Balanced humidity inside a house is important not only for comfort but also for the house itself. When the humidity level—the amount of water vapor in the air—drops too low or rises too high, a variety of problems can develop.

Dry air draws moisture out of everything—from your eyes, lips, and skin to the carpets and furniture. The respiratory system, which relies on moisture to fight off viruses and bacteria, dries out, making you more susceptible to colds, coughs, and asthma. And bothersome static electricity increases. Too-high humidity in the summer is just plain uncomfortable.

During winter, the first signs of overly humid air usually appear when vapor hits the cold inner surfaces of windows and condenses there. Though this normally doesn’t present a serious problem, the same condensation may collect on the inner surfaces of exterior walls (unless the walls are fitted with a proper vapor retarder). This can rot the structure and ruin insulation. Peeling, blistering, or cracking paint—especially if siding is visibly bare in spots—usually indicates the presence of this condition. And, in addition to condensation problems, humid air encourages the growth of mold, mildew, and bacteria.

The most obvious way to reduce humidity is to use an air conditioner or, in a house that doesn’t have central air conditioning, a dehumidifier. Sometimes humidity can be reduced to an acceptable level simply by using kitchen and bathroom exhaust fans. Other methods include venting the clothes dryer outdoors, keeping showers short, and running only full loads in washing machines and dishwashers. You should also eliminate sources of leaks or standing water in the basement or crawlspace.

Tabletop and console humidifiers are relatively inexpensive and portable. Both are filled manually—usually on a daily basis. Tabletop units output from 2 to 4 gallons of water per 24 hours; consoles output 8 to 14 gallons. Some disperse steam or a cool mist; others blow air through a damp evaporative filter. Evaporative humidifiers are installed on forced-air furnace plenums.