How Should an Attic Be Ventilated?

Vent Types

• Intake and outlet vents are two basic types found in most attic ventilation. Intake vents are found in the soffit under the eaves, while outlet vents are installed higher in the attic to allow air to escape. Roof, ridge and gable vents are variations of outlet vents. Ridge vents are the least conspicuous, as they are the same color as shingles and are held back from the roof's gables. Roof vents are watertight devices manufactured of light-gauge metal or plastic and are installed directly into the roof. Gable vents are considered the most efficient type of roof venting. Circular vents are intake devices that are installed when there is no room in the eaves for traditional vents. Starter vents are another example of intake vents.

Power Vents

• Power vents, available in roof and gable types, use fans to force air circulation. Thermostats determine on and off operation. Power vents are good options for attics that are difficult to cool due to architectural features, poor venting design or lack of venting options. Operation of these in winter, however, may cause ice dams to form. A qualified electrician should be used to install power vents

Vent Seletion

• Types selected and placement depends on a number of factors. Architectural details, size of the attic and roof shape determine how many should be used. Soffit intake vents are used in many modern homes, as they offer the best circulation. Typically, a combination of roof, ridge and gable vents is used to create the best overall ventilation.

Calculating Venting Need

• To determine how much venting is needed, the area of the attic floor needs to be determined and should include any space over the garage, as well as soffited overhangs. One square foot of net free vent area is required per 300 square feet of attic floor space. The net free vent area is the portion of the vent opening used to circulate air. Louvers, insect screens or other blockages can reduce the effective venting area. Consult manufacturers' specifications for vents installed to determine the products' net free vent area.