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Overheating and the new Part O

Feeling hot hot hot

The UK experienced its hottest temperature ever recorded, 40.2°C at Heathrow airport on 19th July 2022, 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

This is why Part O of the UK Building Regulation 2021 was created and, since it came into effect on 15th June 2022, applies to new-build dwellings and other places where people live and sleep like co-living developments and care homes (but not hotels).

To comply with Part O, most buildings will use a dynamic thermal model of the proposed building based on CIBSE TM59 overheating standard. This 3D model of the building takes into account aspects such as hourly weather data, solar positions and solar shading as well as natural ventilation from wind pressures on openings and heat absorption from the thermal mass.

What about non-domestic buildings?

Non-domestic buildings may also be required to carry out overheating analysis, for example, following the local policy requirements for new developments in London. This is based on the CIBSE TM52 overheating standard.

Hillcroft College, a project in Kingston upon Thames we are currently working on with Wimshurst Pelleriti is a great example of how overheating simulations can identify potential issues and quantify improvements from interventions. Thanks to this we were able to explore strategic use of exposed thermal mass and increased window openable area to ensure the benchmark level was surpassed.

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

The first thing is to start early and agree the main principles in each occupied area. If engagement is early enough, the environmental concept strategy can inform building form, orientation, shading and fenestration. These discussions will also include aspects such as the window opening strategy and options for exposed thermal mass. Any proposal should synergise with strategies to reduce energy use such as allowing for passive solar heating in winter.

We also encourage optioneering. As soon as the initial layout has settled, we recommend carrying out a study which we typically turn around within one week. An early start means that several design options can be tested.

If you are considering overheating and Part O compliance for a project and would like our help, please get in touch: 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