Berkeley Lab built a roof coating of vanadium dioxide that flips at 67°C; across 15 US climate zones, it outperformed existing coatings in 12 and cut household electricity by 10%

Most people probably don’t give a second thought to their roof until the electric bill shows up. A team led by Junqiao Wu at Lawrence Berkeley National Laboratory might have just found a way to shrink that bill. Researchers have developed a roof coating that keeps homes cooler in summer and warmer in winter without using extra power, according to a study published in the journal Science. The material, called a temperature-adaptive radiative coating, or TARC, has performed better in early tests than cool roofs already on the market.

How vanadium dioxide makes the switch

The secret ingredient is a chemical compound known as vanadium dioxide. In most metals, heat and electricity move through at about the same speed. Vanadium dioxide defies that trend. Wu's team found that its electrons conduct electricity well but not heat, a mismatch most materials do not exhibit. This peculiarity causes the coating to behave as an automatic switch. On its own, vanadium dioxide only makes this switch at around 67°C (153°F): far too hot to be useful on a roof. So the researchers doped the material with small amounts of tungsten, which tunes the switching point down into a practical range: TARC's thermal emittance stays low, around 20%, when temperatures are below about 15°C (59°F), and rises to roughly 90% once temperatures climb above about 25°C to 30°C (77–86°F). There is no wiring, no batteries, or sensors involved. The switch is all down to the physics of the material itself.


Tested on a real rooftop, not only in a lab

Lab numbers only tell part of the story, so the team took the experiment outside. They built a small, postage-stamp-sized TARC panel, just 2 by 2 centimeters, and installed it on the balcony of Wu's own home in Berkeley's East Bay hills, alongside a regular white roof sample and a dark roof sample. After a few days of changing sun and temperature, the panel reflected about 75 percent of sunlight year-round, helping prevent overheating in summer. But its heat release behavior changed with the seasons, just as the underlying physics predicted. Those real-world readings gave the researchers hard numbers to build a larger model.

What happened across 15 US climate zones

Based on the rooftop test, the research team modeled how TARC would perform in 15 US climate zones, from humid coastal regions to arid inland areas. The coating proved to be more energy-efficient than currently available roof materials in 12 out of those 15 climate zones. The most favorable results were achieved in those areas that experienced wide temperature variations: the San Francisco Bay area with hot days and cold nights, and New York City with hot summers and cold winters. Co-lead researcher Kechao Tang said an average American household could reduce electricity consumption by up to 10 percent.

Why this matters even if you don't own a roof

Cooling is already causing stress on the power grids of the world, and this is not an isolated problem occurring in the United States. In fact, the International Energy Agency has warned that global power demand for cooling could rise by as much as 40% by 2030 if the present trend continues. Yet, there is an overlooked cost associated with standard cool roofs: they keep radiating heat away into the sky even in cold weather, which drives up winter heating costs. This is where TARC comes into play, as it changes its behavior based on the season.

For decades, government agencies have favored cool roofs for similar reasons. Cool roofs can help bring down indoor temperatures, decrease a building's air conditioning use, and reduce the heat island effect in cities compared to rural surroundings, the EPA says. This coating could matter to both city planners and homeowners.

Still a lab project, not a product yet

It's not likely that you will find this material in a hardware store any time soon. Now Wu and his team are constructing larger models in order to determine how well the coating will resist the weather over several years. The researchers say TARC could also be adapted for uses beyond roofs, including coatings for phone batteries, satellites, cars, tents, and even clothing, though those applications remain speculative for now.