Attic ventilators typically fall in one three categories, which include powered ventilators, wind-assisted ventilators and gravity ventilators. A powered attic ventilator moves air into and out of an attic with the aid of a fan that is usually electric powered. A wind-assisted attic ventilator uses in-flow and out-flow air portals placed to take advantage of prevailing wind direction of a given house. A gravity attic ventilator uses the tendency of cooler air to fall and hotter air to rise to move air out of an attic.
In each type of attic ventilator, the goal is to maintain reasonable temperatures in the attic area of a house. This is accomplished by exchanging hot air in the attic for cooler outside air. A properly vented attic usually results in a cooler living area in the house as well.
Both wind-assisted and gravity attic ventilators are considered forms of natural ventilation. The gravity attic ventilator usually has two components, including cool air inlets under the eaves of the roof that allow outside air to flow in. An outflow vent is placed in the uppermost ridge of the roofline. Hotter air in the attic rises and escapes through the ridge vent outlet allowing cooler outside air to flow in the cool air inlets. Some gravity ventilation systems have inlets that run the entire length of the roof eave and outlets that run the entire length of the roof ridge line.
A wind-assisted attic ventilator takes advantage of geographical areas where the wind tends to come from the same general direction. Wind blowing broadside against a roof creates positive pressure on that side. The same wind creates negative pressure as it passes over the roof and away from the home. Positive pressure draws outside air into the attic through inlets in the eaves. Negative pressure causes the air in the attic to flow out through outlet vents on the other side of the home.
Powered attic ventilator systems usually are limited to those that use a powered fan to draw outside air into an attic and move hotter air outside. Some powered attic ventilators include a triangular-louvered inflow vent in the gable at the peak of the roof. In the opposite gable, an exhaust fan creates airflow. Powered attic ventilators also use an inlet and outlet configuration similar to a gravity ventilation system. An electric fan near the ridge of the roof draws air into the attic from eave inlets and evacuates the hotter air from inside the attic.