What are the ventilation requirements when replacing windows?

Ventilation falls under Part F of the Building Regulations, and the rules changed significantly with the 2021 edition of Approved Document F, in force in England from 15 June 2022. The old “no worse than before” approach no longer applies as a default. Under the current rules, every habitable room with replacement windows must be provided with background ventilation to the minimum equivalent areas for that room type — and in practice that means trickle vents on the new frames, unless adequate background ventilation is already provided by another compliant source.

The minimum equivalent areas are 8,000mm² for habitable rooms such as lounges and bedrooms, 8,000mm² for kitchens, and 4,000mm² for bathrooms. Ventilation is assessed per room, not per window — so the total trickle ventilation across the windows in a room needs to meet the figure for that room.

The exception is where the room already has another compliant source of background ventilation. If the dwelling has continuous mechanical extract ventilation, habitable rooms (outside wet rooms) need only 4,000mm² in their replacement windows. If there’s whole-house mechanical ventilation with heat recovery (MVHR), or an existing wall ventilator already meeting the equivalent area for that room, additional trickle vents in the new windows may not be required. These alternatives need to satisfy the building control body.

Two points worth knowing as the installer: a homeowner refusing trickle vents — even by signing a disclaimer — is not a valid route to compliance, because the competent installer is responsible for designing the window to meet the regulations. And the work now requires an Installation and Commissioning Checklist, covering background ventilation sign-off, to be completed and handed over on completion.

When you order from us, tell the depot the room types and the ventilation provision in the property, and we’ll make sure your frames are specified with the correct trickle ventilation to meet Part F — far cheaper than retrofitting vents or failing an inspection.

Note, the above is applicable to dwellings in England, Welsh requirements differ and you should check current Welsh Building Regulations.