During the winter, indoor humidity should remain below 40 percent, according to the University of Minnesota, and as outdoor temperature drops, the humidity level should drop, as well. Too much humidity can cause condensation, wet spots on ceilings and walls, and health problems caused by dust mites and mold growth. Too little humidity can cause static electricity, damage to wood floors and furniture, and health problems, such as sneezing, dry noses and an increase in respiratory illness. Adding humidity to a dry house is fairly straightforward, but reducing humidity is a bit more challenging.
Inspect the home to find sources of excess humidity. Furnaces and water heaters, as well as clothing dryers, should have a vent that leads to outside air. Exposed soil in crawl spaces may also cause excess humidity. Lay a moisture barrier fabric over exposed soil or consider sealing it. Frequent showers and many houseplants also add excess humidity to the air.
Adjust the humidistat if you have one. As outdoor temperatures fall, indoor humidity levels should decrease, as well, to prevent condensation and mold growth. If you notice condensation, or tiny beads of moisture, on windows and mirrors, turn the humidistat down. If the air feels dry or your wood floors separate slightly, turn the humidistat up.
Run a humidifier if you lack a humidistat and the humidity seems too low. Use stand-alone, cool mist humidifiers or whole house humidifiers. Clean cool mist humidifiers regularly and don't allow water to stand in the bottom of them, because the humidifiers can put harmful microorganisms or bacteria into the air when not properly maintained.
Run a dehumidifier to remove excess humidity. These machines are more effective during warm weather than during cold weather, but may help somewhat, according to Iowa State University's extension service.
Run exhaust fans in the bathroom when you shower or bathe, and kitchen fans when you cook.