Humidity, Condensation and Mould: What Air Conditioning and Mechanical Ventilation Really Do This Autumn 2026
L'équipe Proclimo
28 Aug 2026 - 08 min read
At the end of August, the shift is abrupt. Nights drop back below 15 °C, we close the windows that had been left open all night since June, and the children go back to daily showers at fixed times. In three weeks, the amount of water vapour stagnating in an average French home can double.
The result always shows up in the same places: a black line in the corner of the bathroom, stubborn condensation along the bottom of the windows in the morning, a musty smell in the north-facing bedroom. And a question that comes back on the forums every year: "can my reversible air conditioner dry all that out?"
The honest answer is: partially, poorly, and often at the wrong time. Because air conditioning dehumidifies as a side effect of cooling — which makes it effective in August and almost useless in November. The real tool for managing humidity in a home is ventilation. And that is precisely the equipment nobody looks at.
Where does the water in your home come from?
Contrary to a very widespread intuition, indoor humidity in a healthy home comes almost entirely from inside. It is produced indoors, by the occupants and their activities.
The orders of magnitude, extensively documented by ADEME and echoed in the guides of the Agence qualité construction (AQC), speak for themselves:
| Source | Water vapour produced |
|---|---|
| Breathing and perspiration of one adult (24 h) | 1 to 1.5 L |
| One shower | 0.2 to 0.3 L |
| Cooking a meal | 0.4 to 0.7 L |
| Air-drying one load of laundry indoors | 1.5 to 2 L |
| Houseplants (per pot, per day) | 0.05 to 0.1 L |
For a family of four, you easily reach 8 to 12 litres of water a day released as vapour. As long as the windows stay open, that vapour leaves. As soon as you close them, it has to find another way out — otherwise it condenses on the first cold surface it meets.
The threshold that matters: 60% relative humidity
ANSES, in its work on indoor air quality and mould, points out that relative humidity sustained above 60-65% creates the conditions for mould to grow on porous materials (plaster, wallpaper, silicone sealant, textiles). Below 30%, you swing into the opposite discomfort: irritated mucous membranes, static electricity, timber moving.
The accepted comfort range is therefore:
40 to 60% relative humidity, at a temperature of 19 to 22 °C during the heating season.
The problem is that almost nobody knows the figure in their own home. A simple indoor thermo-hygrometer placed in the living room, plus a second one in the most exposed bedroom, completely changes the diagnosis: you stop reasoning by smell and by eye and start reasoning from measurements. It is the first investment to make before considering anything else.
What air conditioning actually does to humidity
An indoor air conditioning unit (or an air-to-air heat pump in cooling mode) works by passing the home's air over a very cold heat exchanger. When the air meets a surface whose temperature falls below its dew point, the vapour it carries condenses. That water is collected in a tray and drained through the condensate pipe.
That is why an air conditioner running in summer genuinely dries the air: it can remove several litres of water a day from a home. The effect is real, measurable, and it explains a good part of the comfort you feel.
But there are three major limits
1. Dehumidification only happens if the unit is cooling. In October, when it is 17 °C outside and 20 °C inside, you are not going to run a cooling cycle to dry the air: you would be chilling an already cool home. The dehumidifying effect disappears exactly when humidity becomes a problem.
2. "Dry" mode is not a dehumidifier. Almost every air conditioner offers a mode labelled Dry or marked with a droplet symbol. It runs the compressor in short cycles with reduced fan speed, to condense moisture without dropping the temperature too much. It is useful in a damp shoulder season, but the output remains modest and the room still cools by 1 to 2 °C. It is not a "dry heating" mode.
3. Air conditioning only treats the room it is in. Yet humidity problems concentrate in the bathroom, the kitchen, the utility room and north-facing bedrooms — precisely the rooms that never have a split unit.
In other words: air conditioning is an excellent summer dehumidifier and a poor winter tool. When the problem persists outside the warm season, you need to look elsewhere.
The real lever: ventilation, and the actual state of your mechanical ventilation system
Since the French decree of 24 March 1982, any home built after that date must have general and permanent ventilation: air inlets in dry rooms (living room, bedrooms), extraction in wet rooms (kitchen, bathroom, toilet), and a continuous air sweep between the two.
On paper, a self-regulating single-flow mechanical ventilation system in good condition extracts between 90 and 135 m³/h depending on the home's layout. In practice, AQC regularly finds in its diagnostic campaigns that the majority of inspected installations show flow rates below regulatory values, often for trivial reasons.
The five silent ventilation failures
A mechanical ventilation system almost never fails outright. It degrades, silently, and nobody notices until the first patches of mould.
- Clogged extraction vents. A 2 mm greasy deposit on the rim of a kitchen vent can cut the flow rate by 30 to 50%. It is the most common case, and the easiest to fix.
- Blocked air inlets. Many occupants deliberately seal them in winter "because of the draughts", which breaks the whole air-sweep circuit: no inlet, no extraction.
- Flexible ducts crushed or disconnected in the loft. A duct that has come loose blows into the loft instead of venting through the roof — and creates its own damp spot up there.
- A tired fan unit motor. A unit more than 15 years old gradually loses flow. Its power consumption, on the other hand, does not drop.
- Over-sealed internal doors. A door undercut of less than 1 cm prevents air from flowing from the bedrooms towards the wet rooms.
A simple test you can do this weekend: hold a sheet of toilet paper against each extraction vent. If it stays put, held flat, the flow rate is fine. If it slides or falls, your ventilation is no longer doing its job. It is not a standardised measurement — for that you need a vane anemometer and a professional — but it is a formidably reliable pass/fail indicator.
The September maintenance routine, item by item
September is the right month: air conditioning is finishing its cooling season, ventilation is entering its critical period. Here is the logical order.
1. Clean the vents and air inlets (30 minutes)
Remove each extraction vent (they either unclip or unscrew with a quarter turn), soak them in hot soapy water, and let them dry completely before refitting. Never change the opening setting if there is one: it was calibrated when the home was handed over.
For air inlets in the window frames, a pass with a vacuum cleaner and a brush nozzle is enough, supplemented by a cleaning brush kit for ducts and vents if the ducts are accessible. Plan on doing this twice a year.
2. Service the air conditioning filters before switching to heating
A split unit filter clogged by a season of operation reduces airflow, degrades the COP in heating mode and, above all, maintains a damp, dusty environment that encourages microbial growth on the heat exchanger. Take the filters out, rinse them in lukewarm water, and dry them flat.
If the unit gives off a smell on start-up — the notorious "air conditioner nose" — it is a sign that the tray and the exchanger need treatment. A disinfectant cleaning spray for air conditioners, a foaming aerosol sprayed onto the exchanger with the fins exposed and then drained away through the condensate line, solves most cases. If the smell comes back within a fortnight, the problem is in the tray or the drain pipe: that is a job for a refrigeration engineer.
3. Check the condensate drainage
A partially blocked condensate pipe causes overflows, stains on the wall beneath the indoor unit, and sometimes shuts the unit down on a level fault. On an installation with a condensate pump, listen: a continuous hum or an abnormal clicking signals a pump nearing the end of its life.
4. Treat the cold spots, not just the air
Condensation appears where the wall is coldest: a wall corner, a window reveal, a roller shutter box, behind a piece of furniture pushed against a north-facing wall. Pulling furniture 5 cm away from the wall is sometimes enough to make the problem disappear.
To identify these zones without relying on feel, a thermal camera for smartphones or a simple non-contact infrared thermometer lets you compare wall temperatures. A gap of 4 °C or more between the middle of a wall and its corner indicates a thermal bridge: no ventilation setting will compensate for that in the long run — it is an insulation matter.
Should you buy a dehumidifier?
That is the question that comes up with the first patches. The answer is nuanced.
An electric dehumidifier makes sense in three specific cases:
- a room that is occasionally very damp with no way to ventilate it (cellar, windowless utility room, basement);
- drying out after water damage or building work;
- regularly drying laundry indoors for want of outdoor space.
It is pointless, even counterproductive, when it is used to mask a broken ventilation system or a fault in the building envelope. It draws between 200 and 500 W continuously, produces heat, makes noise, and empties a tank that fills up endlessly because the source has not been dealt with.
If the need is genuine, go for a condensation model sized in litres/24 h for the area concerned, with the option of continuous drainage through a hose — emptying a 3-litre tank every morning always ends up wearing thin.
The special case of homes with an air-to-air heat pump
A home heated by an air-to-air heat pump has one characteristic few occupants anticipate: the air blown in heating mode is dry air. You are not adding humidity, and you are indirectly removing it by raising the temperature. In a recent, well-sealed home with a working ventilation system, it is not unusual to drop below 35% relative humidity in January.
Typical symptoms: dry throat on waking, chapped lips, parquet flooring moving, plants suffering. Here too, measurement beats perception. If the hygrometer confirms it, an evaporative cool-mist humidifier in the main bedroom restores the comfort range — provided you maintain it scrupulously, since a poorly cleaned humidifier becomes a source of biological contamination in its own right, as ANSES regularly points out.
When to call a professional
Some signals call for a proper diagnosis:
| Signal | Likely interpretation |
|---|---|
| Mould that returns despite cleaning and ventilation | Building fault (thermal bridge, rising damp) |
| Wallpaper blistering only at the bottom of the wall | Rising damp from the foundations |
| Zero flow at every vent | Ventilation unit out of service |
| Condensation on cold pipes in summer | No pipe insulation |
| Sewer smell at the vents | Faulty connection or dried-out trap |
For mechanical ventilation, a maintenance contract including flow rate measurement generally costs between €100 and €180 for a single-family home. In a block of flats with collective ventilation, maintenance of the fan unit falls under the communal areas: it is up to the managing agent to have it carried out, and its absence is a frequent cause of generalised under-ventilation across an entire building.
Key takeaways
- Humidity in a home is produced indoors: 8 to 12 litres of water a day for a family of four.
- The target is 40 to 60% relative humidity; above 60-65%, mould takes hold (ANSES).
- Air conditioning dehumidifies only when it is cooling: it is a summer tool, not a cold-season solution.
- Dry mode is a shoulder-season compromise, not a standalone dehumidifier.
- Mechanical ventilation is the real humidity regulator — and it is the most neglected piece of equipment in the home.
- September is the ideal month to clean vents, air inlets and split unit filters, and to check the condensate drainage before the heating season.
A home that smells musty at the start of the school year does not primarily need another appliance. It needs air to be given back the right to circulate.
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