Condensation occurs on cold surfaces when the following three conditions happen at the same time:
Too much moisture in the air.
Inadequate indoor-outdoor air exchange (known as “ventilation”).
The availability of cold surfaces upon which moisture in the air can condense.
Too much moisture in the air may come from:
showers, washing dishes and clothes, cooking, aquariums, standing water, people, pets and plants;
drying laundry indoors;
improperly vented clothes dryers;
earth floor basements or crawl spaces;
improperly set humidifiers; and
humid outdoor air.
Inadequate ventilation may be caused by:
no bathroom exhaust fans, outside-ducted kitchen range hoods, air exchanger or heat recovery ventilator to vent moist air from the home;
broken or disconnected exhaust fans, ventilation system;
exhaust fans not being operated because they are too noisy or ineffective; and
no circulation of air within the home and the rooms in the home.
Cold surfaces may be due to:
very cold weather outside;
inadequate heat or insufficient heat provided to areas of the home (that is, floor vents or baseboard heaters blocked by furniture, spare bedroom heat blocked off if the room is not used regularly, an unheated basement);
wide swings in inside temperature (that is, thermostat setbacks, uneven heat distribution from use of wood stoves, unheated or poorly heated rooms);
poor local air circulation within a room due to furnishings such as beds against the exterior walls; and
older, leaky windows.
When the temperature becomes colder outside, the risks of condensation can increase.
Moisture and Air: Householder’s Guide — Problems and Remedies