In 2023, Princeton researchers tested a naturally ventilated cooling box; it stayed below outdoor air while using radiative cooling and buoyancy-driven airflow

Keeping a building cool without mechanical air conditioning usually means making a trade-off. Thick walls and closed windows hold cold air in overnight, but they also trap stale air, and stale air is exactly what building codes and health guidance try to avoid. Bringing in a steady stream of fresh outdoor air keeps occupants breathing well, yet that same fresh air carries outdoor heat straight into the space, undoing much of the cooling a building might otherwise achieve. In 2023, a group of scientists from Princeton University found a solution to that compromise and built a prototype building that remained cool while being constantly ventilated through the buoyant natural flow of air.

Two model boxes faced off in a California field test

Two model enclosures were fitted to the roof of a shipping container at Topanga Valley, California, mounted on an aluminum frame that lifted them about one-third of a meter (0.34 m) above the container's roof. White, perforated aluminum cladding covered both models, shielding them from direct sunlight while allowing air to flow freely underneath, so that the enclosures' outer surfaces would not exceed the surrounding air temperature.


The first box was used as a benchmark of passive cooling as per industry standards, which involved a combination of thermal mass and ventilation, but only at night time when the outside air would be cooler to get rid of the heat gained during the day, as opposed to allowing air to flow all the time. The test box took the reverse approach: air circulated through it constantly, driven by natural temperature-induced airflow at about seven air changes per hour, and its uninsulated roof was coated with a highly reflective daytime radiative cooling material, not black paint, designed to shed heat to the sky even under direct sun. The box was also heated from within to simulate an occupant's body heat.

Why fresh air usually works against radiative cooling

Until this experiment, researchers had generally assumed that keeping a building's interior below the outside temperature required limiting ventilation, since constant daytime air exchange was thought to bring in too much heat to offset passively. Air changes not only introduce new air but also bring in heat, and the more air changes there are, the harder that heat is to offset using passive means alone. The 2023 paper, Passive radiative cooling to sub-ambient temperatures inside naturally ventilated buildings (Fortin, Mandal, Raman & Craig), published in Cell Reports Physical Science, showed that under the test conditions, an uninsulated radiative cooling roof combined with buoyancy-driven ventilation and thermal mass could offset the heat gained from ventilation rather than being overwhelmed by it. The mechanism works like this: the roof radiates heat to the sky all day, which cools the air just beneath it and creates a temperature gradient between that air and the surrounding space. This gradient generates a buoyancy force, which helps create airflow, drawing cool air in through low openings and pushing warm air out through high ones. In the experiment, this produced about seven air changes per hour without any fans.

This process can be tricky. The roof must lose heat quickly enough over the 24-hour cycle to balance out the heat that will enter through seven air changes per hour, but the box beneath must still manage the internal heat load of a person at the same time. Allowing those three factors, radiative cooling, ventilation rate and internal heat gain, to stabilize into a cool temperature below ambient was what this experiment showed was possible.

A related Princeton project chased comfort in a different way

Further experiments were conducted by the same research group at Princeton University, trying out additional methods of creating comfortable naturally ventilated spaces. In an earlier experiment called the Cold Tube, the team created a pavilion with membrane‑covered radiant panels instead of a radiatively cooled roof, tested in Singapore’s hot, humid climate. The pavilion achieved a mean radiant temperature of 22°C under ambient conditions of 31°C and 65% relative humidity by keeping the cold panels separated from the humid outdoor air behind an infrared‑transparent membrane that prevented condensation. While the Cold Tube uses the method of radiant panels for people cooling directly, not the surrounding air, the mentioned Topanga Valley experiment tried to affect the air temperature inside the completely enclosed ventilated space.

Balancing healthy air with a cool building

According to the U.S. Environmental Protection Agency, ventilation specifically reduces indoor levels of particulate matter, volatile organic compounds and airborne biological contaminants, the reason the scientific community has long been skeptical of sealed, thermal-mass-only buildings as a cooling fix. A tightly sealed building might stay cool, but it also traps whatever contaminants are generated indoors. The Topanga Valley experiment indicates that the trade‑off between fresh air and passive cooling is not inevitable. A roof that constantly radiates heat is able to keep the indoor environment cooler than the outside air without compromising the exchange of air. The problem is that applying this approach to the construction of actual buildings is a topic of further research; however, physics does not consider ventilation and cooling to be incompatible.