Venting an Attic
A well-ventilated attic will maintain a temperature near that of the air outside. During the winter, cold attic air prevents ice damming. In the winter a well-ventilated attic will often have a thick blanket of unmelted snow on its roof, while a nearby roof of the same pitch with inadequate ventilation will display bare spots. Of course thick attic-floor insulation also helps keep the attic from heating up. In the summer proper ventilation keeps an attic from overheating, which can cause serious damage, especially if the hot air is humid. Poorly ventilated attics can reach temperatures of 150 degrees F.
Homes built prior to the 1980s were often built with what is today considered inadequate ventilation. If you have an older home, it may make sense to upgrade the ventilation when you reshingle the roof.
Ventilation means flow-through of air. In a typical arrangement air flows up through soffit vents under the eaves and out through a ridge vent at the peak or through roof vents near the peak. Some older homes have a cupola vent on the roof's peak. In another arrangement gable vents located near the peaks of rake ends pull air through the attic.
Of course air must flow freely. If a soffit vent gets clogged with insulation, it will not distribute air. Some turbine-style roof vents spin with the upflow of warm air flowing through them; if the spinning mechanism gets stuck, they will be less efficient.
Check with your building department or roofing supplier to find out how much ventilation is considered adequate in your locale. Typically if your attic floor insulation has a vapor barrier (look between floor joists for plastic sheeting covering the ceiling below), you should have 1 square foot of soffit vent opening and 1 square foot of ridge or roof vent opening for every 300 square feet of attic floor area. If the insulation is blown in, with no vapor barrier, double the ventilation to 1 square foot for every 150 square attic feet. In a finished attic with insulation pushed against the sheathing, you may need to add rigid baffles to allow air to move behind the insulation.
For a roof vent you can choose either ridge vents (roll-type or rigid) that are covered by ridge roofing or those that have a visible metal cap. Roof vents may be square cap roof vents (skylight types help illumine the attic) or they may be turbines.
Gable vents have louvers to keep out most of the rain; they may not be a good idea on a wall that receives driving rain. Power vents, for either the roof or a gable, add extra venting power when the attic heats up. Some are solar-powered, sparing you the chore of running power to the unit.
Under-the-soffit vents include individual rectangular or circular vents, and a long continuous vent.