Ventilation in a flat: where your responsibility stops and the co-ownership's begins

In a block of flats, ventilation behaves like the water or heating: what you see in your flat is just one piece of a larger system. An extract grille in the kitchen, a duct running through the stairwell, a fan unit on the roof: everything is connected, but not everything belongs to the same owner.

When cooking smells from upstairs arrive through your hood, or the bathroom never seems to dry, the first question is not "which device do I replace?", but "what is mine, and what is the co-ownership's?". You need that boundary clear before you touch anything or spend any money.

What almost always belongs to your private lot

In most Belgian co-ownerships, everything inside the volume of the flat is private. For ventilation this usually means the extract grilles, air inlets, any short flexible or rigid ducts serving only your rooms, and small individual fans in toilets or bathrooms.

These parts can be dirty, badly adjusted or half-blocked by a cupboard or false ceiling. You can clean them, free them and replace like for like, as long as you respect the building rules. But changing airflow also changes the pressure in a shared duct. Before you increase your airflow significantly, it is wise to alert the managing agent or co-owners' committee and check you are not upsetting a fragile balance.

Shared risers, roof fans, collective MVHR: co-ownership territory

Once a duct runs through a structural wall, the stairwell or the roof space, you are almost always in common parts. The vertical riser serving several flats, the roof extract fan, a collective mechanical ventilation or heat-recovery unit hidden in a plant room or under the roof are managed by the co-ownership through the managing agent.

An individual flat owner cannot decide alone to alter or replace these. Routine servicing, checks of total airflow and replacement of a fan or collective unit are funded from shared charges, usually split according to the shares defined in the deed. The co-ownership is also generally responsible if the riser is blocked, a roof fan has stopped or a collective unit has been left without maintenance for years.

Why fixing one flat on a shared riser does not hold over time

Many ventilation problems in flats come from an unbalanced shared riser. One neighbour blocks their kitchen grille because they feel cold, another connects a powered hood to a duct designed for natural extract, a third leaves a tilt-and-turn window permanently ajar. The whole riser changes behaviour.

If you simply boost your own airflow, you may deepen the imbalance instead of curing it. Pressures shift, cooking odours from other floors can be drawn into your flat, and air noise increases at certain grilles. On a collective mechanical system, tweaking only one flat without re-commissioning the unit is like turning a random knob in a system originally designed to work as a whole.

Smells, noise, back-draughts: a three-step way to narrow it down

1. 1Start inside your lot: clogged grilles, filters in small local boxes blocked with dust, furniture pushed over an air inlet, a hood wrongly connected to a shared duct. All of that can be corrected in your flat, without a general meeting.
2. 2Then observe the behaviour of the riser: does the extract vary with the wind, do smells appear only when a neighbour cooks, do you hear air rushing through a grille even when it is closed? These are signs that the issue goes beyond your ductwork.
3. 3Take factual notes to the managing agent: when it happens, which rooms are affected, photos of grilles and visible ducts. That helps the co-ownership decide on a check of the riser or collective unit, instead of focusing the debate on a single "problem" flat.

This approach does not replace a technical survey, but it often separates a local fault (blocked grille, stuck backdraft damper) from a structural defect in the riser or collective system, which can only be addressed at building level.

NBN D50-001: a reference, not a right to demand renovation

The Belgian standard NBN D50-001 gives design airflow values for housing. It states for example 3.6 m³/h per m² of floor area, 75 m³/h extract for an open kitchen and a flat rate of 25 m³/h for a toilet. It also describes the four systems A, B, C and D, which many recent buildings follow in their design.

This standard does not automatically force a co-ownership to upgrade an old riser or install collective mechanical ventilation in an older block. It mainly provides a reference to judge whether current ventilation is realistic. When a report shows flows far below these values, it helps the general meeting to have a more objective discussion about possible works, without guaranteeing that such works will be approved.

What you can do now without waiting for the next general meeting

Between doing nothing and launching a full refurbishment project, there is room for simple, low-risk actions that respect the shared system.

A short checklist often makes a difference:

• Locate every grille and air inlet in your flat and make sure none is taped over, hidden or blocked by furniture.
• Clean visible grilles gently and, if you have small local fans, check they actually run when switched on.
• If you suspect a shared issue, talk to neighbours on the same riser first: their observations help convince the managing agent that the problem is collective, not individual.
• Gather documents: plans if you have them, any previous ventilation report, and photos. They carry more weight at a general meeting than a vague complaint about "bad smells".
• Avoid unilateral modifications such as connecting a new powered hood to a duct whose status you do not know. On a collective system, that type of change can quickly backfire.