Humidity Control

Moisture Control in Homes
Dale Dorman, MS
Extension Housing & Environment Specialist,
Department of Housing and Consumer Economics

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.

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 can not 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 drainage, 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 back-up options when internal moisture sources can not 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.

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.
Normal 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.

humidity control

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:

number of

(crawl space) desired

(room) air changes

Cubic Feet(attic) X per hour



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.

Table 1.
(Pints Water Removed in 24 Hours)

Condition Without Dehumidification Room Area (sq. ft)
500 1000 1500 2000
Moderately damp– Space feels damp and has musty odor only in humid weather 10 14 18 22
Very Damp–Space always feels damp and has musty odor 12 17 22 27
Wet–Space feels and smells wet. Walls or floors sweat, or seepage is present. 14 20 26 32

*Recommendations by the Association of Home Appliance Manufacturers.

Air-to-Air Heat Exchangers

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 home, 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

heat exchanger.