Why You Might Need a Trickle Vent

If you've just fitted new window seals, you might be wondering, "Do I need trickle vents as well?" In a lot of homes, the answer is yes. New seals stop the draughts, keep the heat in, and make the house noticeably quieter. But they can also catch you out.

Those draughts were doing a job. In some rooms, they were the only way fresh air got in. Seal them up and nothing replaces that air, so a few weeks later you're wiping wet windows every morning and wondering where the moisture in the room has come from.

The good news? You don't have to choose between a warm room and a dry one. A trickle vent lets the air back in, in a way you can control.

What's the Difference Between a Draught and Ventilation?

A draught is air you didn't ask for. It sneaks in through a worn gasket, a gap at the corner of a sash, or a door that no longer pulls tight. It's cold, it blows straight at you, and you can't switch it off.

Ventilation is air you choose. A trickle vent, the narrow slot vent fitted along the top of a window frame, lets in a small, steady flow of air high up in the room, where it mixes with warmer air before it reaches you. You can open it, close it, or set it part-way.

Old, tired seals give you plenty of draught and no control. New seals cut the draught out completely. A trickle vent gives you the control back.

Why Does It Matter?

Here's something that might surprise you: more than one in five UK households (22%) report mould or damp on their walls or surfaces. That's according to a YouGov survey of 2,039 adults published by researchers Van Rooyen and Sharpe in 2025.

The same survey shows how rarely we let fresh air in once the weather turns:

• 42% never open the bedroom window at night in winter
• 25% never open the living-room window during the day in winter
• 20% say worries about heat loss or heating costs stop them opening windows

Nobody wants to throw the window open in January. But if the window stays shut, the seals are new, and there's no trickle vent, the moisture from breathing, cooking, showering, and drying clothes has nowhere to go. It settles on the coldest surfaces in the room: the glass and the corners.

Bedrooms and bathrooms come off worst. Of the people in the survey who described where their mould was, half mentioned the bathroom, and 45% mentioned a bedroom. Walls and the area around windows were the most common spots.

Signs Your Home Needs More Background Ventilation

It sounds obvious, but one misty window on a frosty morning is normal. What you're looking for is a pattern that shows up most days.

What to look for:

• Condensation on the inside of the glass most mornings, especially in bedrooms
• Water pooling along the bottom of the frame or on the sill
• Black spots on the silicone, the window reveals, or the gasket itself
• A stuffy or musty smell when you walk into a room first thing
• Mirrors and windows that take ages to clear after a shower

If you're ticking off two or three of these regularly, the room isn't getting enough air.

Do Your Windows Already Have Trickle Vents?

Have a look along the top of the frame or the opening sash. You're looking for a slot with a sliding cover on the inside, usually somewhere between 300mm and 525mm long.

Around half of homes have them. The survey found that 49% of people have trickle vents on at least some of their windows, but the age of the house makes a big difference:

So if you're in a 1930s semi or a Victorian terrace with replacement uPVC windows, there's a fair chance you don't have any. If you do have them, check they're open, clean, and not cracked.

How Seals and Trickle Vents Work Together

Your windows have two jobs to do, and each part handles one of them.

Keeping the weather out is the seal's job. Bubble and flipper gaskets seal the opening sash against the frame, and wedge gaskets hold the glass unit in place. When they're worn, you get draughts, heat loss, and sometimes water ingress. Our Seal Centre helps you identify your profile and order the right replacement.

Letting a controlled amount of air in is the trickle vent's job. It keeps moisture levels down without the cold blast of an open window, and it does it while the window stays shut and locked.

Get both right, and you'll have a room that's warm without being damp or close.

Which Trickle Vent Should You Choose?

We stock three RW Simon trickle vents, all designed for retrofitting to uPVC windows.

• RW Simon Premium Trickle Slot Vent: the go-to choice for most bedrooms and living rooms.
• RW Simon Trickle Vent Hi-Flow+ 525mm: lets more air through, so it suits kitchens, larger rooms, and anywhere that steams up a lot.
• RW Simon Acoustic Trickle Vent: for homes near busy roads or railway lines, as it cuts outside noise while still letting air in. Our guide to noise-reducing acoustic trickle vents explains how it works.

Fitting one is a 15 to 20 minute job if you're happy using a drill. Learn how: our step-by-step guide to fitting a trickle vent takes you through it from marking up to clipping on the cover caps.

Your Seal and Ventilate Checklist

1. Find the draughts. Use the paper test from our winter window draught guide.
2. Replace worn seals. Identify the profile in the Seal Centre, and order a sample pack if you're not sure which one you've got.
3. Check for trickle vents. Make a note of which rooms have them and which don't.
4. Open and clean the vents you have. Slide them open and brush out any dust, cobwebs, or dead insects.
5. Add vents where they're missing. Start with bedrooms, then the kitchen and any room that gets regular condensation.
6. Keep an eye on the windows for a couple of weeks. Less condensation means you've got the balance right.

Shop Trickle Vents with GB DIY

Have a browse through our full trickle vents range, with standard, Hi-Flow+, and acoustic options in a choice of sizes and colours, all with same-day dispatch on orders before 3 pm. If you're still sorting out the draughts, our window and door seals are cut by the metre in black or white.

Not sure which vent or seal suits your window? Use our Part Finder to WhatsApp or email us a few photos, and we'll send you a link to the right part. Or give us a ring on 01623 622205, and we'll point you in the right direction.

FAQs

Will new window seals cause condensation?

New seals don't create moisture, but they can stop it escaping. If a room relied on draughts for fresh air, sealing it up can make condensation more noticeable. Opening an existing trickle vent, or fitting one, usually sorts it out.

Can I keep my windows shut and still ventilate my home?

Yes, that's what trickle vents are designed for. They let a small, controlled amount of air in while the window stays closed and locked. That makes them a good option for bedrooms, ground-floor rooms, and anywhere security is a worry.

Will a trickle vent undo the work of my new seals?

No. A worn seal lets cold air in along the whole edge of the sash, all the time. A trickle vent lets in a much smaller, steady flow high up in the room, and you can close it in very bad weather.

Do I need permission to fit trickle vents?

Adding a trickle vent to your own windows is a normal DIY job. If you're replacing whole windows, Part F of the Building Regulations, which covers ventilation, applies. Our UK home ventilation guide explains the rules.

Source: YouGov survey of 2,039 GB adults (June 2022), published by Van Rooyen & Sharpe (2025), Mendeley Data, doi:10.17632/x3d5xnsypd.2. Figures are self-reported.