The Problem of Concrete Jungles
Cities are full of dark surfaces like asphalt roads and traditional roofing materials, which are excellent at absorbing and storing the sun's energy. During the day, they soak up intense solar radiation, and at night, they slowly release that stored heat,
keeping the air temperature artificially high. This phenomenon, known as the urban heat island effect, can make a city several degrees warmer than its surrounding rural areas. This trapped heat puts a strain on energy grids as air conditioning use soars, worsens air quality by accelerating the formation of smog, and poses significant health risks during heatwaves.
The Science of Staying Cool
Cool coatings are specially designed paints or materials that tackle this problem with a one-two punch of physics. First, they have a high solar reflectance (also known as albedo), meaning they reflect a large portion of incoming sunlight back into space instead of absorbing it. Think of it as the difference between wearing a white shirt and a black shirt on a sunny day. Second, they have a high thermal emittance. This allows them to efficiently radiate away any heat they do absorb, releasing it as infrared radiation that can pass through the atmosphere. A material that excels at both reflecting sunlight and emitting heat will stay significantly cooler than conventional materials.
Enter Bio-Inspired Innovation
While many cool coatings use mineral-based pigments, the term “bio-engineered” points to a new frontier of materials inspired by nature (biomimicry) or made from biological sources. Scientists are studying how organisms in extreme environments stay cool, like the Saharan silver ant, whose unique hairs provide remarkable reflection and heat emission. This has led to the development of coatings that use sustainable, nature-derived materials. For example, researchers are creating coatings with cellulose nanocrystals, which are derived from plant fibers like wood pulp. These crystals can self-assemble into structures that not only reflect sunlight and emit heat but can also produce vibrant colours without heat-absorbing pigments.
From a Single Roof to a Cooler Neighbourhood
When applied to a single roof, a cool coating can dramatically lower its surface temperature—by as much as 28°C in some studies—which in turn reduces the need for air conditioning and saves energy. When these coatings are deployed at scale across a neighbourhood or an entire city, covering roofs, pavements, and walls, the collective effect can be powerful. By reflecting vast amounts of solar energy, they reduce the amount of heat absorbed by the urban environment. This leads to a measurable drop in the ambient air temperature, mitigating the heat island effect for everyone. Studies have shown this can lower urban air temperatures by more than 2°C on average.
Beyond Just Lowering the Thermometer
The benefits of cooler cities extend far beyond simple comfort. Lower ambient temperatures mean a reduced demand for air conditioning, leading to significant energy savings and a smaller carbon footprint from power plants. Cooler air also slows the chemical reactions that produce ground-level ozone, a major component of smog, resulting in better air quality and public health outcomes. Additionally, cooler pavements have a longer service life because they experience less thermal expansion, and their reflective nature can improve nighttime visibility and safety. Some innovative coatings are even being designed to capture clean drinking water from the air as they cool.
