January 22nd 2024

CIBSE TM52, CIBSE TM59 & Part O: Overheating Assessments with IES

CIBSE TM52, CIBSE TM59 & Part O: Overheating Assessments with IES

Overheating is an area of growing concern for building design, with global temperature rises presenting complex challenges when it comes to balancing occupant comfort, health and wellbeing against the need for energy and carbon reductions. The emergence of building regulations in recent years, such as Part O in the UK, and methodologies, such as CIBSE TM52 and CIBSE TM59, are helping designers to better predict the risk of overheating, so that measures can be implemented to mitigate these risks early within the design process.

Here we provide a brief overview of CIBSE TM52, TM59 and Part O, alongside the tools, consultancy services and other resources offered by IES to support accurate building overheating assessments.

What is CIBSE TM52?

CIBSE TM52 provides an adaptive methodology to help predict the risk of overheating in buildings. The approach uses dynamic simulation modelling to assess compliance across three criteria. 

  • Criterion 1: Hours of Exceedance
  • Criterion 2: Daily Weighted Exceedance
  • Criterion 3: Upper Limit Temperature

A building is deemed to be at unacceptable risk of overheating if it fails two or more of these criteria.

TM52 can be applied to any non-domestic building, however specific guidance exists for some building types, e.g. schools (BB101) and healthcare (HTM 03-01).

The benefits of TM52 modelling, beyond simply mitigating overheating risks, can extend to helping projects gain BREEAM credits, supporting successful planning applications under local planning frameworks, such as the London Plan, as well as satisfying other industry and building end user requirements.

What is CIBSE TM59?

CIBSE TM59 provides a methodology specifically for assessing the overheating risks in homes (both new build and retrofit) using dynamic thermal simulation at the design stage.

TM59 recognises that the residential sector faces its own unique challenges when it comes to minimising overheating risks and maintaining the comfort, health and wellbeing of occupants. In particular, the high proportion of glazing frequently found in new and refurbished homes, combined with often inadequate natural ventilation strategies or mechanical ventilation systems, have been found to contribute to heightened overheating risks. Meanwhile, many existing homes are also poorly equipped to deal with rising temperatures, and will most likely require retrofitting in order to adapt to future climate projections.

CIBSE TM59 outlines a dynamic thermal modelling approach which takes account of a variety of factors which can influence overheating in homes. This encompasses everything from the intensity of heat gains, to different occupancy patterns, the orientation, dwelling layout, shading strategy and ventilation method, to help establish whether threshold conditions of discomfort will be reached. The methodology outlines specific criteria relating to specific room types, including living rooms, kitchens and bedrooms, for both naturally and mechanically ventilated homes, to ensure healthy and comfortable temperature ranges are maintained across the year.

The TM59 methodology has been adopted by a number of different building frameworks and regulations as the industry standard for conducting overheating studies on dwellings.

What is Part O?

The Part O building regulation was introduced in 2022 to prevent the risk of summer overheating in new residential buildings. It applies to buildings in England and Wales, however Scotland has also introduced requirements around overheating under its own Section 3, which follows similar criteria.

Approved Document O: Overheating outlines two approaches for demonstrating compliance with Part O requirements:

  • A simplified method, which involves limiting solar gains and maximising natural ventilation potential through window sizing or design.
  • A dynamic thermal modelling method, based on CIBSE TM59.

It is widely acknowledged that the dynamic thermal modelling approach offers greater accuracy when performing overheating assessments and that this route can offer the designer additional design flexibility when compared to the simplified method. This is particularly the case for residential buildings with very high levels of insulation and airtightness, or those that have specific site conditions that are not well represented by the generic criteria outlined by Approved Document O’s simplified method.

While the dynamic thermal modelling method outlined by Part O follows CIBSE’s TM59 methodology, it does include some limits on design choices which are not otherwise stipulated by TM59. Further details on these limits are outlined in Approved Document O: Overheating.

Our recent webinar, Keep Your Cool with the Building Regulations - Part O provides further insight on complying with the Part O regulations and can now be accessed on-demand.

How can IES support TM52, TM59 and Part O?

Our IESVE software is recommended by CIBSE to undertake overheating assessments using TM52 and TM59, and was the first software to be certified for compliance with the Part O regulations.

IESVE’s dynamic thermal modelling capabilities enable the simulation of internal temperature conditions, allowing users to establish whether threshold conditions of discomfort will be reached. TM59 prescribed profiles are also available as pre-loaded templates which users can easily select and apply before beginning their overheating simulation and results analysis.

Our patented VE-Navigator for UK Compliance: Overheating further provides a robust and detailed step-by-step structured workflow for analysing overheating against the requirements of Approved Document O (England 2021/Wales 2022) and Section 3 (Scotland 2022). This innovative tool streamlines the compliance process, taking users through every stage from model setup, through to data assignment and simulation, to ensure that no step is missed. Our unique built-in QA functionality supports improved collaboration and tracking of progress across the design team, while editable thermal templates, automated overheating analysis and report functions help to greatly improve the speed and accuracy of the compliance process.

Our expert consultancy team can also provide a range of dynamic thermal modelling services to support overheating assessments using both the TM52 and TM59 methodologies, as well as to demonstrate compliance with Part O.

For more information on the technology, consultancy services and training courses IES can offer to support TM52, TM59 and Part O, check out the resources below: