Condensation on windows and in conservatories, and the damage it causes to paintwork, curtains, wall coverings and window fittings, are problems frequently encountered in all types of building.

In many homes, traditional open fires have been replaced by sophisticated heating systems, and ill-fitting doors and window frames have been provided with draught excluders. Floors have been completely covered by fitted carpets, while ceiling heights have been lowered and the space between loft joists filled with insulating material.

These modern aids to home comfort have created rooms which are warmer but which often have less ventilation and fewer air changes. The result is that the water vapour produced by normal living activities is no longer able to escape up the chimney or through door jambs, window joints and other outlets.

Some examples of where the water vapour comes from:

Breathing: Two sleeping adults produce 11⁄2 pints of moisture in 8 hours, which
is absorbed as water vapour into the atmosphere.
Cooking: Steam clouds can be seen near saucepans and kettles, and then seem to disappear. The clouds have been absorbed into the atmosphere. The cooker itself may be a source of water vapour; eg. an average gas cooker could produce approximately 11⁄2 pints of moisture per hour.
Washing up: The vapour clouds given off by the hot water are rapidly absorbed
into the atmosphere.

Bathing, laundry, and wet outer clothing: These are often the major sources
of water vapour in the home.
Heaters: A flueless gas heater can produce up to 2⁄3 pint of moisture per hour.
Paraffin heaters produce nine pints of moisture for every eight pints of fuel burned.
Indoor Plants: A frequently unrecognized but nevertheless significant source of
water vapour.
New Property: The bricks, timber, concrete and other materials in an average
3-bedroomed house absorb about 1500 gallons of water during construction.
Much of this is dissipated into the indoor atmosphere during the drying out period

How double glazing helps?

Double glazing is an insulator, designed to reduce the loss of heat by conduction from the inside to the outside of a building. Under average exposure conditions, and provided the room is heated, the room side surface temperature of the inner glass will be higher than would be the case with single glazing. The likelihood of condensation occurring when warm moist air in the room comes into contact with the surface of the glass is thereby reduced. It must be remembered, however, that double glazing is an insulator and not a source of heat; nor does it control the amount of water vapour in the air. When rooms are inadequately heated and there is little heat to retain, double glazing cannot fulfill the purpose for which it was installed.

Condensation on the room side surface of the inner glass means that the temperature of the glass surface is too low given the water vapour content of the atmosphere in the room.

Condensation within the cavity of an hermetically sealed unit denotes a failure
of the seal.