Berkeley Lab engineers have developed a roof coating that can keep a building warmer or cooler, depending on the weather. When it’s warm out, the material will reflect sunlight and heat, but this radiative cooling automatically switches off in winter, reducing energy use for both heating and cooling.

Radiative cooling systems work by drawing thermal radiation (i.e. heat) out of a building, then emitting it to the sky. Because the atmosphere is transparent to these wavelengths, the heat escapes straight out into space. Other versions use reflective surfaces like super-white paints to bounce sunlight and heat away, keeping the building cooler.

But while these systems might work well to keep your house cool in summer, they also keep it cooler in winter. So for the new study, the team developed a coating that could automatically switch to trapping heat instead when the mercury drops. They call the material a temperature-adaptive radiative coating (TARC).

The key to the technology is a strange compound called vanadium dioxide (VO₂). In 2017, the team discovered an unusual property of VO₂ – when it reaches 67 °C (153 °F), the material will conduct electricity but not heat, in apparent violation of known physics.

Now, the team has put this quirk to work. The idea is that when the weather warms up, the material absorbs and emits thermal-infrared light and so keeps it away from the building. But when the weather is cool, the material is transparent to heat, allowing it to pass right through from the Sun to the building.

The team tested the device using 2-cm² (0.8-in²) thin-film patches of TARC, and compared them to samples of commercial dark and white roof materials. Wireless devices measured changes in direct sunlight and temperature.

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