Why Adequate Ventilation is Important
Adequate ventilation in your house is important for two reasons:
Moisture Control – Ventilation will prevent elevated moisture levels within the conditioned space during the heating season. These elevated levels can lead to condensation on window surfaces and give rise to surface mold and mildew, as well as concealed condensation within walls and roof spaces.
Avoiding Indoor Air Pollution – When natural ventilation has been sharply reduced, as in super-energy-efficient houses, it may be necessary to provide fresh air ventilation to avoid build-up of stale air and indoor air pollutants. Special air exchange units with heat-saving features are available for this purpose. The Home Ventilating Institute can give you more information about such heat-recovery ventilators.
A well-insulated attic should be adequately ventilated to prevent moisture accumulation. Attics may be ventilated with a combination of soffit vents at eaves and continuous ridge vents. Attic vents may also be installed in gable faces. Many codes and standards require one square foot of unobstructed ventilation opening for each 300 square feet of attic floor area if a vapor retarder is included in the top floor ceiling. Twice as much ventilation is recommended if there is no vapor retarder. The net free area of a vent is smaller than its overall dimension because part of the vent opening is blocked by meshes or louvers. The openings should be equally distributed between the soffit and ridge vents or between each gable face. Never cover or block vents with insulation. Take care to prevent loose-fill insulation from clogging vents by using baffles.
Whether or not to ventilate a crawl space has been a controversial issue. Most building codes presently require installation of vents to provide ventilation with outside air, but a recent symposium on crawl space design organized by the American Society of Heating, Refrigerating and Air Conditioning Engineers concluded that there is no compelling technical basis for crawl space ventilation requirements. However, if the crawl space is not ventilated, it is crucial that all of the crawl space ground area be covered with a durable vapor retarder, such as heavy-weight polyethylene film.