Joe Doucet develops colour-changing paint to help heat and cool the home

After getting stuck on what colour to paint his house for maximum energy efficiency, designer Joe Doucet has developed a climate-responsive paint that changes from white in summer to black in winter.

Doucet looked to the science behind mood rings to develop his thermochromic house paint, which changes from dark to light when the outdoor temperature hits 25 degrees Celsius.

While the product is still early in development, Doucet says his testing has shown that the colour change can affect a building's indoor air temperature by as much as seven degrees Celsius.

There is "no smart tech, no moving parts, just a simple shift in colour that can cut energy use dramatically", the New York-based designer explained.

Black or white paint only solves part of the problem

Doucet began work on the paint during his own home renovation, while trying to choose an exterior colour based on what would bring the most energy savings.

"Most of the information I found was about what happens to surface temperatures but not how that transfers to the interior," Doucet told Dezeen. "So I decided that I would postpone my decision for a year and run a test."

"I tend to have, like, five tests going on at all times," added the designer, whose pioneering designs include the visually appealing Aviva wind turbine and an anti-terror bench that moves to catch an incoming vehicle.

His tests found that a white house was about seven degrees Celsius (12 degrees Fahrenheit) cooler than a black house in summer, and up to four degrees colder in winter. These results line up with findings in scientific studies.

For those living near the equator, where temperatures are warm year around, or in the far north or south where they're usually cold, this suggests a clear right answer for which colour to paint your house, Doucet concluded.

But what about the vast majority of the world's population, who live in temperate climates and struggle with heating in winter and cooling in summer? For them, Doucet felt, there should be a colour-changing paint.

Paint uses "simple chemical reaction" inspired by mood rings

To create this paint, the designer applied what he describes as a "very simple chemical reaction" also used in mood rings, containing liquid crystals that change colour in response to temperature changes.

The colour change occurs because temperature variations change the molecular structure of the crystals, affecting the wavelengths of light they reflect or absorb.

Following this principle, he developed an additive containing liquid crystals and combined it with ordinary store-bought house paint. However, it failed fairly quickly, as the liquid crystals degraded with UV light exposure.

The real challenge became how to protect the material from degradation – a challenge that Doucet believes he has solved. His testing has shown zero degradation over the course of a year.

The additive works with any paint in any base colour, Doucet says, but it should be white or pale to take advantage of the thermal protection. Below 25 degrees Celsius, the liquid crystal structure within the additive absorbs most of the visible light, so we view the surface as black.

Above 25 degrees, the liquid crystal layer is clear, revealing the base colour underneath. The change in colour is almost instant, so the building is one colour or the other, not in between.

Next stage of development is "uncovering the unknowns"

Doucet is now looking for a manufacturing partner to help him continue testing and bring the product to market.

"The question remains, how long does this effect last?" said Doucet. "And that's where I'm ready to bring it out into the world and partner with a few people. The science is robust but it is about uncovering the unknowns and solving for them."

He thinks the paint could be beneficial for not just homes but schools, factories and other large buildings. His innovation has come too late for his own renovation project, however.

"I'm too impatient," Doucet said. "I went and painted my house black in the meantime."

Usually, white building colours are associated with the albedo effect, which helps cool the outside air temperature and is often associated with white-painted roofs.

However, researchers have also acknowledged the effect on internal temperatures, developing innovations such as an "ultra-white" paint that reflects 98 per cent of sunlight and a graphene-based cladding material capable of changing its infrared colour.

A recent low-tech passive cooling innovation came from British engineer Tom Greenhill, who suggested painting windows with yoghurt to provide temporary protection during heatwaves.

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