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Attic Venting Requirements

Your AirScape needs to breathe

A whole house fan is designed to move air from your house into your attic. And since just about all attics are vented, the pressurized attic air is then naturally pushed outside through those vents. However, without enough venting area, pressure can build up, slowing down airflow and reducing efficiency, potentially pushing hot attic air back into your living space.

Airflow is measured in Cubic Feet per Minute (CFM). The higher the CFM, the more venting area is required to keep the system working efficiently. A rule of thumb is you should have one square foot of "net free venting area" for every 500 CFM of airflow. So at 3000 CFM, you'll want six square feet of venting. Most modern homes easily accommodate this rule, but it is important to check to ensure optimum system efficiency.

Attic ventilation tip

Keep your attic cooler during the day and get even better natural cooling results. By using a combination of ridge venting at the peak and under-eaves vents on the bottom, you can help create a convection current that lets hot air (lighter and more buoyant) out the top and draws cooler air in from below.

More on attic venting calculations

Net free ventilation area can be provided by any combination of gable, eyebrow, roof cap, soffit, or ridge vents, or any other method of ventilating the attic space.

However, the openings of most vents are partially obstructed by grilles, louvers, and/or screens. A vent’s “net free” ventilation area is then the surface area of its opening minus the surface area of any grilles, louvers, or screening covering it. Different types of vents have different ratios of net free area to total area.

Manufacturers typically publish their vents’ net free ventilation areas and/or ratios in their products’ specification documents. If this information is unavailable, a ratio of 50% net free area to total area is usually a good rule of thumb. The most notable exception to this rule of thumb are ridge vents. The industry standard net free ventilation area for ridge vents is 13% of the vent’s length in feet. Thus, a ten foot ridge vent would provide 1.3 sq. ft. of net free venitilation area.

While in our experience most properly constructed homes have adequately ventilated attics, not all do. Because sufficient ventilation is so critical to this fan’s performance, it is important that the home’s existing ventilation be verified before it is installed. Since most attics have multiple vents, often of different types, it is necessary to count each vent, noting its type and size. Apply the appropriate ratio to the dimensions of each vent to find its net free area, and sum these values to find the attic’s total ventilation. An example of how these calculations are made is given in the chart below.


Net free area venting calculation