In parts of the world with hot, sunny climates, it is common to see houses painted white to reflect sunlight and stay cool during the day. White rooftops serve the same purpose. But now engineers at Stanford University have added color to such cooling paints.

Their new paints can be any color of the rainbow, and it could keep buildings cooler in the summer and warmer in the winter, significantly reducing energy use of space cooling and heating systems.

In laboratory experiments, the paints reduce the energy used for heating by about 36 percent and energy needed for cooling by almost 21 percent. Computer simulations showed that applying the paints on exterior walls and roofs of a typical mid-rise apartment building in different climate zones across the United States reduced total heating, ventilation, and air conditioning energy use by 7.4 percent over a year.

Heating and cooling homes and buildings accounts for about 13 percent of the world’s energy use. As air-conditioning use rises in a warming world, this energy burden and its related greenhouse gas emissions are slated to go up.

To reduce this carbon footprint, researchers have been developing passive heating and cooling technologies that do not use energy. These include evaporation-based cooling systems, materials that send infrared heat waves out into space, personal cooling materials that reflect sunlight and dissipate heat.

Paints that reflect heat offer a passive route to cooling not just buildings but also cars and trucks. The new paints reported in the journal Proceedings of the National Academy of Sciences are composed of two layers. The bottom layer is based on aluminum flakes that reflect infrared radiation, while on top is an ultrathin layer that lets infrared waves through and contains tiny particles that can be different colors.

Applied to the outside of buildings, the paints could keep out, and on the interior to keep heat indoors during winter. The team made and tested paints in white, blue, red, yellow, green, orange, purple and dark gray. Each was 10 times better at reflecting infrared light than conventional paints of the same colors.

The paints are also water-repellant, which should make them stable in humid environments and easy to clean with a damp cloth. The paints did not show any performance loss after a week of being exposed to high temperatures of 80°C and lows of -196°C.

Source: Yucan Peng et al. Colorful low-emissivity paints for space heating and cooling energy savings. Proc. Natl. Acad. Sci. 2023.