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Building Regs "Part O" and window specification

18. December 2023

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The introduction of Part O, and how does it affect windows specification

As temperatures across Europe continue to break records, cool and comfortable housing is becoming a priority especially in countries where air conditioning is rarely installed. Building Regulation Part O, launched in June 2022, aims to address this challenge by requiring all new-build homes to actively minimise unwanted solar gain and excessive heat, which for window specification means:

  • Reducing the amount of glazing installed on south-facing facades, and considering the size of glazed screens in habitable rooms
  • Improving cross-ventilation by installing more opening windows
  • Proactively introducing shading strategies

Does this mean small windows? Not necessarily!


For many housebuilders and developers, large windows are a ‘must have’ in new build homes. They enhance ‘kerb appeal’, and can make a valuable contribution to energy efficiency by bringing in more natural light and improving insulation. A return to small and limited windows seems like a backward step, but can be avoided - and Part O compliance still achieved - with strategic planning:

  • Use the Dynamic Thermal Modelling method to calculate overheating. Part O compliance is assessed either by using the ‘Simplified Method’ or by Dynamic Thermal Modelling. This more detailed analysis looks at solar gain seasonally and hourly, and takes into account a range of other factors such as local shade (from trees or other buildings) and low energy and ventilation technologies installed in the home. As a result, Dynamic Thermal Modelling gives housebuilders and architects much greater flexibility when it comes to window design.
  • Make window specification façade-specific, and tailor each window to its position within the façade. This gives precise control over solar gain and more options when planning natural ventilation strategies.
  • Consider windows and shade together, for example by designing homes which combine widows with structural shading solutions, such as deep eaves, balconies or brise soleil, or which exploit local shade.


Finding the ideal balance

Part O is one of many Building Regulations that impact on the compliance of a project. This means that housebuilders must meet the demands of Part F (ventilation) , Part L (energy conservation)  and Part M4 in addition to Part O, and this can throw up a range of challenges. For example:

  • How to maximise ‘free energy’ from positive solar gain without causing overheating?
  • How to ensure good ventilation while minimising the impact of external pollution, acoustic and atmospheric?
  • How to ensure windows are safe and secure without restricting ventilation, especially in family homes or low rise apartment developments?
  • How to make sure windows and doors are operable in accordance with part M4(2) (accessibility) ?

Window systems and Part O compliance - what to look for:

You need to find an experienced window supplier when navigating the complexities of Part O, so look for:

  • Proven in-house technical knowledge - A good supplier wants to help you realise your vision, and will tell you if your ideas won’t work. Look for a supplier with in-house specification advisers who can work with you or your architect from the earliest stages of the planning process.
  • Bespoke manufacture - You can easily achieve façade-specific performance if every window is tailor made (shape, size and performance) - if window design maintains aesthetic uniformity then that’s another bonus.
  • Support in windows specification and design - a good window supplier can help you achieve your targets by for example mixing double and triple glazing and  giving advice about the best windows size and configuration for each elevation of your building.

  • System versatility - The ideal window system will support precise window specification and a faster route to Part O compliance - for example, you should be able to choose from a wide range of glass options, including solar control glass, and select accessories designed to allow safe and effective ventilation, such as lockable window restrictors and trickle vents (both standard and acoustic).


Considering VELFAC?

If you have any question regarding Part O compliance, please contact us. We are ready to help. 




Topics: Architecture, Building Regs

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