When air is cooled, its ability to contain water vapour is reduced and it condenses on certain  surfaces. In most cases this condensed water evaporates again, but sometimes a wall, floor or ceiling may remain wet for days, or even months. This may lead to dimpling of nail heads, sagging of ceilings, rotting of framing timbers and encouragement of mould growth or timber pest activity.

Building design must take many interrelated factors into account if condensation is to be eliminated. Such factors include orientation, temperature and relative humidity differentials, shading, ventilation, methods of heating and cooling, building techniques, use of materials of low or high thermal capacity, function of the building, and living habits of the occupants.

The aim should be to always:

• Keep materials and structures above the "dew point" temperature
• Keep air spaces well ventilated to allow re-evaporation

Moist air should be kept away from cold surfaces (e.g. by using a vapour barrier, i.e. a thin membrane of low water vapour permeability).Ventilation which encourages warm moist air escape is critical and an effective solution that minimizes condensation. This is provided in roofs with airspace above the insulation systems by ridge vents or end wall louvers.

WIKIPEDIA ENTRY FOR DAMP.