What Is a Trickle Vent?
Written by James Walker · Last reviewed 16 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
A trickle vent is a small, closable opening built into a window frame or the head of a window, providing continuous background ventilation without opening the window itself.
Why new windows have them
Building Regulations require background ventilation, and replacement windows are generally expected to provide at least as much as what they replaced.
Modern sealed windows are far more airtight than the ones they replace. Without deliberate ventilation, moisture generated inside a home has nowhere to go, which is why condensation problems often appear immediately after new windows are fitted.
The condensation link
This is the practical point. Homeowners fit new windows, condensation appears, and it is blamed on the windows being faulty.
Usually the windows are working exactly as intended and the house has simply lost the draughts that were ventilating it. Trickle vents, closed or omitted, are frequently the reason.
What to check
Whether trickle vents are included in the quote, they are sometimes omitted for appearance, which can leave the installation non-compliant.
And once fitted, leave them open. Closing them permanently defeats the purpose and tends to produce the mould people then blame on the windows.
How it works
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