Since June 2022, trickle vents have caught out a lot of installers. The rules in Approved Document F (ventilation) changed, and the short version is this: when you replace windows, you usually can't reduce the background ventilation that was already there. Get it wrong and you're looking at condensation complaints, failed sign-offs and callbacks. Here's what actually changed and how to stay clean on every job.
The short answer
On a replacement window, the safe default is: if the old window had trickle vents, the new one must too — and to at least the same equivalent area. If the old window had none, the guidance is still to fit them unless you can show ventilation is provided another way. When in doubt, fit vents.
What changed in 2022
The updated Approved Document F came into force on 15 June 2022. The headline for window installers is the "no worse than before" principle: a replacement window must not make a room's background ventilation worse than it was. In practice, because most windows being replaced are older units that either had trickle vents or leaked air around the frames, the compliant path is almost always to include trickle vents in the new units.
This sits alongside the tighter airtightness we now build to. Modern windows seal far better than the ones they replace, so the controlled background ventilation a trickle vent provides is doing a real job — clearing moisture and stale air that used to escape through draughts.
Equivalent area — the number that matters
Ventilation isn't measured by the size of the slot but by equivalent area (EA), given in mm² — a measure of how much air actually flows through. Document F sets recommended background ventilation levels per room, and habitable rooms typically call for around 8,000 mm² EA, with wet rooms and smaller spaces differing. The key compliance test on a replacement is matching or bettering what was there before.
The practical takeaway: don't eyeball it. Check the EA rating of the trickle vent you're specifying (the manufacturer publishes it) and make sure the vents on the new window meet the room's requirement.
Do you always have to fit them?
Not literally always — but the exceptions are narrow. You can omit trickle vents if adequate background ventilation is genuinely provided another way (for example, a suitable mechanical ventilation system serving that room). What you can't do is quietly drop the vents to give the customer a "cleaner" look and leave the room under-ventilated. If a customer pushes back on the appearance, that conversation is better had — and documented — before the order than after a condensation complaint.
Common ways installers get caught out
- Replacing a vented window with an unvented one to tidy the look — the classic breach.
- Fitting vents with too small an equivalent area for the room.
- Assuming a competent-person scheme sign-off ignores ventilation — it doesn't; F is part of the same Building Regs.
- Not recording the decision where vents are omitted for a valid reason.
How it affects the look
Some customers don't love the appearance of a vent in the frame head. The good news is modern trickle vents are far slimmer and can be colour-matched to the frame, and on many systems they're integrated neatly into the head so they read as part of the profile rather than a bolt-on. It's worth showing the customer an example so appearance is agreed up front.
The bottom line for your jobs
Treat trickle vents as standard on replacement work unless there's a documented reason not to, match the equivalent area to the room, and agree the look with the customer before you order. Do that and ventilation stops being a callback risk and becomes a box you tick without thinking about it.
Every window we supply can be specified with compliant, colour-matched trickle vents to the equivalent area the job needs — across both our uPVC and aluminium ranges. Tell us the rooms and we'll make sure the vents are right on the order, so it arrives ready to fit and sign off.
