This smart coating turns roofs into smart switches: Cuts annual household power bills by up to 10%

High above the East Bay hills, a wireless measurement device sat mounted on a residential balcony, recording real-time data under the sun. Alongside commercial white and dark roof samples, a small 2-by-2-centimeter thin-film device was performing a molecular balancing act.

The material looked like ordinary Scotch tape, but it was behaving like an active HVAC system without drawing a single watt of power.

Developed by a team led by Junqiao Wu, a faculty scientist in Berkeley Lab's Materials Sciences Division and UC Berkeley professor, the material — dubbed a temperature-adaptive radiative coating, or TARC — automatically switches its thermal behavior based on ambient outdoor temperatures.

“Our all-season roof coating automatically switches from keeping you cool to warm, depending on outdoor air temperature,” said Wu. “This is energy-free, emission-free air conditioning and heating, all in one device.”

The flaw in modern cool roofs 

Conventional cool-roof systems rely on light-colored surfaces, membranes, or specialised shingles to reflect sunlight. Through radiative cooling, these systems shed heat by emitting thermal-infrared radiation out toward the sky.

While this prevents heat buildup during summer, standard cool roofs present a significant drawback in cold weather: they continue radiating heat away, causing an "overcooling" effect that drives up winter heating costs.

TARC solves this seasonal conflict by turning off its radiative cooling mechanism when the weather turns cold.

Physics behind vanadium dioxide 

The breakthrough builds on a discovery Wu and his research team made in 2017 regarding vanadium dioxide. In almost all metals, electrons conduct heat and electricity proportionally. Vanadium dioxide breaks that rule, its electrons conduct electricity efficiently while acting as a thermal insulator.

At temperatures below roughly 67°C (153°F), vanadium dioxide remains transparent to thermal-infrared light and does not absorb it. Once it reaches 67°C, the phase-change material switches into a metallic state, becoming absorptive of thermal-infrared light.

To make this phase change relevant for outdoor weather, Wu’s team tuned the switching point, allowing the material to react dynamically within everyday temperature ranges:

Year-round solar reflectance: TARC maintains a steady 75% sunlight reflection regardless of season.

• Warm weather (Above 25°C/77°F): Thermal emittance surges to roughly 90%, shedding excess indoor heat to the sky. 
• Cool weather (Below 15°C/59°F): Thermal emittance drops to around 20%, shutting down radiative heat loss and trapping heat gained from solar absorption and indoor heating. 

Real-world testing and nationwide modeling 

To test the material under real-world conditions, co-lead author Kechao Tang deployed the initial balcony experiment at Wu's home. Data gathered from that test was fed into a comprehensive simulation framework led by Ronnen Levinson, head of Berkeley Lab's Heat Island Group.

Using baseline data from over 100,000 building energy simulations previously conducted by the Heat Island Group, summer intern Finnegan Reichertz modeled TARC’s performance hour-by-hour across 15 US climate zones. Infrared spectroscopy experiments at Berkeley Lab’s Molecular Foundry further verified the modeling.

The team found that TARC outperformed commercial cool-roof systems in 12 of the 15 climate zones, with the greatest benefits showing up in areas with large daily or seasonal temperature swings such as the San Francisco Bay Area and New York City.