CONDENSATION

Under normal atmospheric conditions water vapour is continuously generated by evaporation and is continuously removed from the air by condensation.
The ratio of water vapour retained by a pocket of the atmosphere (its relative humidity or %RH) depends on the air temperature and air pressure.
Warm air is able to carry more water vapour than cold air .Therefore when the air temperature drops the ability of the cooler air to retain water vapour decreases.
A  dew point (100% RH) is reached when the cooling air becomes saturated and is forced to releases excess water vapour as droplets as the vapour reverts from a gas to a liquid.  The condensed vapour is called a condensate.
The diagram to the left indicates, at different temperatures, the relative humidity of a parcel of air having 8.7g of water vapour per 1kg of dry air

Effects of Condensation in Buildings

The air inside buildings often incorporates high level of relative humidity due to the activity of the inhabitants in cooking, bathing, drying clothes etc.   When this moisture laden air comes into contact with cold surfaces such as windows and cold walls it condenses.  Water vapor will only condense onto another surface when that surface is cooler than the temperature of the water vapour.
The condensate is immediately obvious on non-porous surfaces such as glass and ceramics but is, initially, less noticeable on porous surfaces such as plasterboards, wall plaster and timber, which may simply absorb and store the dampness.
Circulating air retains a higher temperature and dew point than stagnant air.  For example a ventilated space is less likely to deposit condensate than a non-ventilated space
Poorly ventilated spaces, such as those found in cupboards, in corners and behind furniture encourage condensation and mould spore germination.  These spaces are often where musty odours accumulate and black mould on walls, ceilings and even on soft furnishings such as curtains and carpets first become evident.   Airborne mould spores are unhealthy and may adversely affect those having respiratory disorders such as the old, the young and asthmatics.

Managing Condensation in Buildings

It is most unlikely that we will change our living habits to reduce kettle boiling, washing and tumble drying or to make our homes less draught-proof.  Therefore condensation is likely to remain a fact of life and our contribution is merely to manage and control relative humidity in the home.
Simply heating the home on its own will only increase the amount of air-borne water vapour with condensate potential.  Condensation control can be achieved with the correct a balance of:

• Heating:-        warm air has higher water carrying capacity (dew or saturation point) than cold air
• Insulation:-     warm walls are less likely to attract condensate
• Ventilation:-    force moisture laden air out of the home to lower the humidity and dew point

Good ventilation is the key.  Consider the introduction of additional ventilation such as trickle vents for windows, extractor fans or heat recovery fans for bathrooms and kitchens or low-cost positive whole-home ventilation units.
Where black mould is an issue, surfaces can be treated with biocide washes to remove moulds and fungi.  Anti-mould paints are available, though it is often more cost effective to supplement standard emulsions with a fungicidal additives to protect against further mould growth.