What the regulation actually says
Approved Document F, volume 1, applies in England from 15 June 2022. The principle on a window swap is simple: ventilation must not be left worse than it was before the work. Tighter frames and better seals cut air leakage, so the replacement has to put back the background ventilation the old draughty unit gave for free.
In practice that means background ventilators on most replacement windows. Frame trickle vents are the usual route, not the only one. A through-wall background ventilator of the same equivalent area is acceptable, and so is existing provision that already meets the standard.
Equivalent area is the figure that matters, not the physical slot size. It is a measured airflow value, so specify vents by their stated equivalent area in mm2, not by how big they look.
Equivalent areas by room
Minimum equivalent areas are set per room type. Habitable rooms and kitchens take the higher figure; wet rooms such as bathrooms take the lower one. Most window trickle vents are rated around 4000mm2 each, so a habitable room often needs two to hit the target. Confirm each value against the current Approved Document F tables before you order, because building control checks against those numbers.
Where continuous mechanical extract is fitted, a reduced background figure can apply. Where the structure makes the full figure impractical, fit as close to the minimum as is reasonably achievable and record the reason.
See energy efficiency for how background ventilation sits alongside the Part L airtightness and U-value side of the same job.
- Habitable rooms and kitchens: higher equivalent area per opening, commonly met with two vents
- Bathrooms and wet rooms: lower equivalent area
- Continuous mechanical extract present: reduced background figure may apply
- Existing vents being replaced: new vent must be no smaller than the original
- Verify every value against the live Approved Document F tables before ordering
Exceptions and the wall-ventilator route
If the room already has a wall ventilator that meets the Approved Document F equivalent area, you do not need to add more after swapping the window. Survey for existing provision before you quote, because it changes the spec and the price.
Two things are not a way out. Windows left on the night-latch do not count as secure background ventilation. Homeowner disclaimers, saying they will fit vents later or do not want them, do not satisfy the regulations. The work has to comply when it is signed off.
Specifying it on trade jobs
Decide the ventilation strategy at survey, not on site. Record the equivalent area you are providing per room and how, whether that is frame trickle vents, a through-wall ventilator, or existing compliant provision you are keeping. That record supports the FENSA or building control sign-off.
Match the vent to the product. Bifold and slider heads leave less room for frame vents, so plan the route early. For aluminium casement windows and tilt and turn windows we supply units with trickle vents factory-fitted to the equivalent area you specify.
Tell us the equivalent area per opening with your order and we fabricate to it. Trade buyers can request a quote with the schedule; homeowners should find an installer to handle the compliance paperwork.