Ventilation
Modern building codes require unoccupied attics to be ventilated to reduce the accumulation of heat and moisture that contribute to mold growth and decay of wood rafters and ceiling joists.One common code requirement is that the total area of attic vents be equal to or greater than 1/150 of the floor area of the attic, with 50% or more of the vent area located in the upper portion of the attic. Various types of turbine ventilators and exhaust fans can also be used to assist with attic ventilation and decrease the required area of passive ventilators.
Read more about this topic: Attic