Can cool coatings combat climate change?

Imagine a paint that could lower your energy bills, an architectural coating that could help your company meet its greenhouse gas reduction targets. That’s the idea behind “cool roof” coatings. A range of such coatings already on the market can reduce the amount of solar heat gain that buildings have to cope with. And a next generation is racing to market in the hopes of doing even better. These coatings include paints that shed more heat than they absorb, even in direct sunlight, that flip between absorbing and reflecting solar energy depending on the season, and that block the transfer of heat between exterior surfaces and interior spaces.

But to make a difference—and a profit for the companies that produce them—these coatings will have to prove that they’re better than the incumbents and better than alternative energy-saving strategies for the tops of buildings, like enhanced insulation and solar roof panels.

An energy hog

According to the US Department of Energy (DOE), buildings account for 40% of energy consumption and 35% of greenhouse gas emissions in the US. Around 40% of that energy use is for space heating, and 10% is for cooling, according to the US Energy Information Administration. In the southern half of the US, about 30% is for heating and 20% for cooling. Developers of cool-roof coatings say they can help building owners, especially those in the US South and other hot regions, make a dent in those numbers.

Calming the demand for air-conditioning could also ease the strain on ailing electrical infrastructure, says Victoria Scarborough, vice president of collaborative innovation at ChemQuest, a specialty chemical consulting firm. Scarborough spent decades scouting and developing new technology for the coatings companies Thompson Minwax and Sherwin-Williams before retiring and joining ChemQuest.

Cool-roof coatings sales are split roughly three ways: 40% for industrial buildings and 30% each for commercial and residential structures. Scarborough says cool roofs are the fastest-growing slice of the global coatings market. The category in the US was worth about $4.5 billion in 2022 and is projected to reach $8 billion by 2028.

Most major coatings producers have cool-roof products, typically in white and silver, as do smaller and specialty coatings firms. “This product category, which enables energy savings, is certainly a megatrend and is expected to continue as the end-user awareness of sustainability increases,” says Jani Rutanen, an R&D leader for coatings at the paint manufacturer PPG Industries.

The firm’s main cool-roof product line reflects about 80% of sunlight. “We are prioritizing this as part of existing product lines, such as the Tikkurila ClimateCooler coating, and as part of new product development,” Rutanen says.

Scarborough says government action—in the form of funding, regulations, and goals around energy efficiency and carbon dioxide emissions—is the biggest driver for the cool-roof trend. At the end of 2022, US president Joe Biden set a policy that any large federal construction project has to be designed to have net-zero greenhouse gas emissions.

The DOE is helping implement that policy with funding such as the $46 million it deployed in August for 29 energy efficiency building projects. In New York City, Local Law 97 will have strict greenhouse gas emission and energy efficiency requirements for any building larger than 2,300 m², starting in 2025.

“They’re requiring that new construction has these net-zero-type goals built in,” Scarborough says. “And part of this big story is the cooling. . . . You’ve got to get rid of the energy drain on the building itself.”

High energy prices, especially in Europe, are another driver for the growth of cool-roof and cool-wall coatings, according to Ulrich Schmidt, global head of technical marketing for the functional pigment maker Eckart. When the German company first launched IReflex, an infrared reflective paint ingredient based on glass-encapsulated aluminum flakes, energy was cheap and the consumer pull for energy efficiency wasn’t enough to tempt coatings makers to develop a new formulation. “We really had a difficult time to convince customers to go that way,” Schmidt recalls.

About a year ago, though, Eckart relaunched IReflex. “The situation changed,” Schmidt says. “We recently faced these harsh increases in the oil and gas prices; there is a much higher willingness in the market to think about new and partly disruptive ideas.”

Two-pronged approach

The basic ingredients in a coating are pigment, binder, and filler, Schmidt says, though the formulator’s toolbox also includes a dizzying array of surfactants, rheology modifiers, and other functional ingredients. Common fillers include clay, silica, and titanium dioxide, all used to add bulk and opacity.

Almost all the solar radiation that a roof or wall absorbs gets converted to heat, so reflecting light is key to heat management. A gray roof reflects only about 20% of sunlight, and conventional reflective roofs deflect about 80%, Scarborough says. “The goal of the newer technologies is to beat that; they want to be in the 98% range.”

One approach to cool roofs is to replace common fillers with performance materials that change the way paint interacts with heat and help achieve so-called passive cooling. These materials reflect most of the light that hits them, so they aren’t warmed much by the sun. By a separate mechanism, they also pull heat out of the surface they sit on by converting it into infrared radiation and emitting it. An effective cooling coating needs to do both these tricks.

Not just any infrared emission will do, though. Water vapor and other atmospheric gases absorb a lot of infrared light, trapping the heat right around the building. For the cooling effect to work, the light has to pass through the air and into deep space. That happens only at wavelengths known as the infrared atmospheric window, or sky window, most of which is between 8 and 14 μm. If a material has strong solar reflectance and sky-window emission, it can achieve a net loss of heat, even in direct sunlight.

The recent star in this category is barium sulfate, thanks to research led by Xiulin Ruan, a mechanical engineering professor at Purdue University. BaSO₄ is cheap and mechanically tough, so it already has some use in coatings as a white pigment and a filler. But the barium-containing paint formulation that Ruan’s team developed differs from conventional white acrylic house paint in two main ways.

First is how much the researchers used. The BaSO₄ serves as both filler and pigment, for a total of 60% BaSO₄ by volume (ACS Appl. Mater. Interfaces 2021, DOI: 10.1021/acsami.1c02368). This is higher than the filler or pigment load in conventional paints.

Second is the physical form of the mineral. BaSO₄ is optically white but has a low refractive index and absorbs some infrared light. The team got around both problems with particle sizes that range from 268 to 528 nm, creating a light-scattering effect similar to what is found in snow, which is opaque white even though water and ice are clear.

With those details dialed in, the BaSO₄-based coating reflects more than 98% of solar radiation and has a strong infrared emission at 9 μm, right in the sky window. In their paper, Ruan and coworkers say the coating has a cooling power of up to 117 W/m². That’s equivalent to one-sixth the capacity of a common 5-metric-ton air conditioner placed on a 100 m² roof.

The paint builds on Ruan’s previous work using calcium carbonate. As with the BaSO₄ recipe, the CaCO₃ content of his earlier paint is a healthy 60%. Particle sizes span about 200 nm to help the coating scatter a broad range of light wavelengths. Between reflection and radiative cooling, the CaCO₃ version has a cooling power of around 37 W/m² (Cell Rep. Phys. Sci. 2020, DOI:  10.1016/j.xcrp.2020.100221).

Ruan has patents covering both systems and is actively commercializing the technology. He declined commenting for this story because he is negotiating with at least one unnamed paint manufacturer.

Passive-cooling paint could be a huge help in hot climates, reducing the demand on electrical distribution systems and making life more comfortable and affordable for millions. Ruan estimates in the papers that commercial coatings based on his patents would cost the same to make as any other paint because BaSO₄ and CaCO₃ are abundant, cheap materials.

In moderate climates, however, people won’t always want to beam their home’s heat into space—in the middle of winter, for example. “It’s going to be a partial solution for some parts of the United States—Phoenix, California, sunny parts of the United States or the world,” Scarborough says. More northern latitudes need a different approach.

A roof for all seasons

Researchers at Harbin Institute of Technology are looking to balance the need to both heat and cool most buildings by taking inspiration from the Namaqua chameleon. The lizard lives in the deserts of Namibia, where temperatures vary between 1 and 38 °C. It copes with that range by changing its skin from black, which helps it fight off the morning chill, to white, to beat the afternoon heat.

The Harbin team paired an infrared-​emissive pigment blend made of BaSO₄, SiO₂, and Al₂ O₃ with a thermochromic pigment based on crystal violet lactone. Below 20 °C, the lactone ring is open and the paint looks gray. The total mixture has reflectance of about 50% and suppresses infrared light emissions, resulting in a net heat gain. Between a temperature of 20 and 30 °C, the ring closes as the chromophore loses a hydrogen to nearby bisphenol A molecules, turning the paint bright white with a reflectance up to 93% and a strong sky-window emission (Nano Lett. 2023, DOI: 10.1021/acs.nanolett.3c02733).

The researchers put their chameleon coating to the test on a series of bedroom-size structures atop a building in Weihai, China. The color-changing paint reduced the inside temperature in summer by about 5 °C relative to commercial white paint, a reduction similar to what was achieved by a passive-cooling paint tested on an identical test structure placed about 2 m away.

In cooler fall weather, the roof with the chameleon coating was a few degrees warmer than ambient air temperature and about 10 °C warmer than one coated with the passive-cooling paint that didn’t change color. But the temperatures inside all the structures weren’t significantly higher than the outside air, which the researchers attributed to 10 cm of insulation under the roof and the low angle of the sun. In winter, the chameleon coating kept the interior 1.2 °C warmer than the single-​color passive-cooling coating did.

The Harbin team estimates that because its coating almost matches the performance of paint like Ruan’s in the summer and turns off the cooling effect in the winter, it can beat the efficiency gains of static-colored passive-cooling coatings in moderate climates. And the team thinks its chameleon coating saves about 20% of year-round energy costs relative to conventional roof paint.

PPG’s Rutanen says the paint industry has its eye on chameleon coatings. “In the case of color-changing surfaces like the Harbin example, there have been discussions among architects and civil engineers because it would be beneficial in terms of the functionality of the building,” he says. But he notes that researchers will need to prove that such coatings can last in the hot sun as the color changes many hundreds of times.

One problem with pigments that reflect almost all visible light is that not everyone wants every surface of their home or business to be bright white. Rutanen says that some areas even ban bright-white roof paint for aesthetic reasons or to protect aircraft pilots from glare. The color works well for flat roofs on row homes, apartment buildings, and commercial developments, but what about walls and other public-facing exterior surfaces?

Cool and colorful

Researchers at Stanford University led by materials science and engineering professor Yi Cui are working on a multilayer paint system that could offer a solution. The bottom layer is an infrared-reflective assembly of microscale aluminum flakes in a nitrile butadiene rubber-co-urea polymer matrix. It is roughly 5–10 um thick and has a reflectance of 85%.

The researchers spray atop that layer a similar polymer mixed with one or more infrared-transparent mineral pigments: blue from prussian blue, Fe₄ [Fe(CN)₆ ]₃ ; red from ferric oxide, Fe₂ O₃ ; yellow from goethite, α-FeOOH; and white from zinc oxide, ZnO. Different combinations of these primary colors afford a wide range of final hues (Proc. Natl. Acad. Sci. U.S.A. 2023, DOI: 10.1073/pnas.2300856120).

The dual-layer system has an infrared reflectance around 80%, according to the paper on the system. That score is lower than the 95% or more achieved by plain BaSO₄ or CaCO₃ but way higher than the reflectance of conventional colorful paints, which the researchers measured at around 10%.

The aluminum flake layer is designed to reflect light and block heat transfer, not maximize sky-window emissions. In lab-scale tests, a blue version of the Stanford system reduced the power needed to keep the inside of a beer-can-size “room” warm in a cold environment by 36% and kept twice as much ice frozen in the back of a model semitruck, in both cases compared with commercial blue paint.

“Although the optical design of our colorful [low-emissivity] paints is not optimized for cooling or heating individually, it provides a more comprehensive year-round energy-saving solution that is aptly suited to a variety of regions,” the researchers write.

The start-up EnKoat is commercializing a suite of coatings that, like Stanford’s colorful system, is intended to provide a thermal barrier rather than a net flow of heat into or out of the building. It’s not insulation per se because insulation slows down heat conduction. Barriers like aluminum or EnKoat’s products instead stop radiant infrared heat from passing through, though some EnKoat formulations also resist thermal conduction.

EnKoat cofounder Aashay Arora says the firm replaces a paint’s fillers with selections from a palette of materials that have low thermal conductivity, high reflectivity, and high thermal storage capacity. The firm is making a different version for each of the seven DOE climate zones in the contiguous US.

Arora says EnKoat uses a mix of polymers and inorganic materials, declining to be more specific. He and cofounder Matthew Aguayo have a patent on the use of encapsulated phase-change materials in coatings, which can avoid a rise in surface temperature by diverting incoming heat to melt a solid.

In a yearlong test in Phoenix, EnKoat applied its paint to the roof of one building and a commercial reflective coating on an identical building next door. Arora says the building with his coating consumed 20% less energy over the year. The firm is now expanding its test program and is in talks with major coatings providers.

From the lab to the roof

Scarborough says several of the new ideas making their way to the market are promising. She watches innovations in paints and coatings carefully as part of her work for ChemQuest, as grant reviewer for the US National Science Foundation, and as a mentor for the occasional start-up that catches her attention—including EnKoat. But having an idea that works in the lab isn’t enough. “You’ve got this great technology—how do you compare to the existing products in the market?” she asks.

The Cool Roof Rating Council, a nonprofit that advocates for and evaluates cool-roof products, lists more than 600 roof coatings with a solar reflectance of 80% or better. “These are some examples of products already in the market, already delivering,” Scarborough says. The organization expanded its scope in 2019 to include cool walls, a smaller but emerging category. “We started with roofs; now we’re moving towards other surfaces,” says Scarborough, who is not affiliated with the Cool Roof Rating Council.

Scarborough is also eager to see how passive-cooling paints and other new thermal management coatings perform over years of hot summers and frigid winters. “Until it’s weathered, you don’t know a thing,” she says.

Austin Trautman, a building science expert, is also withholding judgment on new cool-roof coatings until more real-world data are available. Trautman is the founder of Vali Homes, a builder and green construction consultancy in the US Southwest.

“The whole idea of dark-sky radiation,” another term for passive cooling, “is one of my favorite little pieces of physics that nobody in the general public understands,” he says. “There’s so much cool stuff about it, like that the Persians were making ice in the desert, when the temperature never went below freezing, because they wanted ice cream and cold drinks in the summer. But what actually happens when we put that into the real world? There’s going to be some unintended consequences.”

Trautman’s big concern is moisture. Water tends to condense on any surface cooler than the ambient air, even in dry climates such as Phoenix’s. The heat on a structure’s roof or exterior walls is doing useful work by driving out moisture, he says.

“Moisture is the destroyer of all buildings,” Trautman says, and the savings on energy, cost, and greenhouse gases go right out the window if a surface treatment cuts down the lifetime of a building or substructure. The problem can be overcome, he says. “It’s just a new responsibility.”

He argues that insulation, especially insulation made from natural materials instead of polymer foams, is the most powerful tool for reducing the net carbon footprint of most buildings—though insulation can also trap moisture. “We need to then understand that risk we’re taking and mitigate against it,” Trautman says. “I want my bills to be lower and I want to have higher comfort, but it’s a fairly complicated system.”

Although there are caveats, if reflective and passive-cooling coatings are done thoughtfully, they may be the cheapest way to use a roof to lower emissions and energy costs. Conventional solar panels cost around $540 per square meter, and solar roof tiles like those from Tesla are closer to $1,000 per square meter. Green roofs planted with hardy, drought-tolerant greenery range from $100 to $2,000 per square meter.

In contrast, according to estimates from the Purdue, Stanford, and Harbin researchers and from roofing contractors, a cool-roof coating—offering passive cooling or not—costs $20–$75 per square meter. That’s about the same as standard roof paint.

Like most sustainability measures, paints that reduce the need for heating or cooling are only a partial solution. The best coatings yield a 20% energy savings, which means, at most, a 4% reduction in total US energy consumption.

Cool roofs and walls won’t make a building green on their own, but they could be an easy climate win for a lot of structures, especially when that win can be had for only the cost of a coat of paint.