There are two possible causes why mould appears - one is structural water ingress (rising damp, roof leaks, gutter disrepair) and the other is internal condensation. In the majority of cases, it will be condensation.
It is well known that some houses and flats suffer from condensation. Walls and ceilings, and sometimes floors, become damp and sometimes discoloured and unpleasant as a result of mould growing on the surfaces.
Condensation occurs when warm moist air meets a cold surface, for example similar to when you are in a car and it gets steamed up. The risk of condensation therefore depends upon how moist the air is and how cold the surfaces of rooms are. Both of these depend to some extent on how a building is used. In a room with a cold outside wall, the temperature of which falls below the dew point temperature, it is quite normal for condensation to occur predominantly on the lower parts of the external walls and may be confused with rising damp. If you are the 1st floor, you can be pretty certain you do not have rising damp!
Condensation occurs usually in winter, because the building structure is cold and because windows are opened less and the moist air cannot escape.
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.
Three things are particularly important:
• To prevent very moist air spreading to other rooms from kitchens and bathrooms or from where clothes may be put to dry.
• To provide some ventilation to all rooms so that moist air can escape.
• To use the heating reasonably
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 any sign of misting. Leave the fan on until misting has cleared.
b) If there is not an extractor fan, open the 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 old houses a lot of ventilation occurs through fireplace 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 a room 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, open the upper part, such as top-hung window. About 10mm opening will usually be sufficient. e) Do not use unventilated airing cupboards for clothes drying. f) 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.
a) Try to make sure that all rooms are at least partially heated. Condensation most often occurs in unheated bedrooms.
b) To prevent condensation, the heat has to keep 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 unoccupied and unheated during the day get very cold. Whenever possible, it is best to keep heating on, even if 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 lose 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 house and with reasonable ventilation it is likely to necessary during cold weather to maintain all rooms at not less than 10°C in order to avoid condensation. When living rooms are in use their temperature should be raised to about 20°C.
Any sign of mould growth is an indication of the presence of moisture and if caused by condensation gives warning that heating, structural insulation or ventilation, or all three, may require improvement and this usually can be improved by the occupants of the house.
Using a mild detergent and damp cloth will wipe away mould spores on windows and walls and it may be a case that if you create a lot of moisture in the house, that you will have to keep up to this over the winter months.