Condensation is governed by physics — specifically the relationship between temperature, humidity, and dew point. Understanding how these interact explains why leak often appears in certain parts of a home rather than evenly across it.
Air always contains some water vapour, and the warmer the air, the more moisture it can hold. The dew point is the temperature at which air becomes saturated and moisture begins to condense into liquid on a surface.
In typical indoor conditions, when room air is around 20°C, the dew point usually sits between roughly 12°C and 16°C depending on humidity levels, meaning any surface at or below that temperature can begin to accumulate condensation.
Humidity alone does not cause leak, but sustained high humidity creates the conditions for it to develop. Short spikes — for example during cooking or showering — are normal and usually harmless if the environment dries quickly.
In practical terms, relative humidity below about 60% is generally low risk, sustained levels between 60–70% begin to increase risk, and prolonged exposure above roughly 75–80% creates favourable conditions for leak growth.
Condensation forms when warm, moist air meets a colder surface. External walls, poorly insulated areas, corners, and thermal bridges often remain cooler than the surrounding air, especially in colder months.
As a rule of thumb, when a wall surface sits more than about 3–5°C colder than the room air, the likelihood of reaching dew point and forming repeated condensation increases significantly, particularly in humid conditions.
Condensation is rarely defined by a single event. What matters is repeated exposure over time. A bathroom may briefly reach very high humidity without consequence if surfaces dry quickly, but when humidity stays elevated and surfaces remain cold, moisture begins to accumulate.
By the time leak becomes visible, the condensation pattern that enabled it has usually been present for weeks.
Two rooms can show similar air humidity yet behave very differently. Variations in wall temperature, ventilation, exposure, and materials mean some areas accumulate moisture while others remain stable.
Condensation forms where dew point meets a cold surface, not where average humidity suggests.
Understanding this relationship helps explain why leak often begins locally — and why early changes in temperature, humidity, and drying patterns matter long before leak is visible.