Sweaty windows, peeling wallpaper, mold-covered walls and musty odors are symptoms of a poorly ventilated home. The clothes washer, dryer, shower, wash basin, and stove, among others, produce water vapor. It is an excess of this water vapor that causes problems.

The most effective method of dealing with excessive moisture in the home is to properly exhaust it to the exterior via an automatic exhaust fan. These fans come in various shapes, sizes and styles.

If you're like many other people, you're constantly fighting the mildew battle. This might have something to do with those notoriously long, hot showers teen-agers in the family enjoy. ( Question When does a teen-ager know that a shower is over? Answer When there's no more hot water.) Sound familiar?

The high concentration of water vapor produced by a shower will not only produce an environment in which mildew prospers, it will peel wallpaper and paint, damage wallboard and even cause framing members to rot.

Building codes in many parts of the country require only a vent fan in a bathroom which does not contain an operable window. This fan must fully exchange the air five times in the course of an hour. We believe that an exhaust fan in the bathroom is a must, window or not.

There are several methods of installation and operation that will produce maximum fan efficiency. Whenever possible, an exhaust fan should be installed at the high point of the ceiling. This applies to rooms with ceilings that are vaulted or contain soffits or drop ceilings. Also, the fan should be centrally located and in close proximity to the shower which is the single greatest source of water vapor in the bathroom.

Having a fan is only part of the solution. Properly exhausting it is a major part of its successful operation. Most bath fans consist of a metal housing with a dampered exhaust port. Rigid or flexible plastic or metal pipe should attach to the exhaust port and terminate at a jack located on the roof or, in some cases, at an exterior wall. The ducting should be secured to the housing and jack with at least one screw and thoroughly wrapped with duct tape. An exhaust fan should never be discharged into an attic or crawl space. This could result in major damage in these areas.

You might already have an exhaust fan in your bathroom, and find that it isn't doing an adequate job. This could be due to a variety of things. One of the most common reasons is lack of use. The fan is there, but no one uses it. Often the fan is switched independent of lighting for the space and is unknowingly not switched on, or sometimes purposely is ignored because of the noise it makes. One way to ensure that the fan is used is to tie the electrical for the fan and the lighting together. This can be done at the switch or in attic space. If you are not familiar with electrical work, hire a professional electrician.

Even when the fan is operating properly it still might not be strong enough to adequately exhaust all of the water vapor. This has to do with the fan's power. Fans are rated by the cubic feet per minute (cfm). For example, a 50-cfm fan will move 50 cubic feet of air per minute. An 80-cfm fan is stronger than one rated at 70 cfm. To determine the size of the fan needed for your bathroom, the Home Ventilating Institute suggests the following formula. Take the area of the bathroom and multiply it by 1.1 (assuming an 8-foot ceiling). For example, a 6-foot by 8-foot bathroom would require a fan with a rating of 53 cfm (6 x 8 x 1.1 = 52.8). Remember, this is the minimum recommended size. It never hurts to spend the few extra dollars for a more powerful model. Installation work and cost are generally the same.

One objection to bath fans is the amount of noise they make. Just as fans are rated for capacity of air they move, they are also rated for sound level. The sound-level rating is expressed in "sones". If your fan is too loud you can replace it with a more quiet, lower sone model. Low sound level fans will emit approximately two to three sones, while less quiet models will emit more than five.

Another factor that will influence fan performance is cleaning and maintenance. Periodically vacuum the fan blades or turbine and housing. A drop of machine oil at the aperture will keep it running smoothly.

Good household ventilation means ducting the clothes dryer to the exterior, and exhaust fans in all bathrooms, the kitchen and the laundry.