Indoor humidity

Humidity is usually given as relative humidity, a percentage. EPA advises keeping indoor humidity between 30 and 50 percent. To help prevent mold, EPA also says to keep it below 60 percent if possible, ideally between 30 and 50 percent. University of Minnesota Extension says 25% in winter to 50% in summer is generally considered healthy, comfortable and better for the house. When relative humidity is more than 50%, moisture problems may occur.

The sources differ a little at the edges, but they point the same way: drier in winter, never damp. A moisture or humidity gauge, sold at most hardware stores, shows whether your home is at a good level. The CDC notes that dew point is a better sign of trouble, and that ASHRAE recommends keeping the indoor dew point below 55°F.

Source: U.S. EPA · University of Minnesota Extension · CDC

Winter brings the opposite problem from summer. As the temperature goes down, air is able to hold less moisture. In cold winter weather, relative humidity needs to stay low to prevent condensation and damage to the house's structure, and to discourage mold. So in winter, aim for the low end of the range.

Humidifiers come in two broad kinds. Central humidifiers are built into heating and air conditioning systems and humidify the whole house. Some humidifiers have a built-in humidistat that can be set to the proper moisture level. EPA says not to humidify above 50 percent relative humidity, because higher levels may encourage the growth of biological organisms in the home. Another EPA guide says to set humidifiers to produce less than 60 percent, since above that condensation is likely, which can lead to mold.

Source: U.S. EPA · University of Minnesota Extension

A humidifier is only as clean as the water in it. Follow the maker's directions for cleaning console and furnace-mounted humidifiers, and if yours has a tank, do not let water stand in it for long and keep the water clean. Furnace humidifiers must be cleaned regularly to prevent mold and bacteria. Clean the humidifier as directed at the end of the season, or when it will not be used often.

Water and placement matter too. Water low in minerals helps prevent scale and keeps minerals from being spread into the air. Steam vaporizer and evaporative humidifiers are not expected to spread large amounts of minerals. Do not let the area around a humidifier become damp or wet, and if it does, turn the output down. If you have breathing symptoms you believe are tied to using your humidifier, EPA says to stop using it and contact your physician, even if you follow the care directions.

Source: U.S. EPA

Water on the inside of a window is a familiar sight in a cold month. When warm, humid air meets a cold surface, condensation may form. That is why drops of water form on the inside of a window in cold weather. Condensation on windows can be a sign of high indoor humidity, which has many possible causes and should be looked into. Other signs of too much moisture include mildew, mold, peeling paint and lots of dust mites.

There are a few things to try first. If water condenses on windows, walls or pictures, EPA suggests moving the humidifier, lowering its humidistat or using it less. When you see condensation on windows, walls or pipes, dry the wet surface quickly and cut back the source of moisture. University of Minnesota Extension suggests opening windows just a little for a time in winter, to let warm moist air out and cold dry air in. If the windows keep fogging, a technician can look at your humidifier and ventilation.

Source: U.S. EPA · University of Minnesota Extension

In summer the job flips to taking moisture out. One job of a heating and cooling system is to remove moisture from the air before sending it through the building. In summer, a properly sized air conditioner or a stand-alone dehumidifier can lower indoor humidity. To control a moisture problem, first find the source and remove it, and if you cannot, use ventilation or a dehumidifier.

Bigger is not better for drying the air. Oversized equipment will not run long enough to remove humidity from the air. A unit that is too big cycles on and off often, removing moisture poorly, especially where humidity is high. High airflow in cooling can also leave the air too damp. Even properly sized systems may not be able to control humidity in some cases. That is when a separate dehumidifier can help.

Source: U.S. EPA · University of Minnesota Extension · ENERGY STAR · U.S. Department of Energy · CDC

Basements need their own attention. Bathrooms, laundry rooms and basements are often or always damp, which makes them common places for mold. Water can get in by leaking from outside or seeping through basement floors. Seepage should be stopped quickly, and steps like a sump pump or regrading around the building should be considered to prevent future leaks. Slope the ground away from the foundation so it does not stay wet.

Inside, look at what adds moisture. Clothes dryers, stoves and other appliances that make moisture should vent outside where possible. Rust on a pipe is a sign that condensation forms on it, and that pipe should be insulated. Air conditioners and dehumidifiers can be used when needed. Air cleaners and filters do not fix the cause of mold and musty odors. If you already see mold, the mold page explains what to do next.

Source: U.S. EPA