TM52: What Is It and Do I Need It?

Overheating is a growing risk due to climate change and shifting weather patterns. It can occur as a result of poor design, inadequate management or ineffective building services, with features intended to reduce winter heating demands sometimes leading to excessive summer temperatures. To address this, CIBSE developed TM52, a thermal comfort analysis methodology that helps assess and mitigate overheating risks in buildings.

In this blog, you’ll learn:

• What TM52 is and how it works
• Why thermal comfort is essential in new and existing building design
• How TM52 helps mitigate overheating risks and enhance building performance
• How TM52 differs from CIBSE TM59 and Part O, and which one applies to your project
• How TM52 can be applied to existing buildings

What is TM52?

TM52, The limits of thermal comfort: avoiding overheating (2013), is a methodology developed by the Chartered Institution of Building Services Engineers (CIBSE) to assess the thermal comfort of non-domestic buildings and predict their overheating risk. It focuses on the indoor temperature conditions that occupants experience in these types of buildings, such as offices, schools, hospitals, and other public or commercial buildings.

The methodology applies to “free-running” buildings – those where the peak internal temperature is not controlled by air conditioning, with means for occupants to locally alter conditions. It evaluates both the frequency and severity of temperature exceedance over the warmer months, specifically from May to September.

TM52 sets out three key criteria that help determine whether a building is at risk of overheating:

1. Hours of Exceedance: Limits the number of hours during which the internal temperature can exceed a specific threshold (Tmax) by 1°C or more. This is set at a maximum of 3% of occupied hours from May to September.
2. Daily Weighted Exceedance: Accounts for both the degree and duration of temperature exceedance in a given day, ensuring that extreme heat exposure is not prolonged.
3. Upper Limit Temperature: Establishes an absolute maximum temperature limit of Tmax + 4°C. If a building exceeds this limit, it is considered at unacceptable overheating risk.

If a building fails to meet two or more criteria, it is considered at high risk of overheating and requires intervention to mitigate the issue.

How TM52 Differs From TM59 and Part O

TM52 is one of three related overheating standards, and it’s easy to mix them up if you’re new to this area:

• TM52 applies to non-domestic commercial buildings – offices, schools, care homes, and similar.
• TM59 applies to residential developments – flats, student accommodation, and communal residential schemes. CIBSE published a major revision to this one, TM59:2026, which is worth knowing about if you’re working on residential schemes.
• Part O is the statutory Building Regulation that enforces overheating compliance for new residential developments in England and Wales – TM59 is typically the methodology used to demonstrate compliance with it.

Why is Thermal Comfort Important?

Thermal comfort directly impacts occupant health and wellbeing. Prolonged exposure to uncomfortable temperatures can lead to heat stress, poor air quality, and reduced productivity. In terms of energy efficiency, buildings that lack proper thermal comfort often rely more heavily on energy-intensive cooling systems, which can drive up energy costs and contribute to carbon emissions.

In the context of modern building regulations and sustainability goals, ensuring thermal comfort is now considered just as important as optimising energy efficiency. TM52 provides a data-driven, scientifically backed methodology to assess a building’s ability to maintain comfortable temperatures and reduce overheating risk.

How Does TM52 Work and Mitigate Overheating Risk?

TM52 works by assessing a building’s thermal performance against the three criteria above, using simulations that test whether internal temperatures exceed acceptable limits under normal use and summer conditions. By modelling the building’s thermal behaviour and identifying any areas where temperatures may surpass thresholds, TM52 analysis can highlight where improvements are needed.

The results of a TM52 assessment can inform practical design and retrofit strategies to help mitigate overheating risks. Some common strategies include:

• Shading: Adding external shading devices or solar control glazing to limit solar heat gain.
• Insulation: Improve insulation and thermal mass to maintain stable indoor temperatures.
• Ventilation: Enhancing natural or mechanical ventilation to facilitate effective heat removal.
• Passive cooling: Using techniques like night-time purge ventilation to lower internal temperatures after hot days.

These measures reduce reliance on energy-intensive cooling systems, improving both comfort and sustainability.

TM52 in Practice: Assessing an Existing Building

Overheating isn’t just a concern for new buildings – it’s equally relevant for existing buildings, especially those not originally designed to handle today’s warmer climate. Older buildings often lack sufficient insulation, shading, and ventilation, making them more prone to overheating during summer months.

A TM52 assessment on an existing building typically starts with an onsite survey to record the building’s fabric, glazing, and ventilation as built, followed by a dynamic thermal model to simulate its performance against the three criteria above.

On one existing building we assessed this way, the model showed the building at risk of overheating against TM52’s criteria – precisely the kind of finding this methodology is designed to surface early, before it becomes a comfort or compliance problem.

Where a model shows a room or zone at risk, the same mitigation hierarchy applies as for new designs – passive measures first, mechanical cooling only as a last resort.

What’s Next?

As overheating risk grows with a changing climate, thermal comfort deserves the same attention as energy efficiency – whether you’re designing a new commercial building or retrofitting an older one. TM52 analysis gives you the evidence to act before comfort issues become a bigger problem.

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TM52: Thermal Comfort Analysis

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If you’re looking to ensure your building meets the latest standards for thermal comfort and energy efficiency, get in touch with our team for a TM52 Thermal Comfort Analysis assessment.