Introduction to CIBSE TM59 Overheating Assessment Methodology
In this post we dive into understanding CIBSE TM59 Overheating Risk Assessment in Buildings.
As climate change continues to drive up summer temperatures, ensuring thermal comfort in residential developments has become a critical aspect of building design. The CIBSE TM59 methodology provides a robust framework for assessing the risk of overheating in homes, helping developers, designers, and compliance professionals meet modern comfort and safety standards.
What is Part O of the Building Regulations?
Approved Document O (Part O) of the Building Regulations, introduced in 2022, sets mandatory requirements to reduce the risk of overheating in new residential buildings. It focuses on limiting solar heat gain and ensuring effective removal of excess heat through appropriate ventilation strategies. Part O applies to dwellings, care homes, and other overnight accommodations, and offers two compliance routes: a simplified method based on design limits, and a dynamic thermal modelling approach, often using CIBSE TM59. It also considers external factors like noise, air quality, and security when determining suitable ventilation solutions
What is CIBSE TM59?
Published by the Chartered Institution of Building Services Engineers (CIBSE), TM59 sets out a standardised approach to evaluating overheating risk in residential buildings. It builds upon earlier guidance from CIBSE TM52 and CIBSE Guide A, offering a clear methodology to ensure that homes remain comfortable during increasingly frequent and intense warm weather events.
Ventilation Strategies: The Foundation of TM59
TM59 distinguishes between two primary types of residential ventilation strategies:
- Predominantly Naturally Ventilated Homes
- Predominantly Mechanically Ventilated Homes
The choice of assessment method depends on the ventilation strategy employed, as well as the effectiveness of that strategy in maintaining thermal comfort.
1. Naturally Ventilated Dwellings
This category includes homes that rely on natural airflow, as well as those with mechanical ventilation with heat recovery (MVHR) systems that still allow for effective natural ventilation during the summer.
For these dwellings, TM59 applies the adaptive comfort methodology from TM52 (2013). This approach considers how occupants adapt to warmer conditions over time, and it requires that:
- Each habitable room includes operable windows.
- These windows must meet the purge ventilation requirements of Approved Document F of the Building Regulations.
- Typically, this means an openable window area of at least 1/20th of the room’s floor area.
- Openings must also be secure, quiet, and accessible, ensuring that occupants can manage internal temperatures effectively without compromising safety or comfort.
Key Performance Criteria
For homes assessed under the adaptive comfort methodology, TM59 outlines two critical performance criteria, both derived from CIBSE TM52 (2013):
- Criterion One – Hours of Exceedance (Living Rooms, Kitchens & Bedrooms)
The number of hours during which the indoor temperature exceeds the adaptive comfort threshold by 1°C or more must not exceed 3% of the total occupied hours between May and September. This criterion addresses general overheating risk across all habitable spaces. - Criterion Two – Night-Time Comfort (Bedrooms Only)
To ensure restful sleep, the operative temperature in bedrooms must not exceed 26°C for more than 1% of the night-time occupied hours (10:00pm to 7:00am). This equates to a maximum of 32 hours annually; exceeding this limit indicates a failure.
2. Mechanically Ventilated Dwellings
In contrast, homes where windows cannot be opened—or where natural ventilation is severely limited due to external noise, air pollution, or security concerns—are assessed using the fixed temperature method from CIBSE Guide A (2015a).
This method sets a strict internal temperature threshold, beyond which overheating is deemed to occur. It is particularly relevant for high-density urban developments or buildings located near busy roads or industrial areas.
Where natural ventilation is not feasible—due to design constraints or external factors such as noise pollution or poor air quality—the fixed temperature method is applied. Under this approach:
- All occupied rooms must maintain an operative temperature not exceeding 26°C for more than 3% of the annual occupied hours.
This method is suitable for buildings where occupants have limited control over their indoor environment, and where adaptive comfort assumptions cannot be reliably applied.
Applying TM59 in Practice
At Building Compliance Testing Limited, we apply the TM59 methodology according to the specific ventilation strategy of each dwelling. This ensures a robust, consistent, and regulation-compliant approach to overheating risk assessment.
Our assessments help developers:
- Demonstrate compliance with planning and building regulations.
- Design homes that are resilient to future climate conditions.
- Enhance occupant comfort and wellbeing.
Why TM59 Matters
With the UK experiencing hotter summers and more frequent heatwaves, overheating is no longer a theoretical concern—it’s a real and growing risk. TM59 provides the tools to address this challenge head-on, ensuring that new residential developments are not only energy-efficient but also safe, comfortable, and future-proof.
Noise, Air Quality, and Security Considerations in Part O Compliance
When designing residential buildings to comply with Approved Document O (Part O), it’s essential to balance the need for effective ventilation with the realities of external environmental constraints. Three key factors—noise, air quality, and security—can significantly impact the feasibility of natural ventilation strategies and influence the choice of overheating mitigation methods.
1. Noise Constraints
Part O recognises that in many urban or high-traffic areas, opening windows for ventilation may expose occupants to unacceptable levels of environmental noise. To address this, the guidance includes:
- External noise thresholds that determine when alternative ventilation strategies must be considered.
- Requirements for acoustic assessments where noise levels may exceed acceptable limits, particularly at night.
- Recommendations for acoustically attenuated ventilation solutions, such as trickle vents or mechanical systems, where openable windows are not viable.
2. Air Quality Considerations
In areas with poor outdoor air quality, such as those near busy roads or industrial zones, relying on open windows for ventilation may compromise indoor air health. Part O advises:
- Assessing local air quality data to determine the suitability of natural ventilation.
- Using filtered mechanical ventilation systems where pollutant levels are high.
- Ensuring that any ventilation strategy does not introduce harmful levels of NO₂, PM2.5, or PM10 into the indoor environment.
3. Security Implications
Security is another critical factor, especially in ground-floor or easily accessible dwellings. Part O requires that:
- Openable windows used for purge ventilation must be secure and safe, particularly during night-time hours.
- Where security concerns prevent windows from being left open, secure ventilation alternatives—such as lockable louvres or mechanical systems—must be provided.
- The design must not compromise occupant safety while still meeting the thermal comfort and ventilation requirements.
Have a Project in Mind? Let’s get to Work.
We have explored the CIBSE TM59 methodology for assessing overheating risk in residential buildings. We have learnt how ventilation strategies, thermal comfort criteria, and environmental factors shape compliance.
If you have any questions or need assistance with Part O Overheating, don’t hesitate to contact us at Building Compliance Testing Limited.