The air change rate required in dwellings is not expressed in air changes per hour: the French order of 24 March 1982 sets a minimum total flow in m³/h, from 35 m³/h with one main room to 135 m³/h with seven. Related to the volume of a typical dwelling at 2.50 m floor to ceiling, that works out between 0.53 and 0.37 air changes per hour, and the rate falls as the dwelling grows. The 0.5 ach benchmark everyone quotes is therefore only right for a two-room flat. The calculation always runs the other way, starting from the rate to be held room by room and only then relating it to the volume.
Understanding air renewal: what exactly are we talking about?
Renewal rate: simple definition and units (m³/h, volumes/hour)
The renewal rate describes the amount of fresh air brought in and stale air extracted from a home. It's expressed either as a flow rate (m³/h) or in volumes/hour. 1 volume/hour means the equivalent of the home's air volume is replaced, in theory, in one hour.
Why a minimum is essential: moisture, pollutants, comfort
Without sufficient renewal, moisture sets in. Condensation, mould, odours and discomfort follow quickly, especially after showering, cooking, drying laundry. Pollutants also accumulate, CO2, VOCs, particles. A steady flow protects indoor air quality and stabilises how the home feels, without unnecessary draughts.
Common mistakes on site: confusing extraction, air inlet and airtightness
The table, computed from the regulatory flow
The text reasons in flow, not in air changes. Getting to a rate means dividing by the volume, which requires an assumption on floor area and height. Here is the calculation for typical dwellings at 2.50 m, from the minimum total flows of article 4.
| Dwelling | Floor area assumed | Volume | Minimum total flow | Air change rate |
|---|---|---|---|---|
| 1 room | 30 m² | 75 m³ | 35 m³/h | 0.47 ach |
| 2 rooms | 45 m² | 112 m³ | 60 m³/h | 0.53 ach |
| 3 rooms | 70 m² | 175 m³ | 75 m³/h | 0.43 ach |
| 4 rooms | 90 m² | 225 m³ | 90 m³/h | 0.40 ach |
| 5 rooms | 110 m² | 275 m³ | 105 m³/h | 0.38 ach |
| 6 rooms | 130 m² | 325 m³ | 120 m³/h | 0.37 ach |
The falling curve is not a quirk of the text, it is its logic: the flow is indexed on the number of service rooms to be treated, not on the volume to be swept. A large house has proportionally fewer kitchens and bathrooms than a small flat.
What it changes when you justify a sizing
Two concrete uses. First, if you price from a flat 0.5 ach, you oversize every dwelling above two rooms: on a five-room, the gap runs from 0.38 to 0.50, a third too much flow, so a bigger, noisier, thirstier unit for nothing. Second, the reverse holds in commercial work or in heat loss calculations: there the rate is the input, not the flow, and taking the dwelling's regulatory minimum ignores infiltration and actual use. That is why it sits among the assumptions of a heat loss report drawn up to EN 12831-1, on a par with the base temperature.
The regulatory flow is a health floor, not a comfort target. It says nothing about the moisture the occupants produce or the indoor air quality you are aiming for: that is the point to explain to a client who asks why you are proposing better than the minimum.
Minimum ventilation: the benchmarks to know in 2026 by home type
What ventilation requirements for homes say: principles to follow
In housing, the rule remains general, permanent ventilation. Fresh air enters through the main rooms, stale air is extracted from the kitchen, bathroom and WC. The goal is to ensure continuous renewal without draughts. Air inlets, vents and ducts must stay clear, adjusted and maintained.
House vs flat: minimum flow rates and sensitive points (kitchen, bathroom, WC)
Usual extraction benchmarks read room by room.
| Room | Usual extraction flow | Possible boosted flow |
|---|---|---|
| Kitchen | around 45 m³/h | often 90 to 135 m³/h |
| Bathroom | 30 m³/h | not specified |
| WC | 15 m³/h | not specified |
In flats, watch the shared ventilation system and parasitic air paths. In houses, pay attention to distant wet rooms and lofts.
Energy renovation: when insulation increases the need for controlled air renewal
The more you insulate and make the building airtight, the more moisture and pollutants build up if ventilation doesn't keep pace. A suitable system, humidity-controlled or heat-recovery, allows healthy air without throwing heat outside. Plan settings, balancing and maintenance access from the start of the job, and controlling ventilation by air quality can also help secure flow rates in real-world use.
Sizing correctly to hit the minimum without over-ventilating
Choosing vents and air inlets: where to place them and how to avoid air short-circuits
In housing, fresh air enters through air inlets in dry rooms (living room, bedrooms) and is extracted from the kitchen, bathroom, WC. Keep a continuous air path. Door undercuts, gaps under doors. Avoid placing an air inlet too close to an extraction point, or the air will just turn back.
Settings and balancing: getting the right renewal room by room
Aim precisely for regulatory flow rates. Measure with a flow meter or anemometer, then adjust vent by vent. The goal is stable renewal without draughts. Kitchen at peak, WC and shower room constant, bedrooms lower. Re-check after cleaning filters and vents.
Limiting nuisances: noise, draughts, heat losses
For acoustic comfort, limit duct velocity, take care with elbows, and add a silencer if needed. Track down whistling air leaks. Insulate ducts in cold zones. Over-strong ventilation increases heat losses and draughts. A fine adjustment beats a motor running flat out.
Measuring and justifying air renewal on your jobs
Simple checks: smoke pencil, anemometer, flow measurements (field method)
To validate proper draught, start with a smoke pencil near vents and air inlets. Then measure velocity with an anemometer, and convert to flow rate using the effective section (or a flow hood if you have one). Note the values room by room, kitchen, bathroom, WC, and compare against the manufacturer's target flow rates. To go further, see also diagnostic tools.
Fixing common defects: crushed ducts, leaks, lack of air inlets
If flow rates are low, first look for crushed ducts, elbows that are too tight, or a duct that's too long. Track down leaks at joints, and check the airtightness of connections. Without sufficient air inlets, extraction runs out of steam and renewal becomes irregular.
Recording your settings: commissioning sheet and proof for the customer
Formalise everything in a commissioning sheet.
- Vent settings.
- Damper positions.
- Measured flow rates.
- Photos of the ductwork.
- Equipment references.
A small dated, signed table becomes your field proof in case of after-sales issues, inspection, or a customer question.
Practical cases: hitting the air minimum with the most common solutions
Single-flow mechanical ventilation: key points to guarantee steady renewal
First aim for the regulatory flow rates. Check air inlets in living areas, then extraction vents in the kitchen, bathroom, WC. A clogged vent or a blocked air inlet is enough to break renewal. On site, a check with a flow meter and a smoke test quickly reveal the trend.
Humidity-controlled mechanical ventilation: fine-tuning air while respecting the minimum
A humidity-controlled system works well if it's installed as designed. Short ductwork, limited pressure losses, compatible vents and air inlets. Don't try to "close things off" to gain comfort. You keep the minimum, and the humidity control modulates the rest. Light, regular cleaning of humidity-controlled vents to avoid drift.
Heat-recovery ventilation in renovation: balancing, airtightness and maintenance to keep the right renewal
Heat-recovery ventilation delivers on its promise if supply and extraction flow rates are balanced. In renovation, take care with duct airtightness, limit leaks in the loft, and check pressure after commissioning. Schedule maintenance. Filters to replace, heat exchanger to dust. Without that, renewal drops and noise rises. To go further on the topic, see also heat-recovery ventilation in renovation.
