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Roof and Attic Ventilation
Page 2 of 5

There are many types of attic vents available today. There are static vents, power vents, ridge vents, turbine vents, soffit / cornice vents, gable vents, starter vents, and cupola vents. These all come in a wide variety of sizes, styles and shapes. Some will ventilate better than others depending on the roof configuration, attic size, climate, etc.

To properly ventilate an attic, two types of vents are needed. Intake vents, which are located at the downslope edge of the roof (a.k.a. eaves) and allow fresh air into the attic; and exhaust vents, which are located near or on the ridge line of the roof and allow air to leave the attic. The use of an exhaust vent in conjunction with an intake vent uses the natural forces of wind pressure and thermal effect, collectively known as the Stack Effect, to ventilate the attic space. HINT: Make sure your attic insulation doesn't block the intake vents. If necessary, use baffles to keep the insulation back from it.

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A vent’s effectiveness is measured by its Net Free Vent Area. The Net Free Vent Area is the portion of the opening in the vent that actually ventilates. For instance, a vent can have an opening that measures 12 inches by 12 inches; this would appear as if it would yield 144 square inches of ventilation area. Because of louvers, an insect screen, or some other type of blockage, the actual ventilation area could be as little as 40 percent of that, yielding about 58 square inches of ventilation area. This 58 square inches is what’s known as the Net Free Vent Area, and is the amount that should be used when calculating how much venting you need.

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