Updated
No Trickle Vents After New Windows
Your replacement windows arrived without the small vents the old ones had, and now the rooms feel stuffy and the glass runs wet.
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What's happening?
A trickle ventilator is background ventilation: a small, controllable opening that keeps a slow air exchange going all the time, independently of whether anyone opens a window. The rules matter here because a window replacement is not allowed to leave a home's ventilation worse than it was. New frames are dramatically more airtight than the ones they replace, so if the old window had background ventilation and the new one has none, the house has lost the air path it was relying on — and the first symptoms are condensation on the coldest surfaces and mould behind furniture. This is the most common genuine defect in an otherwise tidy window installation, and it is a compliance failure, not a matter of taste.
What usually causes it?
- Replacement windows specified without ventilators when the windows they replaced had them
- Homeowner asked for them to be left off on the grounds that they let in noise or cold, and the installer complied
- A signed disclaimer used in place of fitting them, which is not a route to compliance
- An indemnity policy taken out instead of meeting the requirement
- Reliance on the window's night-latch position as the ventilation provision
- An existing through-wall background ventilator assumed to be adequate without checking its equivalent area
- A combustion air vent for an open-flued appliance counted as background ventilation, which it is not
Is it dangerous?
Not immediately, but the consequences are real. Losing background ventilation raises indoor humidity and the pollutants that build up from cooking, cleaning products and furnishings, and the ventilation guidance is explicit that poorly ventilated homes carry a high risk of mould. Government guidance on damp and mould treats those as health risks with vulnerable groups at greater risk. There is a separate, sharper point where a room contains an open-flued appliance: those permanently open air vents supply combustion air and must never be blocked or counted as the room's background ventilation.
Can I fix it myself?
Nothing useful, and one thing to avoid. Do not adopt the night-latch as your ventilation strategy — the guidance rules it out precisely because a window locked slightly ajar is not secure. Retrofitting a ventilator means cutting the frame or sash to the right slot dimensions for the required equivalent area, and doing that wrong compromises the frame's drainage and its weather seal. What you can do is check whether the room already has a through-wall background ventilator, which changes the answer.
When should you call a window fitter?
Call a window fitter if your replacement windows have no background ventilation and the previous ones did, and especially if condensation or mould has appeared since they went in. If the installation was done by a scheme-registered installer, raise it with them first — schemes monitor registrants and act on non-compliant work. Call promptly where anyone in the household has a respiratory condition, and where there is an open-flued appliance in an affected room.
What will the window fitter actually do?
- Survey each room for its existing background ventilation and measure what is actually provided
- Confirm whether a through-wall ventilator already meets the equivalent area for that room, in which case no vent is needed in the window
- Retrofit trickle ventilators into the frame or sash at the correct slot size, keeping the frame drainage intact
- Fit noise-attenuating ventilators where the elevation faces a noisy road, rather than leaving them out
- Identify combustion air vents and leave them untouched, adding background ventilation separately
- Confirm extract ventilation in kitchens and bathrooms actually discharges outdoors
What the ventilation guidance rules out
Three shortcuts come up repeatedly when replacement windows arrive without background ventilation, and the published guidance addresses each of them directly.
- A window locked slightly ajar on the night-latch is not an acceptable background ventilation solution, because it does not provide a secure opening.
- A homeowner disclaimer — agreeing to no ventilators, or to fitting them at a future date — is not a way of complying. Work must meet the requirements in full.
- An indemnity policy is not an alternative to meeting the requirements.
- Permanently open air vents serving an open-flued appliance are there for combustion air. They cannot be counted as the room's background ventilation, so additional vents are needed in that room.
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