In most cases, yes. Since the 2022 update to Approved Document F, replacement windows in an existing dwelling generally need trickle vents — and critically, the new windows must not reduce the background ventilation that was there before.
If the window you are replacing had trickle vents, the replacement must have them, and they must be at least as effective. That part is unambiguous and it catches a lot of people out, because installers used to routinely leave them off on request.
If the old window had no vents, the guidance still expects background ventilation to be provided in the replacement unless there is an equivalent provision elsewhere in the room. In practice, most installers will fit vents by default, because a FENSA or CERTASS self-certification depends on the installation being compliant.
Ventilation is measured in equivalent area — the area of a sharp-edged hole that would pass the same amount of air — in square millimetres. Approved Document F sets minimums by room type; habitable rooms in an existing dwelling are commonly 8,000mm², with lower figures for wet rooms.
A trickle vent is stamped with its equivalent area, so this is checkable rather than a matter of opinion.
Nobody wants vents. They are the single most common complaint about a new window installation, and the objection is usually noise or draught.
The honest position: background ventilation exists because modern windows are extremely airtight, and a sealed room with people breathing in it produces enough moisture to cause condensation, mould and, over time, damage to the building. The vents are not a regulatory quirk — they are the reason your new windows do not run with water. Acoustic trickle vents cost a little more and substantially reduce the noise objection.
An installer who quietly leaves the vents off to win the job is passing you a non-compliant installation and a certificate that may be worthless when you come to sell.
Under the 2022 update to Approved Document F in England, replacement windows in an existing dwelling generally require background ventilation, and the replacement must not provide less ventilation than the window it replaces. If the old windows had trickle vents, the new ones must have them.
Only where adequate background ventilation is genuinely provided by other means and the installation still complies. An installer who simply omits them to win the job is giving you a non-compliant installation — which can surface later, when you sell.
They let a small, controlled amount of air in — that is the point, and it is what stops condensation and mould. Acoustic trickle vents substantially reduce the noise transmission and are worth the small extra cost on a busy road.
What U-value your replacement windows must achieve under Part L in 2026 — 1.4 W/m²K for an existing dwell…
Why aluminium windows get condensation, why old non-thermally-broken frames stream with water, and what a…
What a FENSA certificate is, when you need one, what happens if your installer isn't registered, and how …
Free survey, fixed price, no pressure. Or call us and we'll give you a realistic ballpark in five minutes.
