coombes:everitt News Part O – what you need to know…

Part O – what you need to know…

May 8, 2024

Thermal Model

At Coombes Everitt, we have been designing fully-compliant Part O homes since the introduction of the regulations in June 2022 – experience has shown us that early consideration – ideally as early as RIBA Stage 1 or 2 – is key to success.

In response to the regulations, we have invested in dynamic thermal modelling software – IES VE – which enables us to assess any potential overheating risks caused by orientation, glazing and ventilation early in the design process. This means we can optimise schemes ahead of any capital outlay and our clients can be assured of compliance with the Part O regulations.

What is Part O?

Part O of the Building Regulations 2010 came into force in June 2022 – its aim to reduce the risk of overheating in new build developments and encourage the design and construction of buildings that limit unwanted solar gain during the summer months and provide occupants with adequate means of removing excess heat from indoor environments.

The regulations require that a design is checked to ensure that it does not overheat in the summer months. With the increase in insulation levels and increase in new building air tightness, we have benefitted from much better thermal performance in new buildings, reducing the overall heating load and lowering the amount of carbon that’s used to provide energy to a building. This has however increased the risk of summertime overheating and the new regulation has been drafted to tackle this potential risk.

Think early

The requirements brought in by Part O are here to stay and are ultimately of great benefit to the health and wellbeing of future occupiers and the environment. However, if the requirements of Part O are not an early consideration in the design process, this can create the need for additional changes further down the line which can increase the risk of time delays, and additional expense, as well as retrospective changes which may compromise the original design objectives, as well as creating planning complications further down the line.

If the changes have not been considered as part of a planning application, it may be necessary to submit a non-material amendment application, or where the changes required are more significant, a Section 73 application to vary the approved drawings. At this point, new policy considerations may need to be taken into account too – all of which incur extra time and cost.

What must I do I comply?

To ensure compliance, the following amendments may be required:

• Changes to the extent of the openable area of windows.
• Changes to the size or quantity of windows.
• Internal changes to layout to allow for cross ventilation.
• Specification of solar control glass that will reduce the solar gains introduced inside a space.
• External shading devices such as external shutters or overhanging eaves.
• Mechanical ventilation systems.

Leading the way in Part O design

Our investment in thermal modelling software provides an immense amount of design flexibility and enables us to integrate Part O into a design workflow from concept to completion – before they are submitted to planning and way before the design develops into a set of Building Regulation drawings.

Costly and delaying design changes can then be avoided and buildings can be designed as passively as possible right from the outset, without having to ‘fix’ a bad passive design with expensive additional mechanical ventilation or air conditioning/cooling, which will all add to a building’s energy use.

What is thermal modelling?

The simplest way to assess a building is using what is referred to as the simplified method, which is essentially calculating the percentage of glazing on each elevation against tables published in the regulations to ensure that a building does not have excessive glazing that could result in summer time overheating due to solar gain.

This method is incredibly restrictive however in terms of design flexibility and very difficult to pass with more complex building designs.
The alternative method is using what’s called Dynamic Thermal Modelling software to simulate the building throughout the year and check the heat loads within the building for each month of the year.

Sun Path Simulation

The software also allows us to simulate the sun path throughout the year and assess solar shading, which will also consider adjacent buildings that might cast shadows on the building design and external landscaped areas enabling us to hone the design and the building orientation and location to maximise the benefit of sun or design spaces to avoid excessive solar glare or exposure if required. This also allows us to hone the design of shading devices such as eaves overhangs, Briese Soleil’s, Canopies and other features to maximise their effect.


We can also software simulate and calculate the amount of natural daylight that the building designs habitable spaces are exposed to. This allows us to hone the design to maximise the amount of natural light specific rooms can enjoy (such as living spaces, classrooms or office spaces).

BIM – IESVE Workflow

We already design our buildings in 3D using the latest BIM software. We can now seamlessly pass our BIM design models into the thermal modelling environment and assess them at each workstage to hone the design as it develops. Identifying any potential problems early to inform the design as it develops, rather than fix design problems at later stage when they entering into a more technical design phase.

Ultimately the aim is to continue to design beautiful buildings people can enjoy, but also ensure the building design benefits the occupants as passively as possible, to ensure internal spaces are comfortable throughout the year and lower their energy use as part of the design.

For more information on Part O, get in touch. We will ensure that your new home meets the requirements of the regulations and minimise the cost and time impact, whilst enabling design success.