How Do Window Vents Help to Prevent Mould?

Estimated reading time: 5 minutes

Updated: September 2026

Window vents are little openings in window frames that allow for background ventilation. They may also be known as trickle vents, head vents, slot vents, or night vents. When windows are made, a hole is usually drilled through the frame and a cover is placed over it to create the adjustable vent. Window vents are mandatory on almost all new windows, since 2022. So just why was this law introduced? What role do they play in the home and how can you use them to manage air flow, condensation, and prevent mould?

Are all window vents the same?

You will find vents in any window frame, regardless of the material. But there are different types of window vents: They could be ‘through the frame’ where a hole is drilled into the sash and placed at the top of the window frame, above the glazing. If this isn’t possible, you may have an ‘over frame’ design where the vent is built in over the head of the frame or the window. It is also possible to have a ‘glazed in’ vent where it’s fitted within the glazing itself.

What are window vents for?

As we’ve become increasingly more eco-conscious, we’ve worked to create homes that are more airtight. By preventing draughts in our homes, we are warmer, wasting less energy, and saving money. This has had the positive impact of increasing our homes’ energy efficiency (and saving on energy bills in the winter), but this also has the adverse effect of preventing them from “breathing”.

Under UK Building Regulations, Approved Document F last updated in 2022, mandates that trickle vents be mandatory on almost all replacement windows. Trickle vents are designed to allow continuous background ventilation which helps to move air around the home, bringing fresh air in and taking stagnant, damp air out. This prevents moisture build-up and therefore mould developing. Although they should remain open to improve breathability, they are designed with a closure to allow you to control the air flow. However, even closed, you may not get a perfect seal and air can continue to flow through to the room.

Editor note: I had my windows changed over 3 years ago and have learned a lot around the laws and regulations of trickle vents. The window manufacturer and fitter I went with installed trickle vents in every window in my home. However, sections of the house became unbearable in the colder months as draughts would be noticeable walking into rooms, especially around the windows, even with the vents closed. I later learned, through a discussion with Fensa, that one of the rooms in my house had been over ventilated by over 166%! Regulations stipulate that a habitable room – bedroom, living room, office, etc., should have 8,000m2 EA of background ventilation. Hallways don’t need them at all and the requirement is reduced for bathrooms if there are additional ventilation systems installed, such as extractor fans. If you need to have your windows replaced, ensure they understand the current regulations and they take care not to over ventilate. As trickle vents are not 100% airtight even when closed, over ventilation can have an adverse effect and end up costing more to heat rooms through heat loss.

How to prevent mould

If you have trickle vents in your home, the best thing you can do is keep them open at all times. However, in high winds, you may not want to. In this instance, open a vent at one end of your home and open another at the furthest point away. This might be a bedroom window upstairs and the kitchen downstairs, for example. This can help create a gentle airflow without creating a draught.

If you don’t have trickle vents (and you may not on older windows and properties), the easiest thing to do is open the windows – perhaps on catch. Even in the winter, this is really important to do. Especially when we’re inside a lot more, cooking more hot food, and drying clothes inside. Our breath alone creates so much condensation in our homes, let alone these other activities!

Also, wherever possible, keep bathrooms and kitchens well-ventilated as these rooms usually have the most steam within them. Showers, baths, kettles, and cooking food on the hob all create a lot of moisture in the air that contributes to mould growth. Another often overlooked reason for mould build-up is drying washing inside. A bath towel can hold over a kilo of water! So drying them indoors means that’s an awful lot of damp being released into your home.

How to clean away mould

If mould has started to build up, the best thing to do is catch it, and quick! The sooner you can get on top of it, the less chance it has to spread.

There are some mould specific products on the market. When I was a cleaner, my favourite, and one I always recommended to my property clients, was Astonish Mould & Mildew Remover.

Top tip: When cleaning mould away, use paper towels. If you use cloths, it has a chance to stay in your home. Put the used paper towels in a bag and immediately put them in an outside bin. This removes the spores from your property entirely and decreases their ability to re-establish.

Prevention is even easier: Ventilate rooms well and minimise damp air creating activities so mould never even gets a foothold!

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