British Standard BS5250 Condensation
  L19 Taken from British Standard 5250    Explanatory leaflet (paragraphs.2-7)

 2.  WHY CONDENSATION OCCURS. Condensation occurs when warm moist air meets a cold surface. The risk of condensation therefore depends upon how moist the air is and how cold the surfaces of rooms are. Both of these to some extent depend on how the building is used.

3.  WHEN CONDENSATION OCCURS. Condensation occurs usually in Winter, because the building structure is cold and because windows are opened less and moist air cannot escape.

4.WHERE CONDENSATION OCCURS. Condensation which you can see occurs often for short periods in bathrooms and kitchens because of the steamy atmosphere, and quite frequently for long periods in unheated bedrooms; also sometimes in cupboards or corners of rooms where ventilation and movement of air are restricted. Besides condensation on visible surfaces, damage can occur to materials which are out of sight, for example from condensation in roofs.

5.WHAT IS IMPORTANT. Three things are particularly important :-
 (a) To prevent very moist air spreading to other rooms from kitchens and bathrooms or from where clothes may be put  to dry.
(b) To provide some ventilation in all rooms so that moist air can escape.
(c) To use the heating reasonably.

6. REDUCE MOISTURE CONTENT OF ROOM AIR
(a)Good ventilation of kitchens when washing or drying  clothes or cooking is essential. If there is an electric extractor fan, use it when cooking, or washing clothes, and particularly whenever the windows show signs of any  misting. Leave the fan on until misting has cleared.
(b) If there is no extractor fan, open kitchen windows but  keep the door closed as much as possible.
(c) After bathing, keep the bathroom window open, and shut  the door for long enough to dry off the room.
(d) In other rooms provide some ventilation. In older houses a lot of ventilation occurs through flues and draughty windows. In modern flats and houses sufficient ventilation does not occur unless a window or ventilator is open for  a reasonable time each day and for nearly all the time in a room that is in use. Too much ventilation in cold  weather is uncomfortable and wastes heat. All that is needed is a very slightly opened window or ventilator.  Where there is a choice, such as a top hung window, open the upper part. About a 10mm opening will usually be  sufficient.
(e) Avoid the use of paraffin or flueless gas heaters as far as possible. Each litre of oil used produces the equivalent of about a litre of liquid water in the form of  water Vapour. If these heaters must be used, make sure  the room they are in is well ventilated.
(f) If condensation occurs in room which has a gas, oil or solid fuel heating appliance with a flue the installation should be checked, as the condensation may have appeared  because the appliance flue has become blocked.
(g) Do not use unventilated airing cupboards for clothes drying.
(h) If washing is put to dry, for example in a bathroom or kitchen, open a window or turn on the extractor fan enough  to ventilate the room. Do not leave the door open or moist  air will spread to other rooms where it may cause trouble.

7.  PROVIDE REASONABLE HEATING.
(a) Try to make sure that all rooms are at least partially  heated. Condensation most often occurs in unheated rooms.
(b) To prevent condensation the heat has to keep the room  surfaces reasonably warm. It takes a long time for a cold building structure to warm up, so it is better to have a small amount of heat for a long period than a lot of  heat for a short time.
(c) Houses and flats left unheated during the day get very cold. Where ever possible, it is best to keep heating on  even at a low level.
(d) In houses, the rooms above a heated living room benefit to some extent from heat rising through the floor. In bungalows and in most flats this does not happen. Some  rooms are especially cold because they have a lot of  outside walls or loose heat through a roof as well as walls. Such rooms are most likely to have condensation  and some heating is therefore necessary. Even in a well  insulated and with reasonable ventilation it is likely to be necessary during cold weather to maintain all rooms at  10C in order to avoid condensation. When living rooms are in use their temperature should be raised to about 20C."