Ventilation flow rates: what NBN D50-001 requires

When a technician tells you your bedroom “should be at 72 m³/h”, that figure does not come from nowhere. They are applying NBN D50-001, the Belgian standard defining ventilation provisions for residential buildings, used as the reference within EPB requirements.

Understanding it makes you self-sufficient on three points: judging whether a quote is serious, knowing what should be measured in your home, and spotting the rooms that have been forgotten.

The base rule: 3.6 m³/h per m²

A room's design flow rate is calculated from its floor area, at 3.6 m³ per hour per square metre. A 12 m² bedroom therefore gives 43.2 m³/h.

That result is then constrained between a minimum and a maximum specific to the room type. This is where many rough calculations go wrong: they apply the rule without the limits and end up with absurd figures in very small or very large rooms.

Limits by room type

The WC is the exception: its rate is fixed at 25 m³/h regardless of floor area. That is logical — a 1 m² WC and a 3 m² WC create the same need.

Supply, extract, transfer

The standard distinguishes three families of rooms, and that split is what makes the whole thing coherent.

Dry rooms — living room, bedrooms, study — receive fresh air. Wet rooms — kitchen, bathroom, utility, WC — have stale air extracted. Between them, circulation spaces, corridors and stairwells act as a transfer zone: air passes from the former to the latter.

That transfer is not automatic. It requires a free path: an undercut door or a transfer grille. This is the point most often overlooked in renovation. A good system is installed, draught seals are fitted to every door for acoustic reasons, and the system can no longer work — the air simply has nowhere to go.

The standard's four systems

• System A — natural supply and natural extract. Façade vents, vertical flues. No motor, no control over airflow: it depends on wind and temperature.
• System B — mechanical supply, natural extract. Rare in housing.
• System C — natural supply through vents, mechanical extract in wet rooms. The most common in renovation. Exists in demand-controlled form (“C+”), driven by humidity or CO₂ sensors.
• System D — mechanical supply and extract with heat recovery from the extracted air. The best performing, and the most demanding in both design and maintenance.

What the standard does not guarantee

A system compliant on paper can fail badly in service. The standard defines design flow rates; it does not measure what your system actually does ten years later, with clogged filters and two sealed-off valves.

What matters is the gap between the calculation and the measurement. In the homes we assess, that gap is frequently large — not through bad faith, but because nobody ever checked after commissioning. The calculation says what should be; only measurement says what is.