Trickle vents (properly, background ventilators) are small, controllable openings — usually in the head of a window frame or through the wall — that provide a continuous, low-level supply of fresh outdoor air. They're 'background' ventilation: always-available, designed to dilute pollutants and moisture across the home, distinct from the 'rapid' ventilation of opening a window and the targeted 'extract' of a wet-room fan.
What they're for
Background ventilators exist to maintain a baseline of fresh air even when windows are shut and extract fans are off — which, in a typical home, is most of the time. They dilute the slow build-up of CO₂, humidity and indoor pollutants, providing 'make-up' air that lets extract fans work (air has to come in somewhere) and helping keep average indoor humidity down. Part F sets minimum equivalent areas of background ventilation per room.
| Role | Provided by | Purpose |
|---|---|---|
| Background | Trickle vents / background ventilators | Continuous low-level fresh air |
| Extract | Wet-room fans (intermittent or continuous) | Remove moisture/odour at source |
| Rapid / purge | Openable windows | Quickly clear pollutants (e.g. after painting) |
Why people block them — and why that backfires
Occupants frequently tape over or close trickle vents because they feel a draught or hear noise, especially in winter. The result is predictable: with the background air supply cut off and windows kept shut, indoor humidity and CO₂ climb, and condensation and mould follow. The 'draught' that gets blocked is often the very ventilation preventing the mould the occupant then complains about.
Background ventilation and airtightness
Trickle vents are a crude, uncontrolled (weather-dependent) form of ventilation — fine as a baseline in a leakier home, but they pass cold air with no heat recovery and their flow varies with wind and temperature. As a building is made airtight, the right move is not more trickle vents but a designed mechanical strategy: continuous extract (dMEV/MEV) or, in a tight home, MVHR. In a Passive House, the envelope is deliberately airtight and background ventilators are replaced entirely by MVHR.
Getting it right
- In a leakier home: keep trickle vents open and working — they're doing a job, even if you can't see it.
- When replacing windows: ensure adequate background ventilators are included and used, or the house loses its accidental ventilation.
- In a tightening retrofit: shift from trickle vents toward continuous mechanical extract or MVHR as airtightness improves.
- In an airtight home / Passive House: MVHR replaces trickle vents entirely, delivering filtered, heat-recovered fresh air with no uncontrolled openings.
Equivalent area — how background ventilation is sized
Trickle vents are not sized by their physical opening but by their 'equivalent area' — a measure, in square millimetres, of how much airflow they actually pass, accounting for the resistance of the vent. Part F specifies a minimum total equivalent area of background ventilation per habitable room (commonly around 8,000 mm² per room, with higher figures for kitchens and the dwelling as a whole, and the exact numbers depending on the building and edition). This is why simply having 'a trickle vent' is not the same as having adequate background ventilation: an undersized or restrictive vent, or one vent where two are needed, leaves the room short. When windows are replaced, the new units must carry background ventilators of at least the equivalent area provided before — a requirement frequently overlooked, which is exactly how a window upgrade ends up reducing a home's ventilation and triggering condensation.
Acoustic and security-rated vents — solving the real objections
Most reasons people give for blocking trickle vents — noise and security — have proper engineering solutions, so the vents should not simply be omitted. Acoustic trickle vents incorporate a sound-attenuating baffle and lining that maintains the required equivalent area while substantially reducing the noise that passes through, which matters on busy roads and near railways where an ordinary vent can be intrusive. Security has been addressed by the vents themselves being part of the sealed, locked window unit, and by hit-and-miss closable designs that let occupants damp the flow without sealing it entirely. The lesson is that the objection to trickle vents is usually solvable by specifying the right vent, not by removing ventilation the home needs — and where noise or pollution is severe, that is itself a strong argument for moving to a filtered, attenuated mechanical strategy instead.
