How Does Glazing Relate to Building Regulations Part O?

Building Regulations Part O aims to mitigate overheating in a new or renovated building. It outlines ventilation requirements and solar gain, but how does it relate to glazing?


01 May 2023


Simon McAuliffe

Building regulations exist to ensure a new construction is going to be safe, high quality, and its occupants healthy.


They might dictate the amount of airflow in a room, the level of resistance offered by a window to force entry, or, central to this piece all about Building Regulations Part O, the architectural steps to mitigate overheating.

But it’s no trade secret that the documentation outlining building regulations  can feel bewildering at times. Language might be overly complicated, examples unhelpful and advice ambiguous.

In instances where certain regulations have existed for many years, such as Part E (resistance to the passage of sound), the internet is filled with supplementary information, simplified guides and plenty of real working examples.

But for new regulations, such as Part O, you can sometimes feel stranded with only a poorly drawn map for guidance.

As glazing specialists, we wanted to provide a simple, readable guide to Building Regulations Part O, relating to overheating in the world of glass.

We hope that’s exactly what we’ve done here.

First, let’s understand the reason for Part O’s creation by shedding some natural light on its context.


Building Regulations Part O in context


The town of Coningsby in Lincolnshire has a population of 5,500 and is known for little aside from its mediaeval church and bustling RAF base. But in July 2022, it set the record for the highest meteorological temperature ever recorded in the British Isles.


More than double the expected average (between 16 and 20°C), even at the height of summer.

And in all probability, the record for the highest temperature will be broken several more times over the next few years as the world grapples with the effects of climate change. On the continent, where that figure is only marginally above the norm, architects have had generations of experience building accordingly, learning lessons and keeping occupants cool throughout the summer months.

You only have to gaze up at blue-domed sugar cube structures in Santorini or stroll along Spanish streets where external shutters are closed tightly throughout the hottest part of the day to see the science of radiation reflection and the importance of shade in action.

But in the UK, these high summertime temperatures didn’t exist when many of our houses were built, so these architectural methods of keeping cool are only now becoming more important to us.

And legislation has had to adapt.

That’s where Building Regulations Part O comes in, introduced in June 2022 and outlining your responsibility to mitigate overheating specifically in relation to “dwellings, institutions and other buildings that contain one or more rooms for residential purposes.”

In doing so – mitigating overheating in residential buildings – you might think that glass has no place. After all, glazing is used to build greenhouses – specifically designed to increase light and temperature – and we’ve all felt the warmth of a south-facing window on a summer’s day.

Can you still include glazing in your building project whilst adhering to Part O Building Regs? 

The short answer is absolutely yes.

With a little thought and the introduction of some clever technology, you can keep your home bright and cool even when the temperature steps into unprecedented territory. 

But we’re getting ahead of ourselves.

Let’s take a look at Building Regulations Part O in a little more detail.

A comfortable sitting room, necessary to keep cool in summer
Keep comfortable in direct light

What is Building Regulations Part O?


Building Regulations Part O applies to new builds and adaptations of existing residential properties to mitigate overheating, by reducing the occurrence of high indoor temperatures. It does so by outlining requirements for limiting solar gain and removing excess heat.


What is solar gain?


Solar gain, in the context of Building Regulations Part O, is the increase in air temperature caused by direct sunlight. 

Ensuring and demonstrating compliance with Building Regulations Part O can be split into two methods: simplified and dynamic thermal modelling.


Simplified methods


Using simplified methods to ensure and demonstrate compliance with Building Regulations Part O could be seen as a standard, ‘one-size-fits-most’ approach, suitable for smaller, simpler, more uniform buildings and adaptations.

The process for following the simplified method begins with analysis, categorising all residential or part-residential properties into those at high risk of overheating (urban and some suburban areas of London) and those at moderate risk (everywhere else in England).

As part of the UK government document, simplified categorising also takes into account whether the applicable area or whole new build has cross-ventilation (where an opening can be made at opposite sides of a room to let air flow through)

Having specified which camp your building project falls into and highlighted the orientation of its largest glazed façade, the document outlines the maximum area of glazing (by % of floor area) that your rooms can have.

Buildings located in the high-risk territory should also provide shading for glazed areas between compass points north-east and north-west via the south. 


According to the regulations, shading should be provided by one of the following means:

  • External shutters with means of ventilation;
  • Glazing with a maximum g-value of 0.4 and a minimum light transmittance of 0.7; or
  • Overhangs with 50 degrees altitude cut-off on due south-facing façades only.


This simplified method also provides guidance for removing excess heat. It does so by specifying the minimum amount of free space that a window should have. What that means is the total number of openings in a building’s external walls, windows, and roof lights that are used to provide natural ventilation to the building.

As an example of removing excess heat, part of a building in the high-risk zone should have a total minimum free area of either 6% of the floor area or 70% of the glazing area (whichever is greater).

If you have a unique project in mind, particularly one that will make use of Cantifix’s proactive design approach and quality products, it’s unlikely that simplified methods will be suitable. After all, these don’t take into account things like the building’s size, shape, insulation materials, types of glazing used, and whether shading is provided by trees or other buildings.

For your project to comply with Building Regulations Part O, dynamic thermal modelling is likely to be far more suitable.


Dynamic thermal modelling


Undertaking this method requires you to follow the guidance given by the Chartered Institution of Building Services Engineers as part of their published TM59 methodology.

Dynamic thermal modelling involves creating a computer simulation of your building project including elements such as the size and shape of the building, insulation materials, types of glazing used, and surrounding environment.

The simulation then models how the building might behave in different weather conditions, allowing you to identify potential issues with overheating and make adjustments to the design as needed.

Dynamic thermal modelling paints a far more accurate picture than simplified methods, but – as perhaps you’ve already guessed – it can be more expensive to undertake.

A comfortable sitting room, necessary to keep cool in summer

Will getting rid of all glazing from my project make it Building Regulations Part O compliant?


Yes. Probably. After all, caves have a lower temperature than greenhouses.

But aside from breaching several other pieces of building regulations, would your clients really want to live in a cave? Of course not. But they’d probably not be too keen on living in a greenhouse either.

Instead, consideration should be taken to balance the need for natural light with the potential for uncomfortably high temperatures. But as you are likely also aware, not all glass is created equal, and several technologies can reduce solar gain significantly 

Those technologies include:


Low emissivity glass


A microscopically thin metallic coating on the glass reduces the solar radiation that passes through the window without reducing the natural light. This option is ideal for south-facing Victorian side return extensions where temperatures are kept comfortable while maintaining brightness in the space.


Tinted glass


While tinted glass works in a very similar way to low emissivity glass, this technology absorbs solar radiation before it passes through the glass. While it is very effective at keeping temperatures low, it can reduce the amount of natural light in a space.


Spectrally selective glass


Perhaps the most high-tech sounding glazing technology, SSG allows certain wavelengths of solar radiation to pass through the glass and is deployed to achieve a range of aesthetic effects, such as mirrored or coloured.


In summary


Hot summers aren’t going anywhere. In fact, they’re likely to get more intense as we continue to navigate climate change. But that doesn’t mean you should say goodbye to glazing. With a little bit of clever thinking and the right technology, you can keep your home cool and bright, even in the face of scorching temperatures.

So let the sun shine in and beat the heat with glazing that complies with Part O Building Regs!

If you’d like to know more, get in contact with our talented team today.