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Effective ventilation requires a balancing act. You want to "wash" the entire attic by pulling air from the intake vents low on the roof's edge or in the soffit to the vents at the ridge in the high areas.

Evaluate Your Customers' Needs

To specify an attic ventilation system, you should first determine the square footage of your customer's attic. You'll need this figure to make sure you install adequate ventilation. If the roof pitch is 7/12 to 10/12, add 20% to your calculation for vent requirements; 30% for roof pitches of 11/12 and steeper.

If your roof pitch is 7/12 to 10/12, plan to add 20% to your calculation for vent requirements; 30% for roof pitches of 11/12 and steeper.

Vent Requirements

According to most building codes, you need one square foot of vent area for each 150 square feet of attic floor space. The minimum is one square foot for every 300 square feet of attic floor space if there is a vapor retarder or the space is balanced between the ridge and intake vents. A balanced ventilation system means about 50 percent of the required ventilating area should be provided by exhaust vents in the upper portion of your attic with the remaining 50 percent provided by intake vents.

Please remember, building codes specify the minimum amount of ventilation. You may want to increase the requirement to ensure proper ventilation.

Common Mistakes

Too often, builders install products that short-circuit their ventilation system. When designing a ventilation system, avoid these common pitfalls:

Don't use a combination of different types of exhaust vents, like power vents with ridge vents. In this case, competing vents pull air from each other instead of pulling from the intake vents.
Don't underestimate your ventilation needs. Remember that 15 roof vents or 5 turbine vents would be needed to provide the same ventilation as 42 feet of ridge vent.
It is of no use to install exhaust vents without adequate intake. An effective balance of intake and exhaust must be achieved to properly ventilate the home. The flow of air in your attic is limited to the amount of intake.
Don't install a ridge vent that doesn't have an external baffle to increase airflow and protect from weather infiltration.