A CIBSE TM52 Calculation is a technical assessment used to determine whether a non-domestic building is at risk of overheating during summer conditions. It was developed by the Chartered Institution of Building Services Engineers to address a growing issue in modern construction. As buildings become better insulated and airtight for energy efficiency, many now retain unwanted heat, which can lead to discomfort, reduced productivity and in some cases non-compliance with planning or design standards.

Why CIBSE TM52 Calculation Exists

Traditionally, overheating was judged by a single temperature limit applied across the whole year. However, CIBSE identified that this approach was too crude. Occupants respond differently to temperature depending on outside weather conditions, building type and occupancy behaviour. Therefore, TM52 introduced a more realistic, adaptive approach, which considers the actual thermal comfort of people using the space, not just the raw temperature reading.

How TM52 Assesses Overheating

A TM52 assessment is not just a spreadsheet exercise. It is completed using dynamic thermal simulation software such as IESVE or DesignBuilder, which uses hourly weather data and real occupancy patterns to test how the building behaves throughout a warm period. The model understands when the building is likely to be occupied, how much heat the people and equipment generate, and how fresh air enters the space. It then compares the internal conditions against adaptive comfort limits, which adjust depending on the outdoor temperature rather than using a fixed threshold. As the simulation runs, it measures:

Criterion 1 – Hours of Discomfort
This checks how often temperatures rise above the acceptable comfort threshold during occupied hours. If this happens for more than a limited percentage of time, the building is considered at risk.

Criterion 2 – Severity of Overheating
This measures not just how often temperatures are exceeded but how severe the overheating is. Longer and more extreme temperature peaks indicate a higher risk of discomfort.

Criterion 3 – Absolute Temperature Limit
This sets an upper tolerance. If internal temperatures rise more than 4°C above the adaptive comfort temperature at any point, the building automatically fails this criterion, even if the other criteria are met.

A building needs to comply with at least two out of the three criteria to be considered acceptable under TM52 guidance.

When a TM52 Assessment Is Required

Most commonly, a CIBSE TM52 Calculation assessment is requested for:

• Offices and community buildings
• Naturally ventilated designs with large, glazed areas
• Planning submissions where overheating risk has been raised
• Design reviews under sustainability or comfort-focused local policy

Local authorities are increasingly aware of overheating risk and are asking for proof of thermal comfort in line with CIBSE guidance rather than simple reassurance.

How a TM52 Report Helps a Design Team

When we deliver a TM52 report, the most valuable part for design teams is not simply the pass or fail statement, but the insight into which design features are driving the overheating risk. For example, we often see that a classroom is comfortable in the morning but begins to fail during mid-afternoon because there is limited cross ventilation. The report clearly highlights these problem areas so the architect or services engineer can make a targeted adjustment rather than redesigning the whole building.

Design teams often tell us that having clear evidence from a CIBSE TM52 Calculation helps them speak more confidently to planning officers, clients and project managers. Instead of speculation, they have a tested simulation that explains, “This room will overheat on these days unless this specific measure is introduced.” That level of clarity speeds up approvals and avoids late-stage compliance issues.

Why Early Assessment Matters

In some projects we are approached very late in the process when glazing has already been scheduled for procurement or HVAC strategies have been signed off. At that stage, even small changes become expensive. When TM52 is carried out early, small adjustments can be made with minimal impact, such as increasing openable area, adjusting louvre angles or introducing a simple night purge strategy.

Early assessment also helps avoid unnecessary over-specification. Without a TM52 report, design teams sometimes add cooling systems or high-cost shading to “play it safe”. The simulation often shows that a more cost-effective passive measure is enough, which saves money and reduces operational carbon.

Commissioning a CIBSE TM52 Calculation

At Energy Digest, we complete CIBSE TM52 Overheating Assessments for architects, planning consultants and developers across the UK. We build the simulation model, apply the official criteria and provide clear, actionable guidance on how to mitigate any areas of non-compliance. If you have planning conditions to discharge or need to demonstrate robust comfort compliance for design review, our team can prepare the TM52 report on your behalf with a fast turnaround.