An attic must be ventilated to allow fresh air to circulate through the space. With most structures, attic ventilation requirements include both intake and exhaust vents be installed in the attic. Intake attic ventilation requirements include gable and soffit vents, of which there are several styles. Exhaust attic ventilation requirements can range from static vents that simply allow the air to escape as it flows past the vent to electric, fan-driven vents that actually pull the air up and out of the attic.
An unvented attic can suffer from heating and cooling cycles that create condensation. The condensation cannot only erode any attic insulation that may become damp, it can cause the buildup of toxic mold. Most building codes call for particular attic ventilation requirements that are intended to allow fresh air to circulate throughout the attic, thereby eliminating the condensation. Code mandates for particular attic ventilation requirements depend on space and design. Other requirements are based on whether the building is residential or commercial, the type of roof on the building and the building's location.
While the gable vent is one of the more common styles, it is also one of the least effective in removing moisture from an attic. Typical code requirements stipulate attic ventilation requirements be sufficient to completely remove the air in an attic every six minutes during summer months. The gable vent removes only the air near the gables, and does nothing to flush the attic with fresh air. Likewise, static vents are dependent upon number and location as to their effectiveness. Soffit vents can be either round or rectangular, and are typically covered with screen on the exterior to help keep insects and birds out of the attic.
Many new home builders agree that the ridge vent is one of the best at meeting attic ventilation requirements in a peaked roof-style attic. The vent is placed on the peak of the roof, in place of the ridge shingles. The ridge vent is an aluminum cap that draws fresh air in through the soffit vents and expels the air through the ridge of the attic. Power vents can be very effective, however, the air-powered vents work only when the wind is blowing and the electric versions may cost as much to operate as they are potentially saving. As with any object fastened to a building's roof, the potential to leak exists and is an area of concern with the installation of any ventilation system.