Part O: What You Need to Know

Phil Brown, European Regulatory Marketing Manager at Pilkington United Kingdom – part of the NSG Group – explains what architects need to know about the new Part O of Building Regulations, how they differ across the country and why they don’t need to spell the end of the highly-glazed dwellings favoured by homeowners.


The hope is that these new regulations – alongside changes to Part L, which legislates for the conservation of fuel and power in dwellings – will help achieve the Government’s Future Homes Standard, an ambition that all new homes built from 2025 will produce 75 to 80% less carbon than those delivered under current regulations.

These new regulations will significantly impact the type, location and quantity of glass that architects specify, increasing the need for solar control glazing, which allows daylight to pass through a window or facade while reflecting away the sun’s heat to help prevent overheating.

What does it mean for glazing specifications?

The supporting Approved Document to Part O identifies two methods of assessment – the simplified method, which is based on location, dwelling type, glazing area and presence of cross-ventilation, and the dynamic thermal modelling method, which involves the calculation of the risk of overheating in the property.

To use the simplified method, a dwelling must not exceed the maximum area of glazing as a percentage of floor area and the maximum area in the most glazed room, with different percentages depending on the orientation of the largest glazed facade.

This recognises that a ‘one-specification-fits-all’ approach isn’t always appropriate. From a glazing perspective, this could result in solar control glass being installed on a large, glazed south-facing room but low-emissivity glass on a north-facing room with smaller windows.

Approved Document O also sets out a solution for providing an adequate means of removing excess heat from the indoor environment, setting out a minimum free area (geometric open area of a ventilation opening), dependent upon floor and glazing areas and whether the building has cross-ventilation.

Mapping out the differences

Architects also need to be aware that a different approach to mitigation may be required, depending on where the residential building is located in the country.

In recognition of the ‘urban heat island’ effect – whereby urbanisations experience higher temperatures than the surrounding rural areas, particularly at night time – the simplified method in Approved Document O categorises urban and some suburban parts of London as a ‘high-risk’ location. It also advises that parts of central Manchester should follow the guidelines for high-risk areas, given the increased likelihood of overheating in those regions.

Residential buildings in high-risk locations need to satisfy maximum glazed areas and should also provide shading to glazed areas between compass points northeast and northwest via the south. Where shading is required, glazing with a maximum g-value of 0.40 and a high light transmittance of 0.70, such as solar control glass, can be used.

However, with overheating a clear and growing problem in residential buildings across the country, including major cities such as Birmingham, Leeds and Bristol, it raises the question, is the double standard set out in the Approved Document an oversight? Is a failure to acknowledge the risk of overheating outside of London and Manchester a missed opportunity to create more sustainable cities across the country? Maybe, but it doesn’t have to be.

Architects building in these metropolises should consider specifying high-performance solar control glass to help prevent overheating. By specifying not just to meet regulations but exceed them, architects can help futureproof buildings against climate change and reduce the overall carbon footprint of a development. But this isn’t the only location-based difference architects need to consider.

From 23rd November 2022, architects working on new dwellings in Wales must comply with the regulations to mitigate overheating set out in the Welsh Government’s recently-published Approved Document O. Like in England, the simplified method includes a maximum glazed area as a percentage of floor area, depending on the type of building, but permits different percentages of glazing if solar control glass is used. For example, for a glazing g-value of 0.40 or lower, developers can use up to a maximum area of glazing of 35% for dual-aspect residential buildings. The requirements cover all new dwellings in Wales, irrespective of an urban or rural location.

In Scotland, the Technical Handbook 2022 for domestic buildings introduces a new section 3.28 covering overheating, which will be applicable from 1st December. Housebuilders and developers operating in all three nations of Great Britain must be aware of the differences to ensure the glass is correctly specified to meet the new requirements.

Specify smarter, not less

For some architects, Part O is getting them hot and bothered. This is seeing them choose to restrict the amount of glazing they are specifying to avoid having to use solar control glazing or calculate compliance using the more complex thermal analysis method.

However, restriction isn’t the answer that homeowners want, and it can only go so far. Large windows delivering uninterrupted views and plenty of natural light often rank among buyers’ and renters’ top priorities when choosing a property. Natural light in the home is also proven to offer numerous health and wellbeing benefits, including improved mood, boosted immunity and better sleep patterns. It can also help to reduce heating and lighting usage, cutting bills and carbon emissions.

Instead of limiting the amount of glass in their designs, architects should be looking to specify smarter by selecting the correct high-performance glazing for the building while making the most of other shading options.

Specifiers must remember that Part O doesn’t mean a trade off between aesthetically-appealing buildings and overheating. By getting in the know about Part O, including the methods of calculating compliance and the location-based differences in regulations, architects can continue to design the homes that people want to live in, while also meeting the demands of our changing world and vital sustainability ambitions.

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