Building Regulations Part O (Overheating) Explained

Blueprints of new home development
Developing a new home? Part O will impact how your home is designed. Read our guide for more info.

There’s been a lot of buzz around the updates to Building Regulations Approved Document Part O. These came into effect on 15th June 2022 and apply to the development of all new residential buildings.

What does this mean for your new build? Do you have to comply with the overheating rules? How will it affect your current plans? In this blog, we’ll be discussing everything you need to know about the Part O building regulations – let’s get started…

What are the Part O (overheating) building regulations?

Part O is a new Approved Document within the latest Building Regulations which covers overheating in new residential properties. This is approached by ensuring overheating can be prevented and remedied. For example, by limiting the amount of direct sunlight that can enter the property through glazing in doors and windows, excessive heat build can be prevented. Pairing this with a remedy to swiftly remove excessive heat from a room can effectively combat overheating.

The addition of Approved Document O has majorly impacted housing design – particularly the positioning of windows and façades. This can be more prevalent in areas which the Part O regulations deem to be ‘high risk’ – view Appendix C and find a table full of ‘high risk’ postcodes in London.

What can cause overheating?

When direct sunlight enters a building through glazing such as windows and doors, those solar gains can generate heat build in the room it has entered. In a similar way, solar glare bouncing off reflective surfaces (such as metal and water) into a building can also contribute to overheating. These are the main issues that Part O addresses for new build homes, though there are other contributing factors such as surplus heat generated by heating pipes. This is more relevant in communal buildings such as apartment buildings where you may not have control over all of the heating.

In many areas of the UK, there has often been a lack of sunshine outside the summer months, so properties had often been designed to maximise on natural light by including lots of glazing. With temperatures increasing due to climate change, the risk of overheating in homes is on the rise. This is why Part O has been introduced and is essential in ensuring residential properties are being designed to prevent overheating.

How to comply with overheating building regulations

There are two methods which can be used to comply with the Part O building regulations:

  • The Simplified Method.
  • The Dynamic Thermal Modelling Method.

What is ‘the simplified method’?

The simplified method is for restricting solar gains by limiting glazed areas. It requires a specialist consultants to assess the home design and provide a simple pass or fail. Should it fail, the architect will be required to keep working on the design until the issues causing the failure are fixed.  

Part O provides a table identifying how much glass the outside of a building can have based on the total floor space inside. The simplified method is restrictive on window sizes and can be tricky to work to in ‘high risk’ areas in situations like:

  • West-facing properties 
  • Single-aspect properties (A building with windows or openings on just one side).
  • Common areas (such as hallways, stairs, and laundry rooms).
  • Locations with pollution and noise issues

Each property situated in a ‘high risk’ area will likely require window shading between its northeast and northwest orientations. Dual-aspect homes might pose difficulties, as per the overheating building regulations, cross ventilation is only considered valid when it occurs on opposing sides of the property.

Reducing window sizes to meet the criteria may impact daylight needs. Rooflights offer a potential remedy by augmenting daylight, although they aren’t explicitly factored into overheating assessments. If ‘high risk’ locations, alongside adhering to maximum glazing guidelines, you must also provide one of the following:

  • External shutters which can provide ventilation – such as Solar Screens.
  • Glass glazing with a minimum light transmittance of 0.7 and maximum G value of 0.4.
  • An overhang with 50 degrees altitude cut off, exclusively on south-facing exteriors.

What is ‘The Dynamic Thermal Modelling Method’?

The dynamic thermal modelling method provides significantly more design flexibility, as it adopts a comprehensive approach to the entire building. This task should be handled by a professional well-versed in the Chartered Institute of Building Services Engineers (CIBSE) TM59 methodology. Specialised thermal modelling could be considered the most viable path to compliance for:

  • Internal communal areas (such as hallways, stairs, and laundry rooms).
  • Flatted developments.
  • Single-aspect developments and ‘high risk’ locations.

We can support you…

Are you an architect, developer or self-builder looking for oversized solar shading or better ventilation in a new build home to help you comply with the Part O regulations? 

We’re experts in big external blinds with a wide range of solar shading and bug free ventilation screens. 

Get in touch with our team to discuss adding externally fitted automated solar screens of up to 12m wide to your property, to help you comply with the new overheating rules in the Part O regulations.

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