The new approved Document Part O, first published in December 2021, will be formally adopted on 15 June 2022. These new requirements deal with overheating in residential buildings, including dwellings, residential institutions and student accommodation and may have a significant impact on the design of these buildings in the future.

Historically, dwellings were highly permeable with limited or no insulation – this mitigated the risk of overheating. However, as insulation levels and airtightness levels have increased due to changes in legislation, the issue of over-heating of dwellings has become more significant, requiring a method to limit solar gain and remove excess heat from the building.

The Approved Document provides two methods for compliance - the ‘simplified method’ and ‘dynamic thermal modelling’.

The ‘simplified method’ approach will be dependent on the building location (‘moderate risk’ or ‘high risk’) and whether cross-ventilation of the building is possible or not. The key element within this section is table 1.1, which provides a maximum area of glazing for each elevation. For example, in a moderate risk location, on a building with cross-ventilation, the maximum area of glazing on the south elevation is 15% of the floor area with a maximum of 30% in the most glazed room.

This limitation will have a profound effect on many dwelling designs. Many recent developments create rooms with a large area of glazing to take advantage of views and create a light filled interior. With a limit of 15% glazing, this will result in smaller and fewer windows.

The other factor is the removal of excess heat. Table 1.3 lays out the requirements for a ‘free area’ for ventilation. For example, in a ‘moderate location’ a ‘free area’ of 12% of the floor area is required. This would imply that the majority of windows would need to be fully openable, rather than fixed glazing or on opening restrictors. The impact of opening windows on internal noise levels will also need to be taken into account, meaning that in many locations, windows cannot be opened due to external noise sources.

The second method is ‘dynamic thermal modelling’. This method allows for greater flexibility on the dwelling design and creates a far more site specific assessment. This approach also allows the use of secondary measures, such as external blinds and over-hangs to limit solar gain. For the removal of excess heat, the use of mechanical ventilation systems may also be used. This approach will require the input of a suitably qualified consultant using specialist software and would need to be building specific.

The adoption of Approved Document Part O is in the early days, but the long term impact on the design and appearance of residential buildings in the future has yet to be fully investigated by the wider industry.

To discuss your project or property please contact our Architectural team on 0333 920 2220.