Ventilation Ventilation of the home attic is important for two reasons. During the summer, excess heat that builds up in the attic during the day results in high energy costs for cooling. Moisture produced within the home may move into the attic if ceiling vapor barriers are not used. If this moisture is not exhausted from the attic it can condense and cause insulation and construction materials to deteriorate. Thus, temperature and moisture control are the major reasons for providing attic ventilation.
Attic ventilation can be accomplished by gravity ventilators, wind assisted ventilators or power ventilator. Regardless of the method used, the purpose is to provide uniform ventilation of the attic for proper temperature and moisture control.
 Ventilation caused by wind pressure differences requires less vent area to achieve the same ventilation rate as ventilation by gravity. One problem with wind ventilation is that the areas of high and low pressure change with wind direction, causing difficulty in locating inlets and outlets so that ventilation will take place regardless of wind direction. The best system is one in which the outlet is near the ridge and the inlets are in the soffit area. The hip roof is best suited to this system, since there is soffit area on all sides of the house. Outlet vents can be provided by roof louvers, gable end louvers, turbine ventilators or continuous ridge vents. These should be constructed so that rain and snow cannot enter. Water leakage is more likely with the turbine ventilator and gable end louver than with fixed roof or ridge ventilators.
  Natural ventilation is the most common and energy-efficient method of achieving attic temperature and moisture control. This method takes advantage of two principles. First, as air is heated it becomes less dense and rises. Second, wind movement over and around a home creates areas of high and low pressure. If a space has high air outlets in conjunction with low inlets, ventilation occurs as the air within the space is heated. The greater the vertical distance between the outlet and inlet, the greater the ventilation rate will be. Thus, roof outlets should be at or very near the ridge, and inlets should be under the roof overhang or near the ceiling line.
Inlet vents may be of the manufactured type, either slotted or perforated, and installed as individual units or in a continuous strip. The slotted opening seems to resist clogging by fibers and dust better than the perforated opening. Screen wire is sometimes used for soffit vents, but again clogging is a problem. In some localities frequent cleaning may be necessary with either type.
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