Lofts by nature tend to be dark and hold dampness. This is a serious concern, since dark, damp areas make an ideal environment for mould growth. A number of elements can create dampness in lofts, and identifying the problem is the first step to correcting it. By understanding the causes, you will be able to cure the dampness in your loft.
Poor air circulation
Poor air circulation is the primary reason lofts tend to be damp. Air that just sits tends to hold moisture, and over time, the moisture levels in the air build up. When it reaches a saturation point, it will precipitate out in the form of dampness on the interior surfaces. The process resembles that of moisture forming in a cloud. When a saturation point is reached, it will rain. You can correct this by using active ventilation to change out the air.
Dampness forms by temperature differences between the outside and inside air. In the winter time, an uninsulated loft becomes semi-warm from the heat of the house rising in it. The hot air meets the cold walls of the loft, and cools down the moisture in the air. The moisture precipitates, or settles out on the walls of the loft. In the summer, the loft becomes a steam oven. The local government of Derby, in the United Kingdom, recommends insulating the walls and floor of the loft, so that the outside and inside air temperature is the same. The World Health Organization states that this problem is more prevalent in the winter, when temperature differences are greater.
Lack of light
Lack of light contributes to dampness. When light hits a surface, it heats it. This is the principle behind solar collectors that use light on black surfaces to heat the surface. Because light levels in a loft are low, the walls never have a chance to heat up. The end effect is that moisture does not evaporated away, but just sits there.
Combination of problems
A loft usually has a combination of all three problems: lack of fresh air circulation, lack of insulation, and lack of light. The end effect is a "triple whammy" that makes an ideal incubator for mould and mildew growth.