Glossary
Trickle vents and Part F on London window replacements
Background ventilation in window frames. Required by Part F.
Background ventilation in window frames. Required by Part F.
A trickle ventilator is a small controllable opening, usually in the window head or frame, providing background ventilation. Approved Document F recommends an equivalent area of 8,000 mm² per habitable room in an existing dwelling, and replacement windows must not reduce the ventilation that was there before.
Background ventilation requirements rose substantially, and a specific rule was added for window replacement: the new windows must provide background ventilation no worse than the ones removed, and where trickle ventilators were present they must be replaced. Where they were not present, the recommended equivalent areas still apply.
This is the most commonly missed requirement in London window replacement work, and because window replacement is notifiable — normally self-certified under a competent person scheme — it shows up on sale when the certificate is checked.
For an existing dwelling: 8,000 mm² equivalent area per habitable room and 4,000 mm² for a kitchen, utility or bathroom with intermittent extract. New dwellings have their own schedule based on floor area and occupancy. Ventilators should be positioned at least 1.7 m above floor level to avoid draughts.
Yes. Continuous mechanical extract or MVHR removes the need for trickle ventilators, which is why deep retrofits often omit them. Acoustic trickle ventilators are available where external noise is an issue, and are often required by a planning condition on a site near a main road or railway.
A worked Waltham Forest example of trickle ventilator: with 17 conservation areas — the Walthamstow Village Conservation Area and the Lloyd Park Conservation Area among the best known — London Borough of Waltham Forest (Waltham Forest Council) sees trickle ventilator constantly, and an Article 4 direction is in force there, so permitted development cannot be assumed. Where trickle ventilator pushes a Waltham Forest scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Habitable room, existing dwelling | 8,000 mm² equivalent area |
|---|---|
| Kitchen, utility, bathroom | 4,000 mm² with intermittent extract |
| Position | At least 1.7 m above floor level |
| Window replacement rule | Ventilation must not be reduced |
| Alternative | Continuous mechanical extract or MVHR |
| Governing document | Approved Document F |
They must not reduce existing background ventilation, and where vents were present they must be replaced.
Only with a compliant continuous mechanical ventilation system.
Positioned above 1.7 m and correctly sized, they should not.
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