Scientists developed a colored paint that stayed up to 9°C cooler under direct sunlight, and its one-step coating could help buildings beat heat without sacrificing color

One of the more stealthy engineering challenges of the last decade has been how to keep buildings cool without cranking the air conditioning up to 11. A study titled ‘One-step-processed bilayer ethyl cellulose for full-color sub-ambient daytime radiative cooling’ by Liu et al. , published in Nature Energy, shows that passive radiative cooling can cool objects under sunlight without any extra energy input.

The catch? The majority of these materials are white. And white is not always an option for homes, commercial buildings, or anything where appearance is important. Add color, and it absorbs more sunlight. More sunlight absorbed means more heat. That Nature Energy study points out that it’s still difficult to achieve attractive colors in materials without sacrificing solar reflectance. For years, that trade-off felt almost impossible to escape.

Now, a team of scientists may have stumbled upon a real way out. The study published in Nature Energy shows that a bilayer colored ethyl cellulose coating derived from biomass and fabricated in a single casting step and by controlled drying-induced self-stratification has a solar reflectance of 97.0% and sub-ambient daytime cooling of up to 9°C under a solar intensity of 800 W/m². That means the surface can be as much as 16°F cooler than the air surrounding it but still display bright colors.


How it actually works
According to Bioengineer. org's coverage of the research, the material is based on ethyl cellulose, a biomass-derived compound, and is produced through a simple one-step casting process. The strange thing is how its structure changes.

In a single drying step, the coating spontaneously segregates into two layers: a thin, dense top layer that produces color via thin-film interference without contributing to solar absorption, and a hierarchically porous bottom layer that provides high solar reflectance and long-wave infrared emission, according to the Nature Energy study.

The color here does not come from pigment. That’s an important distinction. The Nature Energy study suggests that the precursor concentration can be adjusted to produce different colors without introducing solar absorption, which researchers can tune by controlling the top layer thickness. Traditional pigments work by absorbing some wavelengths of light, which results in converting some solar energy into heat. Instead, the ethyl cellulose coating uses light interference to create color, the same optical principle that makes soap bubbles or butterfly wings iridescent, without absorbing much solar radiation.

The hierarchically porous bottom layer scatters incoming sunlight and, at the same time, emits heat outward as long-wave infrared radiation, which requires no energy input at all, according to the Nature Energy study.

What the field tests showed
Lab results are one thing. Real-world performance is another. According to the Nature Energy study, in field trials in Hong Kong’s humid subtropical climate, the coating outperformed commercially available colored paints and fluorescence-based colored coatings. Humidity is the bane of passive cooling systems, and so this is a particularly encouraging outcome for hot, muggy US cities such as Houston, Miami, and New Orleans.

Why this matters for American buildings
The US is one of the most energy-intensive countries in the world when it comes to cooling. Air conditioning is a massive chunk of summer electricity demand, especially in the Sun Belt. Reflective and radiative roof coatings have been proposed as a cost-effective way to reduce that burden for some time, but their widespread adoption has been limited in part due to aesthetic constraints.

According to a 2022 study ‘Potential energy savings benefits and limitations of radiative cooling coatings for U.S. residential buildings’ published in the Journal of Cleaner Production, radiative coatings are a promising approach to implement passive cooling and provide an eco-friendly pathway to achieve energy efficiency in buildings. The same study, after analyzing 68 locations throughout the US, found that the net energy savings of radiative coatings on residential building roofs are highly climate- and atmospheric-dependent, so performance varies by region, but the potential is high in warm-weather states. Strong passive cooling, and allowing architects and homeowners to choose any color, removes one of the biggest remaining objections to this technology.

Not the first cellulose-based approach, but the most practical
The new coating has been developed from previous work studying cellulose as a platform for radiative cooling. Shanker et al. , in a 2022 study published in ACS Nano, demonstrated that cellulose nanocrystal films that self-assemble into helical periodic structures can provide nonabsorptive, structurally colored reflection, reducing surface temperatures by as much as 9°C under solar illumination. Previous research proved the basic concept, but required a more complicated, multi-step manufacturing process.

The new method using ethyl cellulose stands out for its simplicity. The Nature Energy study finds that the one-step phase-separation method can simplify fabrication, which could make it easier to deploy this technology in the real world. That’s really important for scaling up. The one‐step coating is more likely to be commercially produced than one requiring multiple precision processes.

The bigger picture
Passive cooling isn’t a full substitute for air conditioning, and researchers are cautious about overstating what any one material can do. But with hotter, longer summers across the US, materials that meaningfully reduce heat buildup with no energy input, and without forcing a trade-off between function and looks, deserve serious attention. It is a major advance in sustainable building design to have a roof or wall that helps a building stay cooler, reflects sunlight effectively, and is still available in any color an architect or homeowner wants.