Originally Posted by wikipedia
Reasons for mold in house
In the home, the presence of mold can mean that something is wrong. Mold doesn’t require sunlight; often, sunlight will kill it. Mold does require moisture and food. In houses and buildings, this food source is usually some form of cellulose such as wood or cardboard.
If an entire house has problems with mold year-round, then there is probably too much moisture in the house, coming from a dirt basement or a leaking roof, for example.
If there are mold problems in one specific part of a house year-round, then that part of the house probably needs more sunlight and ventilation. Mold often forms on perimeter walls, because they are coolest, and where ventilation is poorest (in corners, around furniture).
If there are mold problems in a house only during certain times of the year, then it is probably either too air-tight, or too drafty. Mold problems occur in air-tight homes more frequently in the warmer months (when humidity reaches high levels inside the house, and moisture is trapped), and occur in drafty homes more frequently in the colder months (when warm air escapes from the living area into unconditioned space, and condenses). If a house is humidified artificially during the winter, this can create conditions favorable to mold. Also see ventilation issues in houses for tips on preventing mold. [1]