Division of Energy
Residential Energy Efficiency - Moisture Control
The word "moisture" refers to water vapor mixed with air. Most of the moisture generated in the home is dissipated by the movement of moisture-laden air out of the home. As homes become more energy-efficient, the number of paths of escape are reduced, and dealing with moisture becomes more important.
How Moisture Acts in Your Home
Moisture in your home is not necessarily harmful because the humidity of a home affects your comfort. For example, most people will feel cooler in a room at 75oF and 25 percent relative humidity than in a room at the same temperature with 40 percent relative humidity. It follows then, that in the room with the higher relative humidity level, the occupant will be less likely to raise the thermostat setting in winter because he or she will feel warmer, thus there will be savings on the heating bill.
Excessive humidity can contribute to a large number of problems ranging from serious building damage to extreme discomfort in hot weather. Building specialists and homeowners need a thorough understanding of the effect of moisture on the home in order to successfully correct or avoid many problems.
High levels of humidity are often the result of too much moisture vapor generated indoors, usually by bathing, cleaning, cooking and water evaporation and emission. If high moisture levels are a problem, they can be reduced by installing (and using) ventilation fans in bathrooms and laundry rooms, covering exposed earth in a crawl space with a vapor barrier, installing downspouts that flow away from the foundation, and, if possible, sloping the grade away from the house. House plants and pilot lights also add moisture to a home.
During the heating season, the indoor humidity level should hover around 30 percent to 40 percent. One symptom of a high humidity level is condensation forming on cold surfaces.
A more common winter humidity problem is the too-dry home. A house that is dry will seem colder, and static electric shocks occur. Dryness is a symptom of excessive air infiltration.
During the summer, indoor humidity can be controlled by an air conditioner or a dehumidifier.
Homes that are characterized by one or more of the following conditions are more likely to experience excessive moisture accumulation:
- Less than 800 square feet of total living area.
- Less than 250 square feet of living area per occupant.
- Tight wall and ceiling construction and weatherstripping on windows and doors (low level of infiltration).
- Heating systems which use outside combustion air.
- Low-sloped roofs or unventilated attics.
- Cracked heat exchanger in gas space-heating equipment.
- Electrically heated home.
- Unvented appliances.
- Excessive use of a humidifier.
If moisture problems exist, you may have to increase ventilation in your home by using such items as exhaust fans or air-to-air heat exchangers. Please note that these options use energy to operate. So, in terms of conserving energy, it is wiser to try to reduce the source of humidity.
Condensation problems may indicate that your windows are faulty or that your indoor humidity is too high. Condensation will occur whenever the window surface is cool enough to allow moisture in the air to condense on it, which is why some condensation can be expected in the winter - although condensation should be controlled as much as possible since it can damage the window's components. Moisture on the inside of the storm window (or outside pane) indicates that the prime window is allowing air and moisture to leak out to the storm window where it condenses. Stopping these air leaks with caulk and weatherstripping will stop the condensation and ultimately save your window. It is also important to understand that too little humidity is bad for your house. Manufacturers claiming that low humidity (15 percent) is best for windows may be covering for a poor quality product. Good windows should not have excessive condensation at normal humidity levels (30 percent to 35 percent).
Steps to Reduce Excessive Humidity
Recognize that the best way to stop condensation is to reduce the moisture in the inside air. Eliminate or reduce any sources of moisture in your home that you can control.
- Be sure that attic or crawl spaces are properly ventilated. Add a vapor barrier to cover the earth in the crawl space.
- If you have single-pane windows, install storm panels, double-pane insulating glass or triple-glazed panels.
- In winter, provide more controlled ways for moist inside air to get out. Run kitchen and bathroom ventilating fans during periods of moisture production.
- Vent your clothes dryer to the outside.
- Address water problems such as seepage from the outside, leaks in a roof or walls, and leaky pipes.
Be Careful When Installing Insulation
Excessive moisture in the home filters through insulation and causes it to become damp and matted and to lose much of its effectiveness.
To prevent or reduce condensation problems, the side of the insulation exposed to high vapor pressure (warm side in winter) may be covered with material that will impede the natural drive of moisture to flow through the inside surfaces of exterior walls, toward the lower vapor pressure outside. To be effective, such a material must have a high resistance to moisture flow. The material is usually called "vapor retarder."
- Choose wood or vinyl-sheathed wood rather than metal for windows and gliding doors. This will reduce the likelihood of condensation on the frame and sash. Wood is a better insulator than metal. Metal frames without thermal breaks conduct heat readily, so the inside surface of a metal window frame is cold in the winter. When humid air comes into contact with cold metal, condensation, and often freezing, occurs.
- Make sure the attic and crawl space are cross-ventilated. The crawl space should be covered with a vapor barrier. This will prevent water vapor from rising from the soil into your house.
- Make sure your clothes dryer and all gas appliances are vented to the outside. Water vapor is one of the products of gas combustion.
- If you have a basement, take the necessary steps to prevent leakage of soil moisture into the basement. These steps will vary with soil and drainage conditions on your lot.