Condensation occurs when air hits a cold surface. Air has a limit to the amount of water it can hold in suspension and the amount of water in the air is
known as the relative humidity (measured as a percentage). Once air
becomes saturated, the relativity humidity has reached 100%. Now as the
temperature becomes lower, air can hold less and less water.
Condensation occurs when water laden air hits a cold surface, reducing
the temperature of the air. This could be glass in windows, cold tiles or
metal surfaces. These surfaces are either colder than other surfaces in a
room or are of high thermal conductivity so that heat is sucked out of
the air. In any case, the air temperature drops to the extent that it
can't hold moisture any more and it is deposited on the surface.
To
avoid condensation you have several options. Firstly you can vent
moisture laden air (e.g from cooking in the kitchen) so that it doesn't
end up in rooms. You can do this by using extractor fans or simply
opening windows. Portable gas heaters produce lots of water vapour and should be avoided. Another option is to raise the temperature in the room.
This makes surfaces warmer so that moisture doesn't condense out.
Double or triple glazing also helps because the external surface of
inner panes of glass is not in contact with air outside the house.
Removing sources of moisture also helps. This includes house plants and
damp clothing. Clothes in the washer should be kept there with the door
closed until they are transferred to a drier, or hung outside. Drying clothes in rooms or on radiators transfers water to the air where it inevitably condenses out if windows are closed and the room temperature drops. Yet
another way of reducing condensation is to use a dehumidifier. This
appliance works by circulating air over chilled coils. This causes
moisture to drop out of the air (just like it does on your windows) and
collect in a reservoir tank.