Overheating in Buildings: Understanding the Risks and Realities

In September 2025 the UK Met Office announced that the summer of 2025 was the warmest on record for the UK. A 2025 study by the London School of Hygiene & Tropical Medicine and Imperial College London estimated that 1,147 deaths last summer were heat-related. Having those in minds, it is crucial to wonder as professionals in the construction industry: what we can do to create comfortable indoor environments without increasing the carbon footprint of the buildings? At JAW Sustainability we support clients in assessing and mitigating these risks to ensure occupant wellbeing and regulatory compliance.

Why Overheating Matters

Overheating isn’t just about discomfort, it’s a health and wellbeing issue. Prolonged exposure to high indoor temperatures can cause stress, anxiety, sleep deprivation, reduced productivity, and increased risk of heat-related illness and mortality. As building fabric efficiency improves, heat gains become harder to dissipate. The main contributors to internal heat gains are:

- External gains: solar radiation through windows and heat conduction through façades.

- Internal gains: lighting, equipment, and occupant activity.

Beyond personal comfort, overheating has broader implications for building performance, energy use, and long-term sustainability, as people increasingly turn to mechanical cooling to address the issue. A well-designed building can utilise the beneficial solar gain in the winter to reduce the heating, but must also stay comfortable during increasingly frequent summer heatwaves.

How We Assess Overheating in the UK

In the UK, overheating risk is assessed through a combination of regulatory standards and industry guidance. These frameworks ensure that thermal comfort is designed and tested using evidence-based methods.

Key Regulations and Standards:

- Building Regulations Approved Document O (Part O): Sets out minimum overheating mitigation requirements for new residential buildings.

- The London Plan: Requires major developments to demonstrate compliance with the Cooling Hierarchy — prioritising passive design before active cooling.

- CIBSE TM52 & TM59: Provide technical methodologies to assess overheating risk in naturally and mechanically ventilated buildings.

Challenges in Managing Overheating

While the frameworks are clear, real-world constraints often make compliance complex. Designers must balance comfort, energy, cost, and practicality, especially in dense urban environments where various restrictions in the use of natural ventilation can arise. Reasons that the use of natural ventilation may not be possible can be broadly categorised as follows:

Noise, Pollution, and Security:

- Buildings near busy roads or railways face limits on window openings during the night, restricting natural ventilation.

- Air pollution can make passive cooling strategies unsuitable in certain areas.

- Security and safety concerns (e.g., window heights, accessible façades) further limit design freedom.

Design and Density:

- Single-aspect units and highly glazed façades in modern developments increase exposure to solar gain.

- Mixed-use typologies require balancing diverse comfort needs across residential, commercial, and communal areas.

Evolving Climate and Regulations:

- Climate change projections mean today’s 'compliant' buildings may not perform well in future temperature scenarios.

- Regulations continue to evolve, requiring design teams to adapt methodologies and demonstrate long-term resilience.

At JAW Sustainability, we approach each case individually, working closely with the design team from the early stages of building design, offering the most appropriate resolution on a project-by-project basis. Having in mind the unique characteristics of the site, the purpose of the development and the requirements of the local authorities, we come up with a cooling strategy that is tested in many stages of the development to ensure its effectiveness.

In summary: Overheating mitigation isn’t just about compliance; it’s about creating resilient, comfortable, and healthy spaces for the future. By combining robust assessment with practical design strategies, we help clients deliver buildings that perform - even as our climate changes.