Open. Almost always open, and particularly in winter, which is the opposite of what most people assume.
That little slotted vent along the top of the window frame causes more arguments in more houses than anything else we get asked about. Half the household thinks it’s letting the heat out. The other half noticed the windows stopped streaming when it was opened. Here’s what the thing does and why it earns its place.
What it is
A trickle vent is a controlled hole in your window. That’s genuinely all it is.
It sits through the top of the outer frame or through the head of the sash, with a grille outside to keep out weather and insects, and a flap or slider inside so you can shut it. When it’s open, air passes through slowly and continuously whether anyone is in the room or not.
The word trickle is doing the work in the name. This is not ventilation in the sense of an open window. It’s a small, steady exchange of air designed to run in the background all the time.
Why a modern house needs one
An old house ventilated itself whether you liked it or not. Sash windows with gaps you could post a letter through, suspended floors, open chimneys, ill fitting doors. Draughty, expensive to heat, rarely damp, because the air changed over several times an hour with nobody doing a thing.
Then we sealed it all up. Compression gaskets, draught stripped doors, chimneys blocked, lofts insulated. Good news for a heating bill. It also closed off every route the moisture used to leave by.
And you make a lot of moisture. Cooking, showers, baths, washing drying on a radiator, a kettle, and simply breathing, several people, all night, in a shut bedroom.
With no way out, that water ends up on the coldest surface in the room, and in a modern house the coldest surface is nearly always the glass. That’s why it shows on the windows first. The window isn’t causing it, it’s just where the problem becomes visible, which we’ve gone into properly in the article on condensation on the inside of your windows.
A trickle vent gives damp air a route out that doesn’t involve opening a window in January.
The heat argument, honestly
People shut them because they feel cold air off them and reason, sensibly enough, that a hole in the window must be costing money.
There is a real cost and it’s small. A trickle vent has a deliberately modest free area, and what it lets go over a winter is not what’s driving your heating bill. Your loft, your walls and your doors are doing considerably more.
Against that sit two costs that aren’t small.
The first is that damp air is expensive to heat. Water in the air has to be warmed along with everything else, so a humid room takes more energy to bring up to temperature and feels colder at the same thermostat setting than a dry one. Ventilating a little can genuinely make a house cheaper to run, which sounds backwards until you’ve felt it.
The second is what the moisture does when it stays. Water down the glass every morning, pooling on the sill and soaking into the plaster below. Black mould in the corners of the reveals and along the sealant. Mould on the curtains and on the back of the wardrobe against the outside wall. Redecorating a bedroom because of it costs a great deal more than the heat that went out through a slot in the frame.
So the honest comparison isn’t heat against nothing. It’s a little heat against a mould problem.
Where the rules sit in Scotland
Ventilation requirements come under the Scottish Building Standards, and Scotland runs its own system here rather than the English one. Background ventilation in windows has been standard practice for a long time and the requirements have tightened as houses have got more airtight.
The practical point for anyone replacing windows in Wishaw or anywhere else in Scotland is this. Where a room already had background ventilation, a replacement window is generally expected to carry it too. You’re not usually allowed to quietly remove ventilation that was there before. If an installer offers to leave the vents off because you don’t care for the look, question it rather than accept it, and your local authority building standards department will confirm what applies to your job.
If your old windows had vents and your new quote doesn’t mention them, ask why.
Cleaning one
They clog. Nobody cleans them, they sit out of sight at the top of the frame, and over a decade they fill with dust, cobwebs, insects and fluff pulled through by the airflow.
Slide the internal cover fully open and look at the slot behind it. Grey felting is what’s been stopping your air. Vacuum it out with the brush attachment, then wipe the cover and slot with a damp cloth and a bit of washing up liquid. Some covers unclip, most just slide.
The outside grille is usually where the worst of the blockage is, and a soft brush over it shifts most of that. If you can reach it comfortably from the ground, fine. If it’s an upstairs window, leave it. Nothing about a trickle vent is worth going up a ladder for.
Two things to check while you’re there. A vent painted over during decorating is doing nothing whatsoever, and plenty have been. Nor is one with tape or a folded towel over it that somebody put there three winters ago and forgot. Both are common, and both are invisible until you go and look.
Part of the answer, not all of it
Straight about this. If a house is properly damp, opening the trickle vents will help and it will not fix it.
Vents handle the everyday moisture normal living puts into the air. They don’t handle a bathroom with no working extractor, four loads of washing a week drying on radiators, a dead kitchen fan, or rising and penetrating damp coming through the fabric of the building. If the vents are open, the obvious sources are sorted and you’ve still got streaming windows and mould, the problem is bigger than a window, and you want somebody looking at the house rather than the glazing.
Where the glass itself runs unusually cold, single glazing or a sealed unit that’s blown and lost its insulating gas, that pane will gather condensation long before the ones beside it. That part is a window job and we can help with it.
Questions we get asked
Should I close them at night in a bedroom?
That’s the room where you least want to. A shut bedroom with two people in it overnight puts a surprising amount of water into the air, and morning condensation on bedroom windows is among the most common complaints we hear.
Will rain come through?
No. The external grille and canopy are shaped to shed it. Driving rain on an exposed elevation can occasionally get a little past a badly weathered vent, and if that’s happening the vent wants looking at rather than shutting.
They whistle in the wind. Anything to be done?
Common enough. Usually dirt in the slot changing the airflow, sometimes a loose or distorted internal cover. Clean it first. If it still sings, the unit can generally be swapped for a different one.
If you want a hand
If you’re not sure whether your windows have vents at all, or yours look painted shut, a photo of the top of the frame over WhatsApp is usually enough for us to say what you’ve got and whether it’s worth doing anything about. No charge for asking, and we’re on 01698 604 278 through the week.
If a vent has seized shut or the flap has snapped off, we can sort it while we are out doing any other uPVC window repair work.
Related advice
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Browse the rest of our window and door advice for around fifty guides on common window and door problems.
Written by the team at Wishaw Window Repairs. Over thirty years repairing uPVC windows and doors across Lanarkshire.