Indoor Humidity Issues

Humidity is simply vaporized water in the air. Your breath contains hundreds of droplets of invisible water vapor. You can see them when you breathe on a pair of cold glasses.

The term most often used to define the amount of water vapor in the air is "relative humidity." Relative humidity is the percentage of water vapor in the air at a specific temperature, compared to the amount of water vapor the air is capable of holding at that temperature. Warm air holds more water vapor than cold air. When air at a certain temperature contains all the water vapor it can hold at that temperature, its relative humidity is 100 percent. If it contains only half the water vapor it is capable of holding at that temperature, the relative humidity is 50 percent.

If the outside air temperature in winter is 0°F and the relative humidity is 75 percent, that same air inside your 70°F home will have a four percent relative humidity. That’s dry! The Sahara Desert has an average relative humidity of 25 percent.

When air is saturated with water vapor, it has reached the dew point; at this point, water vapor condenses and produces visible water or "condensation." In winter it usually occurs first on windows. When warm, moist air comes in contact with a cold window, air temperature drops and it can no longer hold the water vapor; condensation results.

Desirable Humidity Levels

The human body is comfortable when relative humidity ranges between 20 and 60 percent. In your home, an average relative humidity of 35 to 40 percent is appropriate when the outside temperature is 20°F or above. However, during cold weather, higher humidity ranges may cause structural damage because of condensation on windows and on the inside of exterior walls. As outdoor temperatures fall, condensation problems inside may develop.

The construction of a home also influences how much humidity is desirable. Tightly constructed buildings with properly installed vapor barriers and tight fitting doors and windows retain more heat and moisture. This is where mechanical ventilation becomes important. If a home does not have the proper mechanical ventilation, excess water vapor can move through walls and ceilings, causing wet insulation, peeling paint, and mold on walls and woodwork

The following table shows recommended indoor humidity levels in relation to outdoor temperatures.

How to Gauge Indoor Humidity

  • Drop three ice cubes into a glass, add water and stir. Wait three minutes. If moisture does not form on the outside of the glass, the air is too dry; you may need a humidifier. (Do not perform this test in the kitchen, because cooking vapors may produce inaccurate results.)
  • Frequent fogging of windows may indicate too much humidity. The appropriate relative humidity will allow only slight condensation along the lower edges or corners of windows. More condensation could be damaging.
  • Moisture buildup or mold on closet walls or room ceilings and walls indicates high humidity.

If humidity is too low

Low humidity causes static electricity, dry skin and hair, and itching and chapping. Mucous membranes in nose and throat dry out, increasing your discomfort and susceptibility to colds and respiratory illness. With low humidity levels, body moisture evaporates so quickly that you feel chilled even at higher thermostat settings. Adding a humidifier to your home will remedy these problems. There are three standard types from which to choose.


  • Moisture evaporates into the air from a pan or absorbent plates partially immersed in water and attached to the sides of a radiator or to a warm air heating system; limited humidification capacities.

Portable or room

  • Air circulates through a wet pad or a very fine mist of water is discharged into the room. Water must be manually added regularly; unit may be moved from room to room as desired.


  • Moisture is introduced directly into the air stream circulating through the furnace; water is fed automatically into the unit by a connection to the house water system. A humidistat (humidity control) should be located near the furnace thermostat or in the return side of the duct system. The unit may be built in or attached to a forced air heating system; greatest humidification capacity.

If humidity is too high

High humidity levels produce constantly fogged windows, musty odor and/or a clammy feel to the air. During cold weather, condensation in the lower corners and edges on prime (inside) windows is common. Excessive condensation on prime windows indicates loose storm windows; seal storm windows by caulking and weatherstripping. Condensation on inside surface of storm windows indicates loose prime windows; seal prime windows, also by caulking and weatherstripping.

If you maintain high humidity, expect to have more window condensation. Remember that structural damage may result from extended periods of high humidity.

Exc essive humidity can be temporary. During summer, outdoor humidity is high; your house and its furnishings naturally absorb some water vapor. In fall, when temperatures drop and the air becomes drier, this water vapor is released into the living space and condensation on windows may occur. The situation normally subsides within a short time.

Often the principal source of higher humidity in a home is your family’s living habits

  • One person’s breathing produces 1/4 cup of water per hour.
  • Cooking for a family of four produces approximately five pints of water in 24 hours.
  • Showering puts 1/2 pint of water into the air.
  • Bathing puts 1/8 pint of water into the air.

Adding only four to six pints of water to the air raises the relative humidity in a 1,000 square foot home from 15 to 60 percent, assuming the temperature is constant.

Try these steps to lower humidity in your home

  • Turn down or stop using humidifier.
  • Use range and bathroom exhaust fans while cooking and bathing or open a window for a few minutes to bring in cool, drier air.
  • Cook with pans covered.
  • Take shorter showers with cooler water.
  • Install a fresh air intake duct. Outside air introduced into the home lowers the humidity level.
  • Reduce the number of plants in your home or water them less; they release water vapor into the environment.
  • Vent clothes dryer to the outside.
  • In tightly insulated homes, consider installing an air-to-air heat exchanger.
  • In summer, use a dehumidifier.

A whole-house dehumidifier, can remove 70-150 pints of water per day. It operates on its own, or in tandem with your central air conditioner, to reduce mold and mildew, improves indoor air quality, extends the life of your central air conditioner and help control your energy bills. The drier air provides greater comfort at higher temperatures, so homeowners can raise the setting on their central air conditioners, reducing their energy use.

If these steps do not correct the problem, have appliances checked. A malfunctioning appliance can add water vapor to the atmosphere of your home.