Almost every city in America possibly has a villain right under its nose during summertime: the building itself. Whether it’s brick, stucco, glass, or vinyl siding, whatever material your building may be covered with, by July it has spent several hours absorbing sunlight and radiating heat right into your apartment. That's why the bedroom exposed to the west still feels intolerable around 6 p.m.,in July, when the low-angle evening sun is shining directly into it rather than away from it, and your air conditioner is struggling no matter how hard you try to run it. For more than ten years, researchers specializing in "passive cooling" technologies could not deal with this phenomenon, since their most effective methods required direct access to open sky rooftops, not walls.It looks like Stanford University’s Professor Shanhui Fan and his colleagues; Professor Wei Li’s team from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Science; and Professor Andrea Alu's team from the City University of New York may have finally found the solution for that problem. In their 2024 Science paper titled “Subambient daytime radiative cooling of vertical surfaces,” the researchers note that their new emitter was capable of keeping the temperature of the vertical test surface 2.5°C cooler than the surrounding air even under the blazing midday sun of approximately 920 watts per square meter intensity.Why walls were the missing pieceBut the concept of radiative cooling isn’t exactly new. For decades now, physicists and engineers have been aware of the possibility of developing a special kind of material capable of transmitting the heat directly to outer space through the atmosphere, thus becoming colder than its surroundings regardless of weather conditions and without any external source of energy. However, the problem always lay in the specific shape of these materials. They had to be oriented directly upwards towards the sky, since a wall would face sideways, absorbing additional heat from all the infrared radiation surrounding it.A building painted white. Image credits: Wikimedia CommonsWhat Stanford's team actually builtThe solution suggested in the paper by Xie et al. is an asymmetric sawtooth (AS) emitter, made up of tiny ridges rather than a flat panel, that has the horizontal facets emitting heat into the sky while the angled facets have a coating that reflects energy from the ground and does not absorb it. In comparison to the standard commercially available white paint used in buildings for reflecting solar energy, the emitter was 8.9 degrees Celsius cooler under similar circumstances and 4.3 degrees Celsius cooler than the best available radiative cooling coating material. That is not an insignificant margin.Based on a 2017 report published by the U.S. Energy Information Administration, the energy used for air conditioning represents about 18 percent of the average household energy consumption in the country, and that number reaches its peak level during July and August. Renters in older, poorly insulated apartments, along with homeowners in warmer regions who face rising summer energy bills, stand to benefit most from a building material that does not function as an additional heat source. Walls that remain cooler indicate less movement of heat into the interiors; this means that air conditioning systems will cycle less frequently, further implying fewer expenses and less strain on the electrical grid when it is already strained.The part you may have ignored This remains a lab-scale device tested on a rooftop rather than a product deployed on an actual building. As Fei Xie, the first author of the paper, stated via Phys.org, “Thanks to our design strategy's flexible ability to tune the angular coverage of thermal emission, we can redesign the AS emitter based on practical scenarios. Even if we face the AS emitter towards a hot building wall, it still can achieve subambient radiative cooling."A building with air conditioners. Image credits: Wikimedia CommonsMaking it into a full-scale device, water-resistant and competitive with something like reflective paint, which is far cheaper, would be another problem that the authors did not address in the paper. Even so, the work addresses a problem that had constrained passive cooling research for more than a decade, and may represent a meaningful step toward building materials that reduce, rather than amplify, indoor heat.