Hygrometer
Condensation is the symptom, not the cause
Mould needs three things: spores, something to eat, and moisture. The first two are always present — spores are in every building on earth, and dust, timber, paint and sealant are all food.
So moisture is the only variable, and it is the only one worth acting on.
Visible condensation does not itself cause mould. What it tells you is that the surface has been sitting at 100% relative humidity for some hours, which is comfortably past the threshold where germination becomes possible.
The threshold is at the surface, not in the room
This is the distinction that makes most advice on the subject confusing.
Mould responds to conditions at the material's surface, and a surface can be far damper than the air a metre away. Cold glass, a cold reveal or a cold corner forces the thin layer of air touching it towards saturation, so a room reading 55% overall can easily have an 85% microclimate against the window.
That is why the threshold is quoted in relative humidity rather than in grams per cubic metre — what matters is how close that air is to saturation, not how much water is in the room overall. It is also why relative humidity is the right measure for this particular question even though the dew point is better for most others.
| Surface RH | What can happen |
|---|---|
| Below 75% | Nothing. Below the germination threshold for common building moulds. |
| 75–80% | The most tolerant species can start, given sustained periods. |
| 80–90% | General mould growth becomes possible on porous materials. |
| Above 90% | Rapid growth. Visible condensation puts you here. |
Duration beats the peak
A single cold morning is not the problem. Spores need several continuous hours above the threshold to germinate, and then repeated wettings to establish.
So the question is not "did it get wet" but "how long was it wet, and how often". A window that fogs at dawn and is dry by nine is very different from one still running at midday, even though both looked identical at 7am.
That is also why wiping the glass — which does nothing about either lever — genuinely does help. It shortens the wet window, which is the variable that counts.
Why the frame goes first
Glass is non-porous. Water sits on it, then evaporates, and nothing gets a foothold.
Timber, sealant, paint and plasterboard are porous. They hold water inside the material where it dries slowly, and they offer a surface spores can grow into rather than merely sit on. Gravity puts the water at the bottom of the pane, so the sill and the lower reveal accumulate it.
Which is why mould appears as a line along the bottom of a window rather than a patch in the middle of the glass.
What to do about it
In order of how much they change:
- Shorten the wet time. Wipe it, and get air moving past the surface.
- Raise the surface temperature. Better glazing, open the curtains, do not block the heat.
- Lower the dew point of the air. Extract at the source; ventilate when the outdoor air is drier — which is what the outdoor dew point tells you and why ventilating on a humid night can backfire.
- Remove what is already there, then keep the surface dry so it does not return.
What this site can and cannot tell you
It can tell you what the air outside is carrying, which is what decides whether ventilating will dry things out or make them worse. That is a genuinely useful input and it is the one most people do not have.
It cannot tell you the humidity at a surface in your building. That depends on the construction, the heating, the ventilation and what is going on in the room, and measuring it needs an instrument against that surface. No phone has a humidity sensor, and a weather service knows nothing about your walls.