A trickle vent is a small, closable slot in the head of a window frame that lets a little fresh air in while the window stays shut and locked. Since June 2022 they have become a routine part of replacement windows in England, and the most common question we are asked at quotation.
What the rules say
Part F of the Building Regulations says that when windows are replaced, the home's ventilation must be no worse than before. New windows are far more airtight than old ones, so without vents the house would have less background ventilation than it started with. That is why vents are now expected in most cases.
- If the old windows had trickle vents, the new ones must have vents at least as large.
- If the old windows had none, the new ones should have them, unless the home has another adequate system such as continuous mechanical extract or heat-recovery ventilation.
How big they need to be
| Room | Minimum vent size (equivalent area) |
|---|---|
| Habitable rooms (living rooms, bedrooms) | 8,000mm² |
| Kitchens | 8,000mm² |
| Bathrooms | 4,000mm² |
Where a window is too small to carry a vent of the required size, the largest vent that fits is used.
Will they make the room cold or noisy?
The airflow through a trickle vent is small and enters at head height, where it mixes with warm air near the ceiling, so it is not felt as a draught in the way a gap round an old sash would be. Every vent has a closer, which you can shut in a gale. Acoustic vents are available for homes on busy roads.
The benefit is drier air. Homes produce litres of water vapour a day from cooking, washing and breathing. With nowhere to go it condenses on the coldest surfaces, which leads to streaming glass and, in time, mould. A little continuous ventilation is the cheapest cure there is.
Can I opt out?
Not by simply asking. The installer has to certify that the work complies, and leaving vents out where they are required puts that certificate at risk, which can cause trouble when you sell. If your home has a mechanical ventilation system, tell us, because that may change what is needed.
