Window and façade air vents: why not block them and how to make them bearable

In many Brussels flats the story is the same: a narrow grille in a window frame or façade lets in cold air, carries every sound from the street and creates a draught as soon as the wind picks up. After a few winters, someone tapes it up or stuffs it with foam.

The effect feels positive straight away: less noise, less cold on the sofa. Yet in the background something else shifts. The ventilation of the flat goes out of balance, humidity and indoor pollutants build up, and new problems appear later, somewhere else and in another form.

What that grille in the window frame is actually for

An air inlet is not a random hole "left by the contractor". It is there to bring in fresh air, usually into a living room or bedroom, while stale air is removed elsewhere, typically in the kitchen, bathroom or toilet. Without a planned inlet, any fan or mechanical extract behaves like a straw with your finger on the end: it has to pull air through the weakest leaks it can find.

Physically, the grille creates a preferred path for air, with a defined cross-section and a known route. Air follows this path rather than a tired window seal or an unused chimney. This fresh air dilutes CO₂, water vapour, smells and volatile organic compounds from materials and daily life. When the inlet disappears, the dwelling still "breathes", but through locations you no longer control.

What physically happens when you block an air inlet

Once you obstruct the grille, you change the pressure game inside the flat. A bathroom fan, a cooker hood, a ventilation unit or simply the wind still create a slight under-pressure. Air then enters under the entrance door, through a worn seal or even down a chimney flue that should only evacuate.

Fresh air no longer arrives where it is useful to dilute humidity and CO₂. Levels of CO₂ rise faster in living spaces, and nothing in a typical flat indicates it. Moisture tends to accumulate in the coldest areas: corners of external walls, window reveals, behind wardrobes. That is where paint starts to blister, darker patches appear and later mould follows. A present-day comfort tweak turns into a building defect later.

Cold draught from the vent: where the sensation really comes from

The grille does not generate cold, it lets in outdoor air that is colder and denser than indoor air. That air sinks and spreads along the floor. Your legs feel this thin layer of moving air as an unpleasant draught, especially near a sofa or a desk.

Two effects coincide: a local drop in temperature around the grille, and air speed if the under-pressure is strong because of a fan or wind on the façade. The room thermostat may show a comfortable average temperature while your skin registers local convective cooling. Moving a radiator, blocking the vent with a cushion or improvising a plug do not change this mechanism. They shift or hide it.

Air vent and street noise: why it sounds so loud

From an acoustic point of view, a façade vent behaves like a small duct linking the outside to the room. It passes airborne noise from traffic, conversations, a nearby bar or tram stop, especially in medium and higher frequencies. When the vent sits in an otherwise good window, it often becomes the weak link for noise.

Filling the grille with dense foam does cut part of this sound path, but at the expense of ventilation. If extraction in the flat still runs, the noise may simply reappear somewhere else: whistling through a small frame gap, noisy air in a bathroom valve. It can feel as if you "won" against street noise, when in reality you have degraded the designed air path and created new, sharper noises.

Instead of blocking it: checks and targeted adjustments

The first step is to check that the vent works as intended, not more. A clogged grille or a flap stuck fully open can create a concentrated jet of air. Basic maintenance — removing the cover, cleaning dust, making sure the flap or shutter moves freely and sits correctly — is sometimes enough to reduce the draught sensation significantly.

Then you act on how and where air arrives: spreading inlets across several rooms instead of relying on a single window, adjusting the speeds of a ventilation unit or bathroom fans, or shifting the position of a vent so it does not blow directly onto a bed. For noise, slim-slot vents or ducts sized with sound in mind limit sound transmission without cancelling airflow. In some cases, a small patch of internal insulation around the vent improves comfort without closing the air path.

Practical ways to improve comfort without sealing the vent

Several lines of action can be combined, depending on how the flat is set up and whether you rent or own.

• On airflow: have the airflows checked where possible, adjust ventilation or fan settings, and balance inlets and extracts so air speeds stay moderate at each grille.
• On layout: move a desk, sofa or bed out of the direct path of the incoming air, add a simple deflector to guide the flow towards the ceiling, and make sure doors have enough clearance so air can move between rooms without accelerating under one leaf.
• On the building envelope: seal unwanted leaks around the frame so the planned vent does the work, correct a local cold bridge that over-cools the wall around it, and, if windows or façades are being replaced, opt for vent models that are better adapted to the actual noise on your street.

None of these turns an air vent into a heater or a thick soundproof wall. They keep its role in ventilation while making the airflow and associated noise compatible with a living room or bedroom, which is the real goal behind the idea of blocking it.