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An opening window can form part of an overheating strategy, but its effectiveness also depends on how far it can practically open and whether issues such as security, guarding and external noise affect its usable ventilation area. Approved Document O specifically requires these usability and safety considerations to be taken into account.
Again, there is no single product characteristic that resolves everything. The challenge is balance.
Late changes can have wider consequences
This is particularly important during value engineering and procurement.
Changing a window size, glazing specification or opening arrangement may appear relatively minor, but it can alter assumptions made earlier around thermal performance, solar gain or ventilation.
That does not mean projects cannot change.
It means changes should be treated as design decisions rather than simple product swaps.
For fabricators and installers, that also reinforces the value of understanding exactly what has been specified and why, rather than seeing the window schedule purely as a manufacturing instruction.
A better question for the industry Part L and Part O are often discussed separately because they deal with different regulatory requirements.
In practice, the window does not experience them separately.
It has to perform thermally, manage solar gains as part of the wider design, provide the required ventilation strategy, meet security and safety requirements and remain practical for the occupant to use.
That is why I believe the next step for fenestration specification is less about chasing isolated headline numbers and more about understanding the complete configuration.
The question should not simply be:“ How low can the U-value go?”
It should be:“ Does this window, in this configuration, work for this building?”
That is ultimately where compliance, performance and good specification come together.
The practical challenge is turning these principles into project-specific decisions. VEKA’ s specification team supports architects and specifiers with configuration reviews, technical information and tools such as WinDoPlan, which can be used to explore whole-window U-values, wind loads, sections and 3D configurations.
Request access to WinDoPlan via VEKA Edge: www. vekagroupportal. co. uk / register /
CLEARVIEW-UK. COM NOVEMBER 2026
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