In the context of the building fabric, opening windows are complex products. Their complexity arises from various issues including the number of constituent parts, mechanisms and moving components; the need to maintain weathertightness over repeated use cycles; maintenance obligations; production controls and the necessity to record information required for legal sale under the Construction Products Regulations. Once installed, they may also need to function as safety-critical components, a role that is sometimes overlooked.
This is especially the case where windows also need to serve overheating requirements, purge operations and/or egress alongside other obligations.
If utilising the Approved Documents in the design of windows, it is likely to be covered by sections of the following documents:
- AD B – Fire,
- AD F – Ventilation,
- AD K – Collision impact/falling,
- AD L – Thermal performance,
- AD M – Access/use,
- AD O – Overheating,
- AD Q – Security.
Therefore, a window may be subject to multiple constraints, including constraints that can sometimes compete with each other. For example, it may need to resist horizontal barrier loads in accordance with BS 6180:2011 while also being restricted so that the opening prevents the passage of a 100 mm sphere. The Approved Documents may also allow the restrictor to be unlatched or otherwise defeated to satisfy purge ventilation, maintenance or egress requirements. This can create unsafe situations where a design appears to meet individual Approved Document clauses when considered separately or in specific states but does not adequately satisfy the wider Building Regulations objective of occupant safety.
Additionally, even in a restricted position, many window restrictors are designed to hold a window in place temporarily. They are often not designed to withstand a horizontal barrier load while restricted or wind loading (see ‘Are window restrictors designed to resist adequate loads?’). Following a recent legal case, it should now be accepted that it is reasonably foreseeable that open out vents could be left in the open (restricted) position and that the industry’s assumption that such vents will naturally be closed in high winds has been challenged.
The following is an example that aims to highlight this potential complexity of meeting the requirements across the Approved Documents for a window.
An outward opening window in a dwelling with a cill level 800 mm above finish floor level (FFL) in accordance with Part K, capable of opening to a minimum of 15 degrees, may meet the requirements of Part F for purge ventilation.
If the window as above is used in order to meet overheating requirements in Part O, it either needs appropriate additional guarding introduced to a level up to at least 1100 mm, or it needs to be fitted with a device that will restrict the opening to not allow the passage of 100 mm sphere even when under the appropriate barrier loading from BS 6180:2011 applied at 1100 mm. That is to say, a Part O compliant window requires a barrier present at 1100 mm, whereas a Part F compliant window may not.
In the restricted condition as above, with a cill height of 800 mm from FFL, the restrictor may be defeatable for the purposes of purge ventilation. However, in this defeated condition it can no longer sustain the barrier loading at 1100 mm, while preventing the passage of a 100 mm sphere, which leads to an issue in meeting all relevant parts of the Approved Documents in differing conditions for the same window.
There may be an argument that the purge condition is temporary and therefore not required to carry barrier load at that instance, but this is by no means an objective assertion. This condition is used to highlight that window complexity depends not only on its design location, but also its position(s) in use under differing operational conditions and associated requirements placed on all associated componentry.
The above is intended as a brief overview of some of the issues arising from openable windows. We strongly encourage feedback on this subject related to any issues, concerns and complications arising from the design, installation, maintenance and use of openable windows. This information will be used in conjunction with our ongoing research with a view to produce a Technical Note on the subject in due course to give further guidance.
See also 'Should outward opening windows be used in high rise buildings?'
Duncan Grover - September 15, 2026
A great question, and one that is certainly not limited to windows. Similar tensions between Approved Documents and performance requirements can be found across the entire building envelope. Unfortunately, there is rarely a single solution.
We are often asked to demonstrate compliance with a specific standard or classification in isolation, typically because a particular performance characteristic has been identified as the project’s primary driver, whether that is thermal efficiency, acoustics, fire, safety or security. The risk with this approach is that an overly narrow focus on one characteristic can inadvertently compromise other equally important performance requirements that may receive less consideration during the design process.
Overheating is a good example. It has become a significant challenge in many modern refurbishment projects, often driven by the push for ever-improving thermal performance of the building envelope. While improving energy efficiency is essential, doing so without considering the wider performance balance can create unintended consequences for occupant comfort..
Rather than seeking to optimise a single performance criteria, building envelope design should be supported by a holistic risk assessment approach that considers the interaction between competing requirements. The question should not simply be whether a solution complies with an individual regulation, but whether it creates an acceptable balance of risks and benefits across all relevant performance characteristics throughout the building’s lifecycle.
Too often, building design is driven by demonstrating compliance with individual regulations, with limited consideration given to how the building will actually perform for its occupants in use. Compliance should be the starting point, not the end goal. Successful building envelope design requires informed judgement, understanding the consequences of design decisions, and balancing multiple competing requirements to deliver a safe and functional building.