May, 2005
Inspection News and Views from the American Society of Home Inspectors

Moisture Control in Homes


Part 1 of 3

Two basic types of moisture problems may exist in the home: insufficient or excess moisture. Insufficient moisture in the air can cause dry nasal passages, increased respiratory problems and excessive static electricity in clothing and carpets. Humidifiers or vaporizers will add moisture to the air when it's too dry.
Excessive moisture in the air is a far more complex problem, but one that can be solved. Excessive humidity can cause a number of undesirable conditions-some obvious, but others not always evident or visible. Some indications of excessive moisture in the home are:

  • Condensation, frost or ice on the inside surface of windows.
  • Damp spots on ceilings or inner surfaces of exterior walls.
  • Mold or mildew growth on walls and ceilings.
  • Peeling or blistering of exterior paint.
  • Ice or frost on the underside of roof sheathing in the attic space.
  • Moisture on basement walls and floors.
  • Sweating water pipes.

Air-water vapor properties

In order to solve excess moisture problems, you must first understand the properties of air-water vapor mixtures.

Air is a mixture of invisible gases-dry air and water vapor. Each exerts a separate pressure. The water vapor pressure controls moisture movement through walls, windows or ceilings of homes. The dry air pressure and water vapor pressure together will determine air movement through leaks and openings in the home. A higher pressure will always seek a lower pressure-somewhat like air escaping from a balloon.

The vapor pressure increases as the air's moisture content increases. The amount of moisture that a given volume of air can hold depends on the temperature of that air; the warmer the air, the more moisture it can hold.

Air's moisture content is normally given in terms of relative humidity. Relative humidity is the amount of moisture it could hold at that temperature. Therefore, 50 percent relative humidity means the air is holding half the amount of water it could hold at that temperature. This method of indicating moisture content in the air is misleading because it doesn't give a true picture of the actual amount of water vapor in the air. As an example, air at 35 degrees and 75 percent relative humidity contains about half as much moisture as air at 85 degrees and 25 percent relative humidity.

As a given quantity of air is cooled, the relative humidity of this air increases. If the air is cooled sufficiently, it will reach 100 percent relative humidity. The air is then said to be saturated. The temperature at which saturation is reached is known as the dew point temperature.

Condensation will begin to appear at this temperature. This is why air coming in contact with a colder surface, such as a glass of ice water or cold window, will deposit water droplets on this surface.

Moisture balance

Four factors dictate whether a home's moisture balance will become uneven enough to cause problems. The four balancing factors are source strength, temperature, moisture transfer rate and circulation-ventilation rate. In a home without moisture problems, these forces are typically in balance. These factors are critical to understanding and solving home moisture problems.

Source strength is often the most important factor because moisture problems cannot exist without sources of moisture. Controlling the source of a moisture problem is usually the most productive and cost-effective approach to solving the problem. Examples of indoor source reduction solutions include: fixing plumbing leaks, reducing moisture from domestic activities and reducing the use of a humidifier. Outdoor source reduction solutions include improving drain-age, fixing leaks, and being aware of soaking and puddling from lawn sprinklers. If sources can't be reasonably or affordably controlled, then it is time to try another route.

Temperature differences which promote unwanted condensation should be remedied. Temperature solutions include bringing warm air to cold surfaces through improved heating patterns, insulating surfaces against cold temperatures, installing vapor barriers, and simply being aware of temperature differences and not allowing warm, moist air to contact cooler surfaces such as walls, basements or crawl spaces.

The moisture transfer rate in a home can be altered in several ways. These moisture transfer solutions include sealing air leaks from inside the home, weatherizing before insulating, using vapor barriers in crawl space areas, stopping all exterior leaks and puddling of water, and increasing the moisture resistance of exterior wood.

The circulation-ventilation rate of a home can be adjusted to help solve moisture problems. Venting the moisture out of enclosed areas is the usual solution. Increased circulation and ventilation are also backup options when internal moisture sources cannot be reduced sufficiently. Circulation and ventilation solutions include installing properly sized vents located to promote circulation, using materials that can breathe on the cool side of moisture-resistant surfaces, using spot ventilation in high-moisture areas such as baths and kitchens, using small, efficient fans to move internal air through a house, or using air-to-air heat exchangers to reduce moisture, but keep heat or coolness.

Controlling household humidity

Occupants of households are usually very comfortable when the temperature and relative humidity are maintained within the ranges of 68 to 72 degrees and 25 to 50 percent relative humidity. Maintaining a proper humidity level isn't always easy.

Moisture3.gifNormal household activities such as cooking, cleaning, bathing, washing clothes and dishes, drying clothes, breathing and perspiring can raise the humidity level too high. It has been estimated that the typical family of four converts three gallons of water into water vapor per day. It takes only four to six pints of water to raise the relative humidity of a 1,000-sq.-ft. house from 15 to 60 percent. To avoid the problems of excess moisture, it is necessary to limit or control the amount of water vapor in the house. This can be accomplished by modifying lifestyle habits and by using mechanical means such as exhaust fans, dehumidifiers, and air-to-air heat exchangers.

Reduce moisture vapor production within the house by: 1) decreasing bath time, 2) not boiling water or liquids excessively when cooking, 3) washing only full loads of clothes, 4) using floor coverings that do not require wet mopping, 5) venting clothes dryers to the outside, and 6) opening windows to allow moisture to escape.

Exhaust Fans
Exhaust fans in baths and kitchens will help eliminate moisture before it spreads throughout the house. Fans should be selected for the particular job needed. The fan capacity is measured in the numbers of cubic feet of air it will move per minute-CFMs. Determine the fan size in CFMs needed to do a particular job with this formula:


Crawl spaces and basements need a minimum of 10 air changes per hour. Kitchens require a minimum of 10 to 15 air changes per hour. Bathrooms require a minimum of eight air changes per hour. A hood over a range on a wall should be rated at 40 CFM per linear foot of range top, while one placed over an island would require 50 CFM per linear foot. Attic fans may also be installed to force ventilation. Sizing attic fans is by the CFM formula, with six to eight changes per hour for ventilation.

If the moisture problem is confined to one area such as a basement or unvented storage area, or if the relative humidity inside the home in the summer often reaches or exceeds 60 percent, a dehumidifier can keep these areas dry and free of mildew and odor.

The capacity of a dehumidifier is expressed in pints of water condensed in 24 hours at 80 degrees and 60 percent relative humidity. Individual models have features such as an automatic adjustable humistat, an automatic shutoff and a signal light to indicate a full drip pan.


Air-to-air heat exchangers are sometimes used in tightly constructed homes to lower humidity levels and supply fresh air. One fan forces warm, moist air out of the Moisture2.gifhome, while another fan brings in cold, dry air from outside. The air being moved passes through the heat exchangers. Here, the warm air heats up the cold air entering the house. The units usually run continuously, or are controlled by a time clock. Air-to-air heat exhchangers recover approximately 70 percent of the heat leaving the home, and reduce both heating costs and cold air drafts.
Parts 2 & 3 of this article will follow in the June and July issues of the Reporter.

Reprinted with permission from the University of Georgia, College of Family and Consumer Services

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