Occupants of households are usually very comfortable when the temperature and relative humidity are maintained within the ranges of 68 to 72 degrees and 25 to 50 percent relative humidity. Maintaining a proper humidity level isn’t always easy.

Normal household activities such as cooking, cleaning, bathing, washing clothes and dishes, drying clothes, breathing and perspiring can raise the humidity level too high. To avoid the problems of excess moisture it is necessary to limit or control the amount of water vapor in the house. This can be accomplished by modifying lifestyle habits and by using mechanical means such as exhaust fans, dehumidifiers, and air-to-air heat exchangers.

Exhaust Fans

Exhaust fans in baths and kitchens will help eliminate moisture before it spreads throughout the house. Fans should be selected for the particular job needed. The fan capacity is measured in the numbers of cubic feet of air it will move per minute–CFMs.


If the moisture problem is confined to one area such as a basement or unvented storage area, or if the relative humidity inside the home in the summer often reaches or exceeds 60 percent, a dehumidifier can keep these areas dry and free of mildew and odor.

The capacity of a dehumidifier is expressed in pints of water condensed in 24 hours at 80 degrees and 60 percent relative humidity. Individual models have features such as an automatic adjustable humistat, an automatic shutoff and a signal light to indicate a full drip pan.

Air-to-Air Heat Exchangers

Air-to-air heat exchangers are sometimes used in tightly-constructed homes to lower humidity levels and supply fresh air. One fan forces warm moist air out of the home, while another fan brings in cold, dry air from outside. The air being moved passes through the heat exchangers. Here, the warm air heats up the cold air entering the house. The units usually run continuously, or are controlled by a time clock. Air-to-air heat exchangers recover approximately 70 percent of the heat leaving the home, and reduce both heating costs and cold air drafts.

Landscape Planting

Landscape plants should not block free air flow through the crawl space vents. Plants should be placed beyond the drip edge of the roof, and foliage should be at least 5 feet from the foundation. Finished planting beds and mulches should be lower than the ground level in the crawl space and should slope away from the house. Keep any organic mulch or ground cover at least 12 inches away from the foundation.