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Reducing heat buildup which can reach 150°F on hot summer days, and moisture in the winter, are the two main goals of attic ventilation.
The purpose of attic ventilation is to equalize temperatures inside and outside the attic throughout the year. In summer, this equalization of temperatures helps make the living areas beneath the attic cooler and more comfortable; this can help reduce how hard the air conditioner needs to work, thus lowering its operating costs. This equalization helps reduce heat that will distort and destroy roof shingles and cause the premature deterioration of roof boards, sheathing, siding and insulation.
In winter, equalizing attic and outside temperatures helps to prevent moist air, which seeps into cold areas, from condensing on the underside of the roof surface, beams and rafters. This condensation can cause mildew, rotting of wood and excessive rusting of fasteners and other metal components of the roof structure. Excess condensation creates wet insulation, which diminishes its effectiveness and in sufficient quantities may damage interior ceilings.
Proper attic ventilation can also help prevent the formation of “ice dams” in the winter by equalizing the indoor and outdoor temperatures. Ice dams occur when snow hits a part of a home’s roof warmed by heat loss from the interior. The snow then melts and freezes at the eave area of the roof, where it’s colder. Here the mounds of ice and snow often drip and form icicles.
Powered and static attic ventilators
Effective attic ventilation requires air to exit the attic at or near the peak of the roof and replacement air to be drawn in under the eaves of the house. This can be achieved using a combination of static vents, or the ventilation can be given a boost using powered attic ventilators (PAVs).
PAVs pull air out of the attic and are usually equipped with automatic thermostats, which activate the unit as the temperature rises above the setpoint. They are usually placed near the roof peak on the back slope of the roof, where they are less visible from the street, or high on a gable end wall. A humidistat may also be used to start the unit automatically when the humidity rises above the selected setting in the winter.
Static vents, both intake and exhaust, are not electrically powered and depend on natural airflow for ventilation. Intake vents are necessary whether using a static exhaust system or a powered attic ventilator. Static vents include ridge vents, roof vents, gable vents and turbines.
• Ridge vents are installed at the peak of the roof, often along its entire length. Roof vents cover holes cut in the roof near the peak to let air escape and to prevent rain, snow and insects from entering the attic.
• Gable end vents, generally triangular, rectangular or round in shape, have either adjustable louvers to fit a wide range of roof pitches, or fixed louvers for a specific pitch. They are mounted at the highest point of the gable.
• The intake vents, necessary to balance the systems, are installed under the eaves in the soffits.
To be effective, both intake and exhaust vents must be present to promote air movement. Static ventilators are also available for installation in sidewalls to reduce peeling of siding paint and moisture damage.
The Healthy House Institute (HHI), a for-profit educational LLC, provides the information on HealthyHouseInstitute.com as a free service to the public. The intent is to disseminate accurate, verified and science-based information on creating healthy home environments.
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