How do I clean and maintain window trickle vents?

Trickle vents are closed in most houses that have them, usually within a week of the windows being fitted, and the resulting condensation is then blamed on the glazing.

Safety

Step by step

1. Understand what they are for.Modern windows and draught-proofing made houses far more airtight. Trickle vents replace the background air change that used to happen through gaps, removing the moisture a household produces — several litres a day from cooking, washing and breathing.
2. Open them and leave them open.All year, including winter. The heat loss through a trickle vent is small; the cost of the condensation, mould and damage caused by closing them is not.
3. Find them.Usually a slot along the top of the frame or through the top of the sash, with a small internal cover that slides or pivots. Some are almost invisible until you look.
4. Check whether they are actually open.The internal flap position tells you. Hold a tissue near it — it should move slightly on a breezy day. A vent that appears open but has no airflow is blocked.
5. Clean the internal cover.Slide or unclip it off — most come off without tools — and wash it in warm soapy water. Dust and grease build up in the slots, particularly in kitchens.
6. Vacuum the slot itself.A narrow nozzle or a brush attachment through the opening, from both inside and out. Cobwebs, dust and insects accumulate and restrict it substantially.
7. Check the external hood.Outside there is usually a small canopy directing rain away. Check it is clear, undamaged and not painted over — a blocked external hood means no airflow whatever the inside looks like.
8. Look for paint over them.Decorators paint over trickle vents surprisingly often, sealing them completely. Look closely at the slot and the flap.
9. Check the insect mesh.Many have a fine mesh which clogs. Clean it gently with a soft brush — do not remove it, and do not poke anything through it.
10. Free a stiff flap.Clean the runners and use a dry silicone spray, not oil, which collects dust. A broken flap is usually a cheap replaceable part.
11. Do this annually.Once a year, ideally in autumn before the condensation season. Ten minutes for the whole house.
12. Never tape or block them.It is the single most common cause of condensation and mould in an otherwise sound house, and the damage is far more expensive than the heat saved.
13. Add them if you do not have any.Retrofit trickle vents can be cut into most frames, and many glaziers offer this. In a house with persistent condensation and no background ventilation, it is a worthwhile change.
14. Combine them with the rest of the ventilation.Trickle vents handle background moisture; extractor fans handle the bursts from cooking and showering. Both are needed, and a house with neither will have mould.

Tips

• Open them and leave them open all year. The heat lost is small and the condensation caused by closing them is not.
• Check the external hood as well as the inside. A vent that looks open inside can be completely blocked outside.
• Do it in autumn, before the condensation season starts. Ten minutes for a whole house.

Common mistakes

• Closing or taping trickle vents to stop draughts — It removes the background ventilation the house depends on, so moisture accumulates and causes condensation and mould — usually blamed on the windows themselves.
• Only cleaning the inside — The external hood and mesh block with dirt, cobwebs and paint. A vent open inside and blocked outside provides no ventilation at all.

If it doesn't work

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