Passive Radiative Cooling: A Natural Alternative to Air Conditioning

In many regions of the world, demand for air conditioning is skyrocketing. According to the International Energy Agency, cooling systems today account for nearly 20% of the electricity used in buildings, which represents about 7% of total global electricity consumption.

Researchers are therefore exploring passive solutions capable of keeping buildings cool without relying on mechanical energy. Among these, passive radiative cooling has, over the past decade, emerged as a credible option.

The idea seems simple: every hot surface emits infrared radiation. The atmosphere, however, has a “window” that allows part of this radiation to escape directly into space. By developing materials that can reflect sunlight while strongly emitting radiation within this window, it becomes possible to keep a surface cooler than the surrounding air, even in broad daylight.

This approach was first demonstrated by a team at Stanford University in 2014, a discovery widely covered by scientific and technology media such as Wired.

Ever More Efficient Materials

Modern research on passive radiative cooling truly began with work published in Nature in 2014.

The team led by Shanhui Fan at Stanford designed an experimental optical device based on a multilayer stack of specially engineered coatings. This prototype combined an exceptional ability to reflect nearly all incoming solar radiation with strong infrared emission.

When installed and tested outdoors on campus, this coating demonstrated that a surface could remain cooler than the surrounding air at midday under a clear sky. As Wired reported at the time, this was a world first: natural cooling achieved without energy or water, solely through material properties.

Since this breakthrough, research has accelerated. In a review published in Sustainability (2024), researcher L. Guo describes several families of materials now being investigated for radiative roofing:

• Ultra-reflective paints: developed at Purdue University, they are described as the “whitest paints in the world” because they reflect more than 98% of sunlight. They keep surfaces cooler than the surrounding air.

• Special polymer films: based on polydimethylsiloxane or porous polyethylene, they are inexpensive, effective, and easy to apply over large areas.

• Nanostructured ceramics: studied by Lin and his team in Science (2023), they achieve near-total solar reflection and strong infrared emission, while being robust enough for industrial-scale production.

From the Lab to the Field

Several pilot projects confirm the effectiveness of this technology beyond the laboratory:

• At Stanford, prototypes installed on buildings showed that radiative roofs could significantly reduce air-conditioning use.

• At Purdue, the ultrawhite paint was tested on pilot rooftops. According to the Rocky Mountain Institute (2023), these surfaces achieved a noticeable drop in temperature and a reduction in demand for mechanical cooling.

• In hot regions, recent trials reported in Sustainability (2024) and in the work of Zhao (2025) confirm a substantial decrease in energy consumption. In some cases, air-conditioning demand was reduced by about one-quarter to one-third.

These examples demonstrate that passive radiative cooling is not just a scientific curiosity: it is a technology already proving its effectiveness in real-world settings.

A Global Environmental Lever

The advantages identified by researchers and specialized organizations are numerous:

• Direct reduction of air-conditioning demand: cooler surfaces reduce the heat entering buildings and ease the load on mechanical systems.

• Fully passive operation: as the RMI points out, no energy input is required.

• Urban-scale benefits: L. Guo highlights that multiplying radiative surfaces could mitigate urban heat islands, a growing issue in dense cities.

• Scalability: ceramics studied by Lin show potential for mass production at reasonable costs, opening the door to widespread use.

The impact extends beyond individual buildings. Guo explains that large-scale deployment of such surfaces would limit heat buildup in urban environments and reduce cities’ overall energy demand. Zhao (2025) emphasizes that this approach opens the way to climate-friendly cooling strategies, easing the burden of air conditioning on power grids. The RMI adds that this technology could become a tool for environmental resilience in the face of increasingly frequent heat waves.

What perspectives for tomorrow?

The integration into green certifications is well underway: the RMI recalls that these roofs could soon join sustainable building labels alongside solar panels and green roofs.

Technological coupling opens horizons: Guo mentions for example the association with photovoltaic panels, which would benefit from cooling to improve their efficiency. Other researchers are considering combinations with green roofs, in order to combine biodiversity and thermal performance.

Priority deployment in hot regions represents a key opportunity. In his 2025 publication, Zhao insists on the fact that in tropical and desert countries, air conditioning can represent up to half of the total electricity consumption. The potential impact of a passive solution would therefore be major.

Beyond overall figures on energy savings, several studies highlight tangible effects noticeable in daily life. For example, tests conducted at Purdue University and reported by the Rocky Mountain Institute (2023) show that even under the most unfavorable conditions, radiative roofs deliver at least a 4 °C reduction in surface temperature compared to a conventional roof. While this difference may appear modest, it already translates into a clear decrease in indoor heat and improved comfort without relying on air conditioning.

As for adoption, researchers emphasize that implementation is very similar to existing construction practices: paints and films can be applied like traditional coatings and do not require special equipment. Lin et al. (Science, 2023) also underline that the materials tested demonstrate robustness and durability comparable to conventional solutions. These elements help address concerns often associated with new technologies: this is not a complex system, but rather a natural evolution of roofing materials, already ready for large-scale application.

Conclusion

In just a decade, passive radiative cooling has moved from theoretical concept to practical demonstration.

The pioneering work at Stanford published in Nature in 2014, followed by its coverage in Wired, paved the way. Innovations such as Purdue’s ultrawhite paint (described by the RMI) and the hierarchical ceramics studied by Lin in Science confirm its technical feasibility. The reviews by Guo and Zhao highlight a rapidly growing field, with research advancing quickly.

If challenges related to durability, aesthetics, and standardization are overcome, this technology could emerge as a disruptive alternative to traditional air conditioning. It offers a simple and elegant solution: using the sky as a free and universal air conditioner.

Sources

[1]Our World in Data. (2022). Air conditioning causes around 4% of global greenhouse gas emissions and this will change in the future. Our World in Data. 

[2] Raman, A. P., Anoma, M. A., Zhu, L., Rephaeli, E., & Fan, S. (2014). Passive radiative cooling below ambient air temperature under direct sunlight. Nature, 515, 540-544.

[3] Wired. (2014). Stanford engineers invent high-tech mirror to beam heat away from buildings into space. Wired.com.

[4] Guo, L. (2024). The Review of Radiative Cooling Technology Applied to Building Roofs: High Solar Reflectivity and Energy Saving in Building Sector. Sustainability, 16(16), 6936. MDPI.

[5] Zhao, Z. (2025). Progress in passive daytime radiative cooling from spectral design to system integration. PhotoniX.

[6] Lin, L. (2023). Hierarchically structured passive radiative cooling ceramic with near-perfect solar reflectance and infrared emittance. Science, 382(6667).

[7] Rocky Mountain Institute (RMI). (2023). Clean Energy 101: Passive Daytime Radiative Cooling. Really Cool Roofs. RMI.org.

Would you like to learn more about Passive Cooling Solutions ? Feel free to check out our article   'Passive Cooling Solutions: Designing Spaces that Stay Comfortable Naturally ' to explore the topic further.