Moisture movement and condensation effects

Moisture movement

Moisture moves through building materials in two forms.

1. Liquid movement: by capillary action (rising damp), by gravity (percolation through roofs and cracks), by wind pressure (driving rain) and by hydrostatic pressure (basements).
2. Vapour movement: by diffusion from higher to lower vapour pressure, usually from the warm, humid side to the cold side, and by air movement carrying vapour through leaks.

If vapour meets a surface below the dew point temperature, it condenses to water.

Types of condensation

1. Surface condensation: Occurs on the visible inner surface of a wall, window or ceiling that is colder than the dew point of the room air. It is common in kitchens and bathrooms in winter.
2. Interstitial (concealed) condensation: Vapour passes into the wall or roof and condenses inside the material, where the temperature falls below the dew point. It is unseen and more harmful.
3. Condensation in cavities (roof voids, cavity walls).

Effects on building materials

• Damp patches and stains, peeling paint and plaster.
• Mould and fungal growth, bad smells and health problems.
• Rotting of timber and corrosion of reinforcement and steel.
• Frost damage and spalling of masonry.
• Wet insulation loses much of its insulating value.
• Dripping water damages furniture and finishes.

Control

Ventilate humid rooms; insulate to keep surfaces warm; use a vapour barrier on the warm side; use dehumidifiers and extract fans.