All homes need ventilation—the exchange of indoor air with outdoor air—to reduce indoor moisture, odors, and other pollutants. Contaminants, such as formaldehyde, volatile organic compounds (VOCs), and radon, that may cause health problems can accumulate in poorly ventilated homes. Inadequate ventilation prevents unpleasant odors from being removed. Excess moisture generated within the home needs to be removed before high humidity levels lead to physical damage to the home or mold growth.
Ventilation techniques and strategies have become more sophisticated as homes have become more complex. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE) requires that natural or natural plus mechanical ventilation provide a whole-house ventilation rate of 0.35 air changes per hour (ACH), but not less than 15 cubic feet per minute (cfm) per person.
Natural ventilation—uncontrolled air movement into a building through cracks and small holes (infiltration) and through vents such as windows and doors—is the traditional method of allowing fresh outdoor air to replace indoor air. Nowadays, because of central heating and cooling, as well as the desire for privacy, people tend to make little use of windows for ventilation, so infiltration has become the principal mode of natural ventilation in homes. Unfortunately, a home’s natural infiltration rate is unpredictable and uncontrollable, because it depends on the home’s airtightness, outdoor temperatures, wind, and other factors. Homes with low infiltration rates may lack sufficient ventilation for pollutant removal. Homes with high infiltration rates may experience high energy costs. Infiltration may also allow contaminated air to enter from a polluted area such as a garage or crawlspace, or may not ventilate the house uniformly.
Whole-house ventilation—use of one or more fans and duct systems to exhaust stale air and/or supply fresh air to the house—can better control the exchange of indoor air with outdoor air. Whole-house ventilation may be exhaust-only(relying on leakage into the building for fresh make-up air), supply-only (relying on air leakage from the building to exhaust stale air), or balanced systems that include both exhaust and fresh air intake components.
Spot ventilation—the use of localized exhaust fans (e.g., kitchen and bath fans) to quickly remove pollutants at their source as they are generated—is an important tool to improve air quality and ventilation effectiveness, whether natural or whole-house ventilation strategies are used. Spot ventilation strategies are often integrated with whole-house ventilation designs. In addition to its whole-house ventilation requirement, ASHRAE recommends intermittent or continuous ventilation rates for bathrooms and kitchens as alternatives to operable windows: 50 cfm (intermittent) or 20 cfm (continuous) for bathrooms and 100 or 25 cfm, respectively, for kitchens.
What About Noise?
Noise is a major reason people avoid using ventilation fans. When purchasing a ventilation fan, look for models that have the ENERGY STAR® label. To be ENERGY STAR®-qualified, a ventilation fan must meet stringent noise requirements of sound levels lower than 4.0 sones. A sone is an indication of loudness that measures the way a person perceives a sound. The lower the sone, the lower the level of noise. Small bathroom fans (less than 76 cfm) must have sound levels of no more than 2.0 sones to be labeled as ENERGY STAR® compliant, while large bathroom fans must have sound levels of no more than 1.5 sones. Bath fans with noise levels of 1 sone or less are also available and should be considered to eliminate noise problems. For more information, see Residential Ventilating Fan Sound Levels.