What is the minimum required design flow rate?
The design flow rate is the air flow, in m³/h, that the ventilation system in a room must at least be designed for. The regulations set a minimum per room type, so that every room gets enough fresh air or can discharge stale air. The system must be able to reach that flow rate in all rooms at the same time at nominal fan speed.
The calculation rule per room type
For most rooms the minimum flow rate is 3.6 m³/h per m² of floor area, with a lower and an upper limit per room type. A toilet and an open kitchen have a fixed flow rate. Hall, storage and dressing room have no requirement.
| Room | Direction | Per m² | Minimum | Maximum |
|---|---|---|---|---|
| Living space | Supply | 3,6 | 75 | 150 |
| Bedroom | Supply | 3,6 | 25 | 72 |
| Playroom | Supply | 3,6 | 25 | 72 |
| Study | Supply | 3,6 | 25 | 72 |
| Closed kitchen | Extract | 3,6 | 50 | 75 |
| Open kitchen | Extract | fixed 75 m³/h | ||
| Bathroom | Extract | 3,6 | 50 | 75 |
| Shower room | Extract | 3,6 | 50 | 75 |
| Laundry room | Extract | 3,6 | 50 | 75 |
| Wet room | Extract | 3,6 | 50 | 75 |
| Toilet | Extract | fixed 25 m³/h | ||
Supply and extract
The regulations divide rooms into two groups. In the dry rooms (living space, bedroom, playroom and study) fresh air comes in: supply. In the wet rooms (kitchen, bathroom, shower room, laundry room and toilet) air is extracted: extract. The total for both is shown.
The kitchen-diner and the open kitchen
A closed kitchen counts as a wet room, with an extract of 50 to 75 m³/h. An open kitchen has a fixed extract flow rate of 75 m³/h. If the kitchen is part of the living space, as in a kitchen-diner, that room is treated as living space (supply) and also gets the fixed extract of an open kitchen.
The area on the floor plan then applies to the whole. It is not split and no division is guessed: you get a notice to check whether the kitchen part of the area should be deducted.
Who prepares the ventilation report?
In Flanders there are two documents: the ventilation pre-design, before the works, and the ventilation performance report, after completion, with measured flow rates. Only a certified ventilation reporter may prepare them.
This does not replace that report: the calculation of the minimum required flow rate per room is done, and you copy those numbers yourself.
How the rules are applied
The floor plan is read, the rooms and their area are recognised, the room type is determined and the rule above is applied. You get a table with the minimum required flow rate per room and the totals for supply and extract. If a room or area is not recognised, nothing is guessed: you fill it in.
What is not checked: whether supply and extract are in balance, whether there are transfer openings, and what the design flow rate per valve, the ducts or the noise will be.
Source and check
The numbers come from the EPB-pedia of the Flemish government, section ventilation, and have been checked against a second, independent source. Last checked on 7 October 2026. We have not yet checked whether the numbers differ per building permit year: always check the rule that applies to your application.