A home does not grow mould because it is damp, it grows mould because a surface has dropped below the dew point of the air touching it. That point is worked out from two readings a survey produces in thirty seconds, room temperature and relative humidity. At 19 °C and 60 %, it is 11.1 °C. Anything colder than 11.1 °C in that room condenses, whatever the quality of the heat source.
The table to take on survey
The dew point is the temperature at which the surrounding air becomes saturated. It depends only on the temperature and relative humidity of the room, never on the fabric.
| Room temperature | Dew point at 50 % RH | at 60 % RH | at 70 % RH |
|---|---|---|---|
| 16 °C | 5.6 °C | 8.2 °C | 10.5 °C |
| 17 °C | 6.5 °C | 9.2 °C | 11.5 °C |
| 18 °C | 7.4 °C | 10.1 °C | 12.4 °C |
| 19 °C | 8.3 °C | 11.1 °C | 13.4 °C |
| 20 °C | 9.3 °C | 12.0 °C | 14.4 °C |
What the table shows, against intuition: raising the temperature of a room without touching its absolute humidity also raises its dew point. Heating more warms the surfaces, which moves the risk away, but if relative humidity stays constant because ventilation keeps pace with vapour production, the dew point rises with the set point. That is why a better heated but poorly ventilated home can grow more mould than before.
The working threshold is not visible condensation either. Fungal growth starts once relative humidity at the surface reaches 80 %, which happens roughly 3 °C above the dew point. In practice a surface less than 3 °C above the table value is already a surface at risk.
The three readings that make the diagnosis
On survey, this class of defect is diagnosed with three readings, taken in the affected room and not in the living room.
- Room temperature and relative humidity, with a thermo-hygrometer, at 1.50 m above floor level and away from walls.
- The surface temperature of the suspect wall with an infrared thermometer, in the coldest corner, not mid-wall.
- The extract rate at the wet room terminals, with an anemometer or flow hood, against the design figures.
The gap between the third reading and the first explains almost every case. An extract that fails to hold its rates lets absolute humidity rise, and the dew point with it, and the same wall that was not condensing starts to condense without any heating setting having changed.
What the health references actually say
The World Health Organization published its Housing and health guidelines in November 2018, recommending that indoor temperature be kept above 18 °C in temperate climates, and identifying people over 65 as the priority group. It is a health reference, distinct from the energy-saving heating limits set in national regulations, which pursue a different aim.
On the consequence side, the French agency Anses issued an opinion and a collective expert appraisal on mould in buildings in October 2016. Two conclusions are directly useful on survey: the effects on respiratory health are proven, covering the development and exacerbation of asthma and allergic rhinitis, and 14 to 20 % of French dwellings show visible mould. The agency also notes that fuel-poor households and overcrowded homes are the most exposed.
Those two references let you record a finding in writing without stepping outside your role. You are not making a medical diagnosis: you are recording a surface below the dew point, naming the reference threshold, and dating the reading.
What you write down, and what it commits you to
The subject turns contentious as soon as work has been done. Insulation fitted in a home whose ventilation was not holding its rates moves the dew point and manufactures the defect: your intervention changed the balance, and the causal link is easy for an expert to establish.
Three written records protect you, and all three happen before signature:
- the dated readings of temperature, relative humidity and surface temperature, before work;
- the caveat on the existing ventilation, with measured rates set against design rates;
- the heat loss calculation establishing the output from the fabric.
That last document is the one most often skipped. A heat source chosen on the old appliance or on a rule of thumb per square metre cannot be defended when the home fails to hold its set point at design outdoor temperature. With Argile, the output is set from the calculated heat loss, not from the old appliance, and that calculation is what stands.
Treating in the right order
Both levers act on the same gap, and one of them alone is never enough.
Lowering the dew point means removing the vapour produced, so restoring the extract rates. It comes first because it is the cheapest and produces a measurable effect within hours. The rates aimed at still have to be the right ones, and the fresh air rate calculated room by room is what sets them.
Raising the surface temperature means insulating the cold element or correcting the thermal bridge that chills it. That is the durable action, and it is what deals with the radiant asymmetry the occupant reads as a draught when it is in fact a fabric defect.
Between the two, the choice is made on the readings, not on impressions. If the surface temperature is sound and relative humidity is high, the subject is ventilation. If humidity is normal and the wall is cold, the subject is the fabric. If both are degraded, the order is ventilation first, fabric second, heat source last, recalculated on the post-works heat loss.
The rest of what gets observed, efflorescence, plaster releasing, timber darkening, comes from the same mechanism applied over time, and belongs with the documented defects of degraded fabric. The balance to hold between temperature and humidity is treated separately in hygrothermal comfort.
What to take away
- Mould is triggered by the gap between surface temperature and dew point, not by a feeling of dampness.
- At 19 °C and 60 % RH the dew point is 11.1 °C, and the risk starts 3 °C above that.
- The WHO sets 18 °C as the health threshold, distinct from regulatory heating limits.
- Anses establishes proven respiratory effects and 14 to 20 % of homes with visible mould.
- Three dated readings before work, ambient, surface and extract rates, decide the outcome of a dispute.
