Damp, mould and the dew point

Black spots in the corner of a bedroom. Water running down the windows in the morning. A musty smell behind the wardrobe. In Kashmir these usually start in December and are often blamed on leaks. More often, the cause is condensation.

What the dew point is

Air always carries some water vapour. Warm air can carry more than cold air. When warm, moist air touches a surface that is cold enough, it can no longer hold all its water, and some of it condenses into liquid on that surface.

The temperature at which this happens is the dew point. For a room at 21 °C:

• at 50% humidity, the dew point is about 10 °C
• at 60% humidity, it is about 13 °C
• at 70% humidity, it is about 15 °C

Any surface in the room colder than the dew point will collect water.

Why Kashmiri homes reach it

Two things push winter homes towards condensation:

1. Cold surfaces. Uninsulated walls, single glazing, window frames and concrete slab edges all get cold on the inside in winter.
2. Moist air. Cooking, bathing, drying clothes indoors and simply breathing all add moisture. With the windows shut against the cold, it has nowhere to go.

Put a wall surface at 12 °C next to air at 21 °C and 60% humidity, and you have condensation. Keep it up for weeks and mould follows.

Leak or condensation?

Leaks tend to appear after rain or snowmelt, often near roofs, pipes or the plinth. Condensation follows the cold: corners, behind furniture, around windows, and it is worst on the coldest mornings. We check which one it is before recommending a fix, because the cures are different.

The three fixes that work together

1. Warmer surfaces. Continuous insulation and better windows keep inside surfaces well above the dew point.
2. Controlled ventilation. Heat-recovery ventilation removes moist air continuously while keeping most of the heat.
3. Keep water out. Good waterproofing stops rain and snowmelt adding to the problem, and breathable details let walls dry.

Fixing only one rarely solves it. A dehumidifier, or repainting with anti-mould paint, treats the symptom for a season. Warm surfaces and fresh air fix the cause.

Dew points from the Magnus formula. Real conditions vary room by room.