As you are probably aware condensation is a build up of bacteria and moisture in the air. This moisture comes from many sources such as breath, sweat, bathing, cooking and drying clothes.

The hotter the air, the more moisture it can hold. The colder the air the less saturation. This moisture, especially coming from humans, carries bacteria. It is this bacterium that causes the black/green/yellow mould that is the visible signs of condensation. Typical areas for mould growth is master bedrooms and bathrooms.

Evaporated water condenses on contact with cold objects. In a room this is generally the exterior wall, which becomes colder with the weather and rain saturation (each porous house brick can hold up to 1pt of water. Obviously a wet brick is colder than a dry one). The window also provides a perfect place for condensing water creating droplettes and puddles on sill. This also is an early sign of condensation.

During the winter months, October-March, people are less likely to open windows as exterior air is cold this means that every day insufficient ventilation occurs, air saturation becomes greater until eventually air must condense.

To remove condensation one of 2 changes must occur:

Walls to be insulated:
(1) A styrene backed plaster board can be fitted covering exterior walls.
(2) a chemical sealant can be sprayed onto the surface of the exterior wall from the outside so as to stop rain saturation.
(3) any signs of rising damp must be rectified.