When most people think about heatwaves, they picture scorching pavements, crowded beaches, and soaring temperatures outdoors. Yet the greatest threat from extreme heat is often not outside - it is inside our homes, schools and offices.

In the UK, we spend around 90% of our time indoors. As climate change increases the frequency and severity of heatwaves, overheating within buildings is emerging as one of the most important public health and building design challenges of our time. While a hot summer day may feel enjoyable, prolonged exposure to high indoor temperatures can have serious consequences for health, wellbeing, and energy consumption.

Professor Lucelia Rodrigues looks at the hidden impact of overheating buildings and shares strategies to deal with the issues in the face of ever-increasing summer temperatures.

Heatwave 800x

Victoria Park in London during a summer heatwave. Image courtesy of Adobe Stock.

The hidden impact of overheating

Most people have experienced the discomfort of trying to sleep during a hot spell. A bedroom that remains warm into the night can lead to restless sleep, fatigue, poor concentration, and reduced productivity the following day. However, overheating is far more than an inconvenience. Even relatively small increases in indoor temperature can affect both physical and mental wellbeing. High temperatures increase the risk of dehydration, place additional strain on the cardiovascular system, and can worsen existing health conditions. When heat persists over weeks rather than days, the risks become significantly greater, particularly for older adults, young children, and people with underlying health conditions.

As summers become hotter, ensuring that buildings remain safe and comfortable is becoming a critical aspect of climate resilience. According to the UK Health Security Agency (UKHSA), in the summer of 2022 England experienced five separate heatwave episodes, with, temperatures exceeding 40°C for the first time. These were associated with an estimated 2,985 heat-related deaths. With climate models predicting more frequent and intense heatwaves in the coming decades, these events cannot be regarded as exceptional - they must be planned for as part of normal building design and urban development.

Are UK buildings prepared?

The uncomfortable reality is that many UK buildings were never designed with warm or extreme summer temperatures in mind. For decades, building regulations focused primarily on preventing heat loss during winter. This approach made sense historically, but it has resulted in a housing stock that often performs poorly during hot weather. Well-insulated and airtight buildings can trap heat indoors, causing internal temperatures to rise to uncomfortable or even dangerous levels.

Only in 2022 did overheating regulations formally become part of England’s building regulations. Until recently, future climate projections were rarely considered during the design process. The good news is that designing buildings to stay cool does not necessarily mean increasing costs. Effective design can create buildings that remain comfortable throughout the year while minimising energy use.

Many warmer countries already incorporate cooling strategies that could be adopted more widely in the UK. Features such as shading, planting, well-designed ventilation, thermal mass, and thoughtful orientation can significantly reduce overheating risks without relying heavily on mechanical cooling systems.

Why cities feel even hotter

Heatwaves are particularly challenging in urban areas because of the 'urban heat island' effect. Research by organisations including the UK Green Building Council and the US Environmental Protection Agency suggests that densely built urban areas can be up to 12°C warmer than surrounding rural environments.

Several factors contribute to this phenomenon:

  • Concrete, brick, and asphalt absorb and retain heat.
  • Dense development restricts airflow.
  • Limited vegetation reduces natural cooling through shade and evapotranspiration.
  • Transport, industry, and buildings generate additional waste heat.

As a result, cities often remain hot long after sunset, preventing buildings from cooling naturally overnight and increasing overheating risks.

The hierarchy of cooling: a smarter approach

The most effective way to prevent overheating follows a simple principle: prevent heat from entering the building before attempting to remove it. Effective measures include:

  • Installing external shading devices such as shutters, blinds, awnings, and brise soleil.
  • Using strategically planted trees, green walls, and vegetation to provide natural shading.
  • Reducing internal heat gains from lighting, appliances, and equipment where possible.
  • Improving thermal performance through appropriate construction materials and thermal mass.
  • Keeping windows closed during the hottest periods of the day when outdoor temperatures exceed indoor temperatures.

These strategies can dramatically reduce cooling demand before it becomes a problem. Once temperatures begin to fall, buildings should be designed to discharge accumulated heat. This can be achieved through:

  • Night-time ventilation.
  • Cross-ventilation strategies.
  • Stack ventilation using natural air movement.
  • Mechanical ventilation systems - where natural ventilation is insufficient.

Removing heat during cooler periods helps a building start each day at a lower temperature, improving comfort without significant energy consumption.

Only after passive measures have been maximised should active cooling systems be considered. Highly efficient technologies such as reversible air-source heat pumps can provide cooling alongside low-carbon heating. Air conditioning can still play an important role in certain circumstances, particularly during severe heatwaves, but its usage should be minimised through good design.

When passive measures have already reduced heat gains, air conditioning systems operate for shorter periods, consume less energy, and impose a much lower environmental impact.

Is air conditioning the answer?

As temperatures rise, it is understandable that many people look to air conditioning as the solution. However, widespread dependence on conventional air conditioning presents significant challenges.

Rising energy demand, and consequent rising carbon dioxide emissions: According to the International Energy Agency, cooling technologies already account for around one-fifth of energy used in buildings globally. Because much of the world's electricity is still generated from fossil fuels, increased cooling demand contributes to greenhouse gas emissions, which further intensify climate change and future heatwaves. The UK electricity network is also not currently designed for universal air-conditioning use. Large-scale adoption would substantially increase peak energy demand during hot weather.

Refrigerant emissions: Many cooling systems use fluorinated gases as refrigerants. These substances can have global warming impacts hundreds or even thousands of times greater than carbon dioxide if they leak into the atmosphere. Research suggests that approximately one-third of cooling's climate impact comes from refrigerant leakage, making effective refrigerant management essential.

Waste heat and the urban heat island: Air conditioners cool indoor spaces by extracting heat and releasing it outdoors. While this improves comfort inside a building, it increases temperatures outside. In dense urban environments, thousands of cooling units collectively release substantial amounts of waste heat into the surrounding air. Studies from cities including Lyon, Hong Kong, Beijing, Sao Paulo, London and Taipei have found that this can raise outdoor temperatures, increase cooling demand, and create a feedback loop where more cooling leads to even more cooling demand. The result is a cycle that can worsen urban heat islands over time.

Building for a hotter future: an opportunity

The transition to low-carbon heating presents an opportunity. As the UK moves away from gas boilers and towards electrified heating systems such as heat pumps, cooling requirements should be considered simultaneously. The goal should not be to eliminate cooling technologies altogether, but to use them intelligently within buildings that have already been designed to resist overheating.

By learning from warmer regions and incorporating climate resilience into every stage of building design, the UK can create homes and workplaces that are healthier, more comfortable, and better prepared for future heatwaves. The challenge is not simply keeping people cool today, it is ensuring our buildings remain safe and resilient in an increasingly warmer world.