How much attic ventilation does your house need? Here in the Pacific Northwest, a certain amount of outside air can benefit a roof with asphalt shingles. Ventilation that allows air to circulate can reduce the excess moisture that may lead to mold, mildew or other types of damage.
With the moist climate in Washington and Oregon, ventilation can limit or prevent condensation beneath the roof. Moisture damages roof sheathing and may lead to rot in your home’s structure. Asphalt shingle manufacturers’ warranties often require minimum ventilation standards.
Ventilation does more than prevent moisture accumulation. It can also reduce interior attic temperatures in warm weather. This can make a difference in how hot a house gets during summer.
Ventilation doesn’t have a significant effect on the temperature of a roof’s surface. Research indicates that other factors have much greater bearing on exterior temperatures. For instance, the color of a roof makes a difference in absorption of solar heat. Light-colored roofing materials reflect sunlight. The darker a roof, the more it heats up.
Types of Ventilation
There are two main methods of attic ventilation. The most common method, passive ventilation, is based on natural air convection due to the rising of hot air. Intake vents installed low in the attic allow cooler air to replace warmer air. Warm air is expelled through vents situated higher in the attic.
This type of ventilation works best with what is commonly called “balanced ventilation.” With balanced ventilation, the size of the lower vents at the soffits or eaves is about equal to the size of the higher vents at the top of the attic.
The use of wind-assisted exhaust vents can promote passive ventilation. As long as you have adequate air intake, these types of vents can significantly increase the amount of air exhausted from your attic. The exhaust vents take advantage of the wind outside to induce a drop in pressure across the vents. This forces warm air up and out.
The second method, powered or forced ventilation, uses fans to pull up the hot air at the ceiling and expel it through the roof. As with passive ventilation, the intake vents have to be adequate to the task of allowing in enough air to offset the air that is expelled.
Powered vents shouldn’t be used with passive vents. Ideally, a home should have one type or another.