White aerogel keeps roofs cool in full sun without electricity

A feather-light white “blanket” could help roofs stay cool in the midday sun without using any electricity. Engineers at City University of Hong Kong made a white aerogel (a solid that is mostly air) that sits on top of a cooling roof coating, and in outdoor tests it made that coating up to 5.2 °C cooler, Tech Xplore reports.

Why it matters: air conditioning is one of the biggest energy bills in hot places, and it gets worse every summer. A roof that cools itself, for free, all day, means less work for the AC, lower bills and less strain on the power grid on the hottest afternoons.

First, how can a roof cool itself?

The trick is called radiative cooling. Every warm object gives off heat as invisible infrared light. Most of that light gets trapped by the air, but one band of infrared slips straight through the atmosphere and out into cold space. Scientists call that band the “atmospheric window”.

Special white coatings take advantage of it. They bounce back almost all sunlight, and at the same time they shine their own heat out through that window. Done well, a surface can end up colder than the air around it, even at noon. The experts call that “sub-ambient daytime radiative cooling”, or simply a roof that is cooler than the weather.

The catch: one surface doing two jobs

Until now, the same exposed surface had to do everything: reflect the sun and send heat to the sky. But as soon as it gets colder than the air, the warm air around it starts heating it back up, like an ice cube on a summer table. The researchers call this “parasitic heat gain”, and it limits how cold these roofs can get.

The new idea is to split the jobs. The cooling coating stays underneath and keeps sending its heat out. On top goes the new layer, which the team calls WITA (white infrared-transparent aerogel). It works like a sunshade that is also a window:

It reflects 96.6% of sunlight, so the sun’s heat stays out.
It lets up to 94.5% of the infrared in the atmospheric window pass straight through, so the heat underneath can still escape to the sky.
Being mostly air, it insulates very well (a thermal conductivity below 0.035 W/m·K, in the range of common insulation foams), so warm outside air can’t creep back in, CityUHK explains.

Gloved hands hold a light white aerogel slab in sunlight, next to red, yellow and violet sample squares on a white lab bench. — The aerogel is mostly air, so it also stops warm air from heating the roof · Illustration by DIGITAL WOXTER TECHNEWS.

What the tests showed

The aerogel is made in a single step from ultra-high-molecular-weight polyethylene, a super-tough cousin of everyday plastic bags. In outdoor tests in Hong Kong it boosted the cooling of a standard radiative-cooling coating by up to 5.2 °C. The team says it even let ordinary roofs do better than existing cooling materials on their own.

On a computer model of a four-storey apartment block in Abu Dhabi, a cooling-coated roof already saved 14.6 gigajoules of energy a year compared with a normal roof, roughly 4,000 kWh. Adding the aerogel on top raised those savings by about 55%.

It also looks tough. After 75 days of strong artificial UV light at 40 °C, which the team says equals about two years of Hong Kong sunshine, it reflected and let heat through almost exactly as before. With a water-repellent treatment it sheds water and keeps itself clean, it didn’t catch fire in a flame test (it only shrank), and there are red, yellow and blue versions that keep working. So cool roofs don’t have to be plain white.

What it can’t tell us yet

The 5.2 °C is a best case on top of a cooling coating, not a promise that your attic will be 5 °C cooler. The 55% figure comes from a building simulation, not from real apartments. And the team hasn’t said yet what it would cost or how to make it by the square kilometre. The study, led by Professor Edwin Tso Chi-yan with first author Wenqi Wang and partners at Xi’an Jiaotong University, is published in the journal Energy & Environmental Science.

What you can do today

Aerogel roofs aren’t something you can order yet. But the basic idea works now: light-coloured “cool roof” paints, good insulation and shading all keep heat out in the first place. And if you want smarter control of the cooling you already have, our home climate gadgets guide looks at a fan that also heats and purifies, and a thermostat that learns your routine.

Clever materials are having a moment: we have also covered a plastic that breaks down even in the deep sea and alloys that stay tough at -253 °C.