Mould's three conditions
Mould needs three things: moisture, the right temperature and a food source. A closed-up holiday home has all three in abundance. Paint, timber, textiles and dust are already a food source; a Mediterranean winter provides an ideal temperature. That leaves one controllable variable: moisture.
When relative humidity stays above 60% for a long period, mould starts. Above 70%, the timescale is measured in weeks.
Condensation: moisture collecting on the wall
Moisture in the air turns to liquid when it meets a cold surface. That is condensation. It is why mould doesn't appear everywhere, but always in the same places: corners on external walls, behind cupboards, window sills, the north wall of the bedroom.
Those points are a few degrees colder than the rest of the room; with no air circulation, moisture collects there and stays.
Why wiping doesn't work
Wiping a mould stain with bleach removes the colour on the surface, but it doesn't kill the mycelium inside the fabric and it doesn't change the moisture source. Two months later it comes back in the same place.
A permanent fix is two steps: first cleaning the surface and grout with a biocidal product, then removing the cause of the moisture. Without the second, the first only buys time.
Ways to lower humidity in an empty house
In order of priority:
- Regular ventilation — airing every room through for 15–20 minutes on each visit is the single most effective method
- A dehumidifier — if the electricity can be left on, a humidistat-controlled unit holds humidity around 55%
- Chemical moisture absorbers — useful for small volumes and inside cupboards, not enough for a whole house
- Pulling furniture 5–10 cm off the wall — allows air to circulate behind it
- Leaving cupboard doors ajar — prevents stagnant air
Measuring humidity doesn't give an opinion, it gives data
When humidity is measured room by room and logged on every visit, after three months you have a curve. Which room is the problem, which month the moisture rises and whether the measure you took worked — none of that is visible without the data.
That is why our reports record the humidity value room by room on every visit: fighting mould is not a one-off intervention, it is a monitoring job.