Recent record-breaking temperatures and the extreme weather conditions anticipated in the climate emergency mean it is vital to make new dwellings safe and comfortable in hot weather.
Clearly, a significant number of dwellings have become unbearably hot, to the extent that they are a threat to life – for example, a UK government report recorded 2,985 excess deaths due to the heatwave in 2022.
Given the clear risk to life, bringing in measures with the aim of preventing overheating seems proportionate, necessary and welcome.
Ensuring practical and effective heat mitigation
Overheating: Approved Document O was published in 2021 and came into effect on 15 June 2022, and only applies to new dwellings. The premise of the guidance is to minimise excessive heat caused by solar gain in the building, and to mitigate any such heat build-up that does occur.
The document identifies two ways of assessing compliance with these requirements, the first defined as the simplified method and the second as the dynamic thermal modelling (DTM) method.
Regardless of the methodology used though, Overheating: Approved Document O also sets out guidance to ensure that any openings included to help mitigate heat can, and will, be used effectively. These should not just be a theoretical number in the modelling that will, in reality, not be used by the occupants.
To ensure the openings are useable, there is also guidance on security considerations for open windows, particularly at night, and on how effective windows will be in noisy or polluted areas, for example near busy roads.
Fall protection conflicts with other guidance
Having covered the unintended consequences of the revised Fire safety: Approved Document B, in terms of construction issues and balcony design, in this series of articles, I was aware that Overheating: Approved Document O had also given rise to some unexpected problems.
For instance, it contains guidance on safety issues such as protection from falling; but it sets higher standards than those in Protection from falling, collision and impact: Approved Document K, which covers such protection in all other situations.
The rationale behind this is that to mitigate overheating under Overheating: Approved Document O, windows will need to be open for extended periods – and that this assumes the risk of falling is greater.
The guidance in the document requires that where any window can be opened more than 100mm, the sill or guarding height is a minimum of 1,100mm above the internal finished floor level (FFL).
However, this is considerably higher than the Protection from falling, collision and impact: Approved Document K minimum of 800mm, although this may be addressed in future revisions of the document.
Overheating: Approved Document O may also conflict with Fire Safety: Approved Document B on fire safety, which states that any window used for the purposes of escape must have a maximum sill level of 1,100mm above FFL; in two-storey dwellings, it is quite common that the upper windows are designed for escape as part of the fire strategy.
Therefore, when Overheating: Approved Document O was published, any window used to mitigate overheating – but also as an escape window – had to have a sill height at a simultaneous maximum and minimum of 1,100mm.
When the frequently asked questions (FAQs) on Overheating: Approved Document O were published in June 2022, FAQ 16 stated that, where a window is used for escape purposes, a build tolerance of ±100mm would be reasonable.
Therefore, the window sill in this situation could be 1,000mm above the FFL. However, FAQ 16 does go on that to say that the Overheating: Approved Document O expects the 1,100mm guarding height to be achieved. Clearly these are contradictory requirements, and it is almost impossible to fulfil both of them.
'To mitigate overheating windows will need to be open for extended periods – but that means the risk of falling is greater'
Accessibility requirements must also be met
Approved Document M: access to and use of buildings, volume 1: dwellings contains guidance on the heights at which various controls found in a typical home should be mounted, such as switches and sockets. The purpose of this guidance is to ensure that wheelchair users can comfortably reach any that are used on a regular basis.
The guidance requires window controls to be a maximum of 1,400mm above the FFL, with a limit of 1,200mm for category M4(3) dwellings.
This requirement – when combined with the minimum sill height of 1,100mm in Overheating: Approved Document O – can restrict the maximum window head height, that is, the top of the window, that can be achieved.
This is because a typical side-hung window normally requires latching within about 100mm of the centre point of the frame. As the handle can be a maximum of 1,400mm above the FFL to meet Approved Document M: access to and use of buildings, volume 1: dwellings – and the sill is a minimum of 1,100mm above the FFL to meet Overheating: Approved Document O – the latch will only be around 300mm above the sill.
If this is assumed to be around 100mm from the centre of the frame, the overall window height would only be around 700mm. Given the maximum sill height of 1,400mm, this means the head of the window would be at a maximum height of 1,800mm – which is very low, given that it would normally be 2,100mm or more.
If alternatives to a simple latch are used to open the window – such as the multipoint espagnolettes often found in tilt-and-turn window arrangements – the handle can be lower than the midpoint of the frame, which would offer more flexibility and the possibility of a greater window head height.
An alternative option would also be to mount the latch at the bottom of the frame, thus bringing it well within 1,400mm above the FFL and make a top-hung window open out.
Fulfilling both Approved Documents M and O
There are four ways of meeting both sets of requirements.
The first is to limit the opening of the window to 100mm by using fixed restrictors, as discussed in FAQ 20 in Overheating: Approved Document O.
This would mean the minimum sill height is 800mm, as shown in Protection from falling, collision and impact: Approved Document K. This option can be used if the restricted opening still provides adequate ventilation and overheating mitigation, while the windows must not be used for escape purposes.
The second option is to use a window that opens inwards, with a sill that is a minimum of 600mm above the FFL and has 1,100mm-high external guarding, as illustrated in 2015's BCA Technical guidance note 16.
The third option is to use a window that opens outwards with a 1,100mm-high sill, in conjunction with an advanced opening mechanism, such as tilt and turn or multi-point espagnolette.
The fourth option is not to use windows for overheating mitigation at all. This would only be possible by ascertaining that the building would still comply with Overheating: Approved Document O by using the DTM method.
Mitigation could then be achieved either by mechanical means, such as an air-source heat pump with cooling capability, or ventilation through louvres or other secure and controllable openings.
Although the introduction of Overheating: Approved Document O has caused some headaches and limited some design options, with a bit of creative thinking it is possible to meet its requirements without coming into conflict with Fire safety: Approved Document B and Approved Document M: access to and use of buildings, volume 1: dwellings.
In the fourth and final article of the series, I will discuss the impact of the revised Ventilation: Approved Document F, in particular the greater adoption of mechanical ventilation with heat recovery in new dwellings.
Tony Hall is a retired former technical design director at Levitt Bernstein
Contact Levitt Bernstein: Email
Related competencies include: Health and safety, Legal/regulatory compliance