Poor ventilation causes many problems, not the least of which is substantial reduction in the lifespan of asphalt-based roof coverings.
General Venting Requirements
Roofs with unconditioned attic spaces require some type of ventilation.
All attic spaces require ventilation at a minimum rate of 1 square foot of venting per 150 square feet of attic area. This may be reduced to 1 square foot of ventilation per 300 square feet of attic space where most of the vents are high on the roof and air flow is induced from a lower point, as is the case with ridge and soffit vents.
The "1-in-300 Rule" may also apply where a vapor barrier is installed on the warm side of the ceiling.
These basic rules apply both to traditional attic spaces and to enclosed areas where the ceiling material is applied directly to the underside of the roof rafters, as one would find with a cathedral ceiling.
The primary reason for these requirements is to allow moisture-laden air to be evacuated from the attic space, and also to attempt to balance the temperature of the roof coverings and sheathing with that of the outside air.
There are many methods employed to achieve adequate venting, among them:
gable vents, which are screened openings in the gable ends, allowing cross-ventilation;
turbine vents, which are wind-powered vents that promote air flow out of the roof area;
passive vents, which are used to provide some air flow between the sheathing and ceiling areas on flat or low-slope roofs;
soffit and ridge vents, which are installed so air can be drawn from cooler air at the soffit and exhausted through the ridge vents. This style is the most common in new construction and is generally considered to be the most efficient;
powered vents use a thermostat or a switch in the attic space to energize the fan when the attic air reaches a pre-set temperature; and
combination venting, which refers to employing two or more of these methods described, and, in some areas, using through-the-roof vents installed a few feet below the ridge line. Complements of InterNachi, www.