Thermochromic coatings: A materials solution to the UK's overheating challenge

For much of modern history, the challenge facing UK homes has been keeping heat in. Building regulations, insulation technologies and construction practices have largely focused on improving thermal efficiency and reducing heating demand during the colder months.

However, a new challenge is emerging. As average temperatures rise and heatwaves become more frequent, many homes are increasingly struggling with the opposite problem: overheating.

What was once considered a relatively minor issue in the UK is becoming a growing concern. In 2022, temperatures in the UK exceeded 40°C for the first time on record, while studies suggest that summertime overheating may become increasingly common as the climate continues to warm.

At first glance, the solution may seem obvious: install air conditioning. Yet widespread adoption of air conditioning presents its own challenges, including increased energy consumption, higher carbon emissions and significant retrofit costs.

As a result, researchers and building designers are increasingly exploring technologies that prevent buildings from overheating in the first place.

Why are UK Homes Overheating?

The causes of overheating are surprisingly complex.

Climate change is an obvious factor, with hotter summers increasing cooling requirements.

Changes to building design have also contributed. Modern homes are generally far more airtight and better insulated than older housing stock. While this significantly improves energy efficiency during winter, it can also make it more difficult for excess heat to escape during summer.

Urbanisation presents an additional challenge. Dense city environments often experience the “urban heat island” effect, where buildings, roads and other infrastructure absorb and retain heat during the day before releasing it overnight. This can prevent homes from cooling naturally, particularly during prolonged heatwaves.

Recognising these risks, the UK introduced Part O of the Building Regulations in 2022, establishing requirements intended to reduce overheating in new residential developments through measures such as solar gain control and enhanced ventilation. The result is a growing focus on passive cooling technologies: systems that reduce indoor temperatures without relying heavily on energy-intensive mechanical cooling.

Thermochromic Glazing

One of the largest sources of unwanted heat gain in a building is its windows [1]. Solar radiation passing through windows can significantly increase indoor temperatures, particularly in modern developments with large windows. Once sunlight enters a room, it is absorbed by surfaces such as floors, walls and furniture, which then re-emit the energy as heat. Because this heat is emitted at longer infrared wavelengths that are less readily transmitted through glass, it can become trapped inside the building – a phenomenon commonly referred to as the greenhouse effect.

While external shutters and shading systems are highly effective at reducing solar gain, they remain relatively uncommon in UK housing and can be challenging to retrofit. This has driven considerable innovation in glazing technologies designed to limit the amount of solar energy entering the building in the first place.

One particularly promising area is thermochromic glazing, which automatically changes its optical properties in response to temperature.

The leading thermochromic material is vanadium dioxide (VO2) [2]. VO2 undergoes a reversible metal-insulator (MIT) phase transition between its monoclinic and rutile phases. Below the MIT transition temperature (68 °C for pure VO2), monoclinic VO2 acts as an insulator and transmits near-infrared (NIR) solar radiation. Above the transition temperature, rutile VO2 is metallic and becomes significantly less transmissive to NIR radiation, reducing the amount of solar heat entering the building.

Since a substantial portion of the sun's energy reaching the Earth's surface lies in the NIR region, controlling NIR transmission offers a powerful way to modulate solar heat gain whilst maintaining visible daylight. During cooler conditions, NIR radiation is admitted into the building, helping to maintain comfortable indoor temperatures. During hotter periods, a greater proportion of heat-carrying solar radiation is blocked from entering the building before it can contribute to overheating.

Despite its promise, several technical challenges remain. The transition temperature of pure VO2 is higher than is desirable for most buildings, meaning researchers have investigated various dopants, including interstitial dopants such as lithium [3] and substitutional dopants such as tungsten [4] to lower the switching temperature. Other challenges include improving visible light transmission through the VO2 coating, increasing durability, and reducing manufacturing costs. Micro and nanoengineering solutions such as core-shell structures and multilayer films have been proposed to solve these challenges [4].

Looking Ahead

The UK's housing stock was largely designed for a cooler climate. As temperatures continue to rise, keeping homes comfortable during summer may become just as important as keeping them warm in winter.

While air conditioning will undoubtedly play a role in some settings, the long-term focus is increasingly shifting towards passive and low-energy cooling solutions.

Thermochromic glazing illustrates how advances in materials science are enabling buildings to respond dynamically to their environment, helping to regulate temperature without consuming electricity or requiring active control systems.

As climate adaptation moves higher up the agenda, technologies such as thermochromic coatings may become as familiar in future homes as insulation and double glazing are today. The question is no longer simply how to keep buildings warm, but how to keep them comfortable year-round.

References

1. R. Baetens, B. P. Jelle, and A. Gustavsen, “Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: A state-of-the-art review,” Solar Energy Materials and Solar Cells, vol. 94, no. 2, pp. 87–105, Feb. 2010, doi: 10.1016/j.solmat.2009.08.021.

2. C. Jiang, L. He, Q. Xuan, Y. Liao, J. G. Dai, and D. Lei, “Phase-change VO2-based thermochromic smart windows,” Dec. 01, 2024, Springer Nature. doi: 10.1038/s41377-024-01560-9.

3. Y. Chen et al., “Electric-field control of Li-doping induced phase transition in VO2 film with crystal facet-dependence,” Nano Energy, vol. 51, pp. 300–307, Sep. 2018, doi: 10.1016/j.nanoen.2018.06.070.

4. Y. Bleu, F. Bourquard, V. Barnier, A.-S. Loir, F. Garrelie, and C. Donnet, “Towards Room Temperature Phase Transition of W-Doped VO2 Thin Films Deposited by Pulsed Laser Deposition: Thermochromic, Surface, and Structural Analysis,” Materials, vol. 16, no. 1, p. 461, Jan. 2023, doi: 10.3390/ma16010461.