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Overheating and Part O of the Building Regulations

Addressing rising temperatures

Last July, the UK experienced its hottest temperature ever recorded, 40.2°C at Heathrow airport, beating the previous record of 38.7°C set at Cambridge’s Botanic Gardens in July 2019.

As building designers, it is increasingly important to master the art and science of reducing overheating because, as we know, this is just the beginning.

Insights from the latest UKCP dataset show a potential average temperature increase of up to 6 degrees by 2100 in a high emission scenario, vs just 2 degrees in a low emission scenario, with much higher summertime increases compared to winter1.

Overheating doesn’t just mean uncomfortable conditions or reduced productivity. For the most vulnerable it can be more serious and it has been estimated by the Committee on Climate Change (CCC) that heat-related deaths in the UK will treble by 2050 to 7,0002.

2 climate change

The new Part O

To ensure better living conditions and thermal comfort, the UK Building Regulation 2021 introduced "Part O." Since its implementation on 15th June 2022, it applies to new-build dwellings and places where people live and sleep, such as co-living developments and care homes (excluding hotels).

Complying with Part O typically involves using a dynamic thermal model (based on the CIBSE TM59 overheating standard) for most buildings. This 3D model takes into account various factors like weather data, solar positions, solar shading, natural ventilation from wind pressures on openings, and heat absorption from the building's thermal mass. It helps assess and optimize the building's design to prevent overheating issues.

What about non-domestic buildings?

Overheating analysis may also be necessary for non-domestic buildings, particularly when local policies, such as those in London, require it for new developments. The analysis is typically based on the CIBSE TM52 overheating standard.

An excellent illustration of the benefits of overheating simulations is the ongoing project at Hillcroft College in Kingston upon Thames, which we are working on alongside Wimshurst Pelleriti. By conducting these simulations, we identified potential issues and quantified the improvements achievable through interventions. This allowed us to strategically utilize exposed thermal mass and increase the window openable area, surpassing the benchmark level for optimal thermal comfort.

4 Hillcroft college

The New Part O is harder to meet than TM59

In recent years, all major new-build schemes in London have used the CIBSE TM59/52 method to demonstrate a minimum overheating standard has been met. As such, schemes that have been through the planning process in London should already be tested and meet the standard. However, the new Part O introduces additional constraints meaning that schemes that have passed TM59 at planning may not pass the Part O variation of TM59 during construction and will require retesting. Part O also introduces additional acoustic and security requirements.

Going Beyond the Standard

The standard represents the minimum acceptable performance, but there are many ways to go beyond the basics and provide better performance. Hillcroft College is a great example and active cooling was designed out using innovations such as wind catchers on the roof to induce wind-driven flow in classrooms. The project is set to achieve BREEAM Excellent.

3 Hillcroft graph V2

Future Proofing

It may not always be cost effective to implement all of the desired overheating interventions. However, good design should allow for additional solutions to be easily retrofitted in order to future proof buildings and avoid active cooling. For example, the ability to retrofit window shutters or low-energy solutions such as evaporative cooling. Improvements can be as simple as planting trees that will offer shading in the future, or creating a low cost trellis network for leafy climbers to spread and develop, which is particularly effective on west facades.

The key: getting it right from day one

It is important to propose and test overheating performance early on in the design process. Once construction has started retrofitting solutions for schemes that fail to meet the Part O standard will prove very difficult – especially for schemes with large amounts for glazing on the south and west façade. Tree planting and greenery, albeit very effective solutions, cannot be taken into account by Part O modelling.

How to get started with Part O and TM59

Getting started with Part O and TM59 involves several key steps to ensure compliance and optimize building design for thermal comfort. Here's a guide to help you begin:

1. Start Early and Agree on Principles: Engage in early discussions to establish the main principles for each occupied area. By aligning the environmental concept strategy with building form, orientation, shading, and fenestration, you can lay the foundation for effective thermal performance. Consider window opening strategies, options for exposed thermal mass, and how the proposal can synergize with energy reduction strategies like passive solar heating in winter.

2. Embrace Optioneering: Once the initial layout is established, conduct a study to explore different design options. This process, which we typically complete within a week, allows for testing and comparison of various approaches. Embrace optioneering to identify the most effective strategies for mitigating overheating risks.

3 Collaborate with Experts: Seek guidance from experienced professionals who can provide insights and expertise in Part O compliance and TM59 standards. Engaging with specialists early on can help streamline the process and ensure optimal design solutions

Are considering overheating and Part O compliance for a project? Let's chat: hello@integrationuk.com

1. UK Climate Projections 2018 (UKCP18)
2. www.theccc.org.uk/wp-content/uploads/2019/07/Outcomes-Heat-preventable-deaths-case-study.pdf