Sometimes, humidity or dampness (water vapor) in the air can supply enough moisture for mold growth. Indoor relative humidity (RH) should be kept below 60 percent — ideally between 30 percent and 50 percent, if possible. Low humidity may also discourage pests (such as cockroaches) and dust mites.

Humidity levels can rise in a building as a result of the use of:

• Humidifiers
• Steam radiators
• Moisture-generating appliances such as dryers
• Combustion appliances such as stoves
• showers 
• heat escape from the attic floor to roof deck 

Heat escape from the living space is a big contributor to mold growth as a result of cold air from the soffits or roof vents  hitting warm air from the living space that leaked out through the attic floor seams. This is evident from the exterior when icicles are present. Often the attic sheathing ( roof deck ) will  have mold, why? because the heat escape from the living space is creating both condensation and warming of the roof deck.