In 2024, scientists tested a zigzag-shaped wall prototype; it was up to 3.1°C cooler than a conventional radiative-cooling wall in outdoor tests

For many Americans, air conditioning is one of the biggest summer expenses, and it is a two-way relationship with rising temperatures. As the summer temperatures keep rising, so do electric bills, and subsequently, most of us simply turn the thermostat down and pray for rain. A 2024 study, however, suggests that building design itself could help reduce the amount of heat a wall absorbs in the first place.

Columbia University engineers designed and tested a wall with a pattern of zigzags that can passively cool itself without consuming electricity. The peer-reviewed study titled “Realizing optimal radiative cooling walls in building-energy nexus via asymmetric emissivity,” in the journal Nexus, reported that their design achieved a daily average temperature reduction of 2.3°C compared with a conventional wall coated with a radiative-cooling material, with the difference reaching 3.1°C when the ground temperature was around 56°C.

But what's happening with these walls anyway


The team, Yuan Yang, with Qilong Cheng as the paper's first author, designed and fabricated what they call an emissive-reflective zigzag (ERZ) wall. Imagine a wall made up of repeated ridges, like a paper fan folded up and laid on its side. Each ridge has two faces, one of which faces upward and radiates infrared energy toward the sky. The other is designed to reflect thermal radiation coming from the hot ground.

That's important because a conventional flat wall cannot simultaneously optimize both directions of radiative heat exchange. The researchers' zigzag structure uses geometry and asymmetric emissivity to give the wall a different thermal behavior depending on which direction it faces. The approximately 60-degree geometry helps the upward-facing surface maintain a strong view of the sky while the downward-facing reflective surface rejects radiation from the hot ground.

The numbers, sans the hype

The Nexus paper noted that the zigzag wall was 2.3°C cooler on average over a full day than the conventional radiative-cooling control wall during outdoor field testing. When the ground temperature reached approximately 56°C, the temperature difference increased to 3.1°C, corresponding to a relative cooling power of 67 watts per square metre compared with the control wall.

The researchers conducted summer field tests to measure the prototype's performance, alongside simulations examining its potential impact on building energy use. That distinction matters: this was a physical prototype tested outdoors, but the broader building-scale energy benefits were evaluated through simulations rather than demonstrated in an occupied apartment building.

Fair enough for all the doubting heads: a few degrees of temperature difference may not seem like too much. But reducing the temperature of a building envelope can matter because walls continuously exchange heat with their surroundings. In hot urban environments, surfaces such as pavement and roofs can become substantially warmer than the surrounding air and radiate heat back toward buildings. A reduction in wall temperature could therefore reduce the heat load that a building's cooling system has to handle, although the study does not suggest that a zigzag wall can replace air conditioning.

Why this is important

Here's the part that’s closer to home. According to the U.S. Energy Information Administration, 88% of U.S. households used air conditioning in 2020, while air conditioning accounted for about 19% of household electricity consumption that year. The broader energy picture is significant, too. The International Energy Agency says buildings account for around 30% of global energy demand, while cooling demand continues to grow as temperatures rise.

That does not mean a passive wall can do the work of an air conditioner. An AC system actively removes heat from indoor air and, depending on the system, can also dehumidify and circulate that air. The zigzag wall does something much simpler: it changes how the building's exterior exchanges radiant heat with the sky and the hot ground. If incorporated into a building envelope at scale, that passive cooling effect could potentially reduce the cooling load, and therefore the electricity required for mechanical air conditioning.

Nor does this mean that zigzag walls are about to become standard features in American apartment buildings. The researchers demonstrated a prototype and used building-scale simulations to assess potential energy impacts; the technology still requires further development and real-world evaluation before widespread adoption. The study itself also discusses limitations and possible improvements, while Columbia's technology-transfer office describes the zigzag wall as a technology intended to reduce radiative heat transfer from the ground and the need for electrical cooling.

Passive cooling technology has been around for years, but this particular approach offers an intriguing architectural twist: instead of adding another electrical device to a building, it changes the shape of the building itself. As extreme heat becomes a more familiar part of summer life, sometimes the smartest cooling technology may be hiding in the geometry of the walls around us.