What humidity causes mould?

The number that matters is at the surface

Most guidance says "keep humidity below 60%", and that is sound advice with a hidden step in it.

Mould responds to water activity at the material — how much moisture the surface itself holds in equilibrium with the air touching it. The germination threshold for common building moulds is around 80% relative humidity at that surface, with the most tolerant species starting nearer 75%.

The 60% figure is a proxy. It is the room-air level that, in a typical building with typical cold spots, keeps surfaces under 80%. Whether it actually does depends on how cold your cold spots are.

One room. One humidity reading. Five completely different microclimates, and only the last two are where mould actually appears.

That is the whole reason mould grows behind furniture, in corners, and along window reveals rather than evenly across a wall.

Duration matters as much as level

Germination is not instantaneous. Spores need several continuous hours above the threshold, and then repeated episodes to establish visible growth.

So the question is never "did it get damp" but "how long, and how often". A bathroom that spikes to 90% for twenty minutes after a shower and then clears is not a mould problem. A north-facing corner that sits at 85% every night from November to March is.

This is why improving air movement helps so much even without changing the average humidity — it shortens the time surfaces spend above the threshold.

What actually controls it

Two levers, exactly as with condensation, because it is the same physics:

Raise the surface temperature. Insulation, heating, and not blocking airflow to cold walls. This is usually the more effective one, because it targets the specific cold spots where the problem actually is.

Lower the dew point of the air. Extract at the source, ventilate when the outdoor air is drier, dry laundry elsewhere.

Temperature is not a third lever. Common moulds grow from about 5 to 35°C, which covers every building anyone lives in.

The ventilation question, again

"Ventilate more" is the standard advice, and whether it works depends on something most people cannot see: is the outdoor air drier than the indoor air?

In winter, almost always — cold air holds very little water however high its percentage reads, so bringing it in and warming it produces dramatically drier air.

On a mild damp spell, often not. Outdoor air at a 60°F dew point will not dry anything indoors, and ventilating with it can raise the indoor dew point instead.

The outdoor dew point is the figure that settles it, and it is what this site shows.

Health, briefly

WHO guidance treats persistent indoor dampness and mould as a genuine respiratory risk, associated with wheeze, cough, asthma exacerbation and respiratory infection. That is the level at which it is worth taking seriously.

Identifying species from appearance is not something to attempt, and the practical response does not depend on it: remove the growth, and stop the surface being damp. If the dampness is extensive or keeps returning after the surface conditions are addressed, that is a building problem rather than a humidity one.

What this site can and cannot tell you

It reports the air outside, which is what decides whether ventilating will dry a space or dampen it. That is a real and useful input.

It cannot tell you the humidity at a surface in your building — that needs an instrument against that surface, and no phone has a humidity sensor.