrounding air.
Why radiative cooling paint is more than white paint
White roofs have been used in warm countries for centuries because pale surfaces absorb less sunlight than dark ones. Modern “cool roofs” apply the same principle with reflective membranes, tiles and coatings.
Passive radiative cooling goes further. A surface can look brilliantly white without being particularly effective at emitting heat through the atmospheric window. Equally, a strong infrared emitter will still become hot if it absorbs too much sunlight.
The Purdue paint combines both properties. It contains a high concentration of barium sulphate, a white compound also used in cosmetics and photographic paper. The particles come in different sizes, allowing them to scatter a broader range of the wavelengths contained in sunlight.
Viewed normally, it is simply an exceptionally white surface. At microscopic scale, particles throughout the paint are redirecting incoming light before it can be converted into heat.
There are limits to how far this can be taken. Adding more particles can increase reflectivity but also makes paint more likely to crack or peel. A laboratory record is of little value if the material cannot survive years on a roof.
Other developers use porous polymers, ceramic particles or carefully engineered stacks of thin materials. Some products can be rolled or sprayed onto a surface. Others come as films or rigid panels. Radiative cooling is therefore becoming a family of technologies rather than one miraculous tin of paint.
Radiative cooling for supermarket refrigeration
A reflective roof coating can reduce the amount of heat entering the building below. But the technology does not have to work directly on the building.
California-based SkyCool Systems uses rooftop panels covered with a specialised optical film. Fluid circulates through the panels, sheds heat and then helps an existing refrigeration or air-conditioning system work more efficiently.
That approach has been tested at supermarkets in California, where refrigeration operates day and night and represents a substantial part of the electricity bill.
At a supermarket in Red Bluff, an array of 80 SkyCool panels helped its refrigeration system reject an additional 30 to 45 kilowatts of heat. According to a California Energy Commission project report, maximum energy savings reached 15 per cent. The installation also removed the need to spray water over the rooftop condenser during hot weather.
A second installation in Milpitas saved almost 20,000 kilowatt-hours during the project. Depending on the outside temperature, the panels either cooled the refrigerant before it entered the condenser or cooled it further after condensation.
These results put the technology in perspective. Radiative surfaces commonly provide cooling power measured in tens of watts per square metre, although some experimental materials have exceeded 100 watts under favourable test conditions. Mechanical cooling can move much more heat when required, but needs power to do so.
Radiative cooling is therefore less a replacement for conventional equipment than a quiet assistant, reducing the amount of work that equipment must perform.
A robot applying cooling paint to power lines
High above the ground, a small robot grips an electricity cable and crawls along it. As it moves, it leaves behind a thin coating designed to change how the metal interacts with sunlight and heat.
British startup AssetCool developed the system to address a problem facing electricity networks around the world. Power lines become hotter as more current passes through them. When a conductor overheats, it expands and sags. High temperatures also increase electrical resistance and can accelerate the ageing of components.
These thermal limits help determine how much electricity a line can carry safely.
AssetCool says its SE02 coating reflects more than 80 percent of solar radiation and has a thermal emissivity of 0.93, allowing the conductor to release heat more effectively. According to the company, this can increase a line’s continuous carrying capacity by as much as 30 percent.
The idea is attractive. Instead of replacing a cable or constructing another transmission route, a grid operator may be able to obtain additional capacity from infrastructure already in place. A material that cools itself is no longer merely an energy-saving finish: it could become part of the electricity grid.
The limits of passive radiative cooling
The atmosphere’s infrared escape route is not always equally open. Clouds and water vapour absorb radiation, weakening the cooling effect. The technology generally performs best beneath clear, dry skies and less strongly in humid or overcast conditions.
The surface also changes over time. Dust, soot and biological growth can reduce reflectivity. A coating that performs impressively when newly applied may absorb more sunlight after several years beside a busy road.
That produces some decidedly practical questions. Can it be cleaned without damaging its optical properties? How often will cleaning be necessary? Will the installer guarantee its cooling performance, or only that the paint remains attached? How should buyers compare products tested in different climates?
Colour introduces another compromise. A surface appears coloured because it absorbs some wavelengths of visible light. Absorbed light becomes heat, so maximum performance still favours white or very pale materials. Coloured radiative coatings are being developed, but brighter and darker shades generally sacrifice some cooling ability.
Nor does a colder roof guarantee an equally large reduction in indoor temperature. Insulation, ventilation, building design and local weather all affect how much difference occupants will notice.
From laboratory experiment to commercial cooling
The physics behind passive radiative cooling is well established. The commercial test is more mundane: whether coatings remain affordable, clean and effective after years of sun, rain and neglect.
The first attractive markets are likely to be places where heat already carries a measurable cost. Supermarkets pay continuously for refrigeration. Data centres and factories need to protect temperature-sensitive equipment. Electricity companies lose transmission capacity when conductors become too hot.
In these settings, the technology does not have to transform the temperature of an entire building. A modest but continuous reduction may be enough to save electricity, extend equipment life or postpone an expensive infrastructure upgrade.
Air conditioning cools a building by using energy to move heat elsewhere. Radiative materials change how much heat the surface absorbs and how readily it can release it.
The next cooling system may not sit on the roof. It may be the roof itself.
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