In 2023, scientists built a rooftop tile that opens its cooling vents when hot and closes them when cold, using no electricity and cutting cooling costs 3.1-fold

Air-conditioning bills often rise in summer. One reason is that buildings absorb heat during the day and then need energy to remove it. Buildings are not very good at regulating when to keep heat in and when to let it out. Roofs are exposed to sunlight throughout the day. They absorb this heat whether we want them to or not. Then the cooling system has to correct the problem after it happens. A team of engineers wanted to fix this problem at the beginning. They wanted to make the building’s surface responsive so it could switch on and off on its own. According to a study, "Passively adaptive radiative switch for thermoregulation in buildings" published in the Cell Press journal Device, engineers at the University of California, Santa Barbara built exactly that. They made a tile with tiny fin-like parts that open when the surface gets hot and close when it cools. The tile does not need any electronic sensors, batteries, or external wiring to work.

A tile is doing a thermostat's job without a single wire

The tiles are compact enough to arrange in rows across a rooftop. Each tile has a row of slats on top. The slats are black on the side that faces out and white on the underside. When the slats are closed, the black surface gets sunlight and heat. This is useful in the morning. The tiles warm up quickly when it's cold outside. When the slats open, they show the underside. This white surface reflects sunlight. It sends heat back out to the sky. This can help a building stay cooler on a hot afternoon.


Wax powers the switch, not wiring

The louvers move because of a wax-based actuator within each tile, a passive component sometimes called a wax motor, which runs without electricity or electronic controls. Wax is a phase-change material because it expands when it melts and shrinks when it solidifies. When the tile temperature rises to about 65°F, the wax melts, expands, and pushes a piston to open the louvers. When the temperature decreases, the wax shrinks, closing the louvers automatically without any external energy source. Because the switch responds to the tile's own surface temperature rather than the surrounding air, a sunny but cold morning could, in principle, still trigger the slats to open early. The researchers note this is one of the behaviors future testing will need to characterize across different climates and seasons.

Older passive designs were too sluggish to be useful

The concept of constructing self-adjusting surfaces in response to changing temperatures without requiring any energy input was already developed. Older versions of these surfaces worked only when the temperature difference was greater than 27°F (15°C). This is a significant temperature variation, much higher than what we experience on any given day. The researchers' adaptive tiles can switch states with a temperature change of about 5.4°F (3°C). That means that these UCSB surfaces do not require extreme temperatures to operate; they can perform with the normal daily temperature variations.

The number that matters: a 3.1-fold cut in cooling costs

The key numbers come from outdoor testing conducted on the UCSB rooftop, where researchers held the building's target indoor temperature steady so they could directly compare energy use between the switching tile and a standard, non-switching tile under identical conditions. The study reports that the switching tile lowers cooling energy costs by 3.1-fold and heating costs by 2.6-fold relative to a similar, non-switching tile, based on outdoor test results. According to the United States Energy Information Administration, cooling alone accounted for 14 percent of electricity use, or roughly 170 billion kilowatt-hours, among commercial buildings in the United States in the agency's most recent Commercial Buildings Energy Consumption Survey (CBECS), conducted in 2018.

Still a prototype, not a product on anyone's roof yet

The four-inch prototype tile tested on a university rooftop is very different from tiles sold at hardware stores. The researchers say it is an early prototype, and more testing is needed to see how it performs in sun, rain, and freezing weather. In addition, the researchers want to find out how costly it would be to manufacture these tiles in large numbers, but the outdoor results so far show that a passive material can reliably respond to temperature swings using nothing more than a small amount of enclosed wax. The test suggests the idea works: a material can respond to temperature changes using a small amount of enclosed wax.

Why the research matters

Rising summer temperatures are driving up cooling needs across the country. The aging power grids are already under strain when temperatures peak. That has increased interest in materials that can reduce heating and cooling needs without electricity or electronics. As power grids strain under rising cooling demand, a passive tile that needs nothing more than sunlight and a bit of wax to regulate a building's temperature is a reminder that some of the most effective climate solutions may be the simplest ones.