VENTILATION

VENTILATION

 All houses need ventilation to remove stale interior air and excessive moisture and to provide oxygen for the inhabitants. There has been considerable concern recently about how much ventilation is required to maintain the quality of air in homes. While it is difficult to gauge the severity of indoor air quality problems, building science experts and most indoor air quality specialists agree that the solution is not to build an inefficient, “leaky” home.

Research studies show that standard houses are as likely to have indoor air quality problems as energy efficient ones. While opening and closing windows offers one way to control outside air for ventilation, this strategy is rarely useful on a regular, year-round basis. Most building researchers believe that no house is so leaky that the occupants can be relieved of concerns about indoor air quality. The researchers recommend mechanical ventilation systems for all houses.

For example, consider a 2,000 square foot home, with 3 bedrooms, and assume an occupancy of 4 people. The amount of ventilation would be 50 cfm:

7.5 cfm x (3 + 1) + 1% x 2,000 = 30 cfm + 20 cfm = 50 cfm

Increasing the number of occupants or increasing the square footage of the home would increase the necessary ventilation requirements.

Air leaks are unpredictable, and leakage rates for all houses vary. For example, air leakage is greater during cold, windy periods and can be quite low during hot weather. Thus, pollutants may accumulate during periods of calm weather even in drafty houses. These homes will also have many days when excessive infiltration provides too much ventilation, causing discomfort, high energy bills, and possible deterioration of the building envelope.

Concerns about indoor air quality are leading more and more homeowners to install controlled ventilation systems for providing a reliable source of fresh air. The simplest approach is to provide spot ventilation of bathrooms and kitchens to control moisture (see Figure 7-10). Nearly all exhaust fans in standard construction are ineffective—a prime contributor to interior moisture problems in homes. Bath and kitchen exhaust fans should vent to the outside, not just into an attic or crawl space. General guidelines call for providing a minimum of 50 cubic feet per minute (cfm) of air flow for baths and 100 cfm for kitchens. Manufacturers should supply a cubic feet per minute (cfm) rating for any exhaust fan.

The cubic feet per minute rating typically assumes the fan is working against an air pressure resistance of 0.1 inch of water column—the resistance provided by about 15 feet of straight, smooth metal duct. In practice, most fans are vented with flexible duct that provides much more resistance. Most fans are also rated at pressures of 0.25 to 0.30 inches of water column—the resistance found in most installations.

Many ceiling- or wall-mounted exhaust fans can be adapted as “in-line” blowers located outside of the living area, such as in an attic or basement. Manufacturers also offer in-line fans to vent a single bath or kitchen, or multiple rooms. Distancing the in-line fan, Figure 7-11, from the living area lessens noise problems.

While improving spot ventilation will certainly help control moisture problems, it may not provide adequate ventilation for the entire home. A whole house ventilation system can exhaust air from the kitchen, all baths, the main living area, and bedrooms.

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