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Moisture control is an important part of pest management. All living creatures on Earth require three things to live; Food, Water, and Shelter. Moisture can provide enough water for many insects and moisture may also soften wood or other objects to help insects obtain shelter. Therefore, the reduction of moisture should always be a major part of a pest management program. Most of these procedures are to decrease the moisture in the air. This will cause the moisture in the surrounding wood to be reduced as well.
There are several areas that are especially prone to moisture penetration. The area of largest concern is the crawl space. A crawl space is an area located under a structure. Cement crawl spaces are low areas, usually less than four feet high, with cement covering the floor (usually called a "rat slab"). Dirt crawl spaces, however, are areas with no cement floor covering. Dirt crawl spaces do not have to be low-lying areas. Some people might call a basement without a cement floor a basement, but for pest management purposes, this, no matter how high the floor joists, is still considered a dirt crawl space.
Dirt crawl spaces can be the greatest source of moisture entering a structure. They may allow up to 20 gallons of water per square foot, in the form of water vapor, to enter the structure. This kind of moisture can lead to all kinds of insect, and other arthropod, infestations.

There are several ways to eliminate this:
1. Cover the dirt floor. This can done by putting a "rat slab", or cement floor, inside the crawl space. However, if this is not an option, because of expense or accessibility, then an alternative is to lie 3 - 5 mil thick plastic sheathing across the floor. Be sure to cover the entire floor and put weight along the edges. If more than one sheet is used, be sure to overlap the ends by at least a foot, and either tape with duct tape or weigh this down. An added benefit to the plastic is that it will be easier and cleaner to crawl around down there if you need to.
2. Take care of any water leaks that may exist. These water leaks may come from your plumbing or may be coming from the exterior. Either one, even if it only happens when it rains, can be a major source of moisture and should be taken care of.
3. Ventilation is a key component of moisture control. Vents should be installed to provide adequate ventilation. Usually this means at least 3 - 4 vents for every 150 square feet or so, with a minimum of 4. The vents should, however, be the kind that can be closed for the winter, in colder climates. The best type of vents are automatic vents that automatically open when the temperature goes above 70 degrees and close when the temperature goes below 62 degrees.
4. Another important consideration is to use dehumidifiers. This will decrease the moisture dramatically in the air.

Cement Crawl spaces and basements should be looked at the same way with respect to moisture control. With the exception of the putting down of plastic or rat slab on the floor, all other moisture control steps should be pursued. In a finished basement, the use of vents may not be aesthetically appropriate. Therefore, using more than one dehumidifier may be the best approach.

Within the living areas of the house, moisture tends not to be a problem because of the central heating that most houses have today. The central heating tends to dry out the house. However, if there is a problem, dehumidifiers will probably be sufficient.
The attic can also provide a source of moisture. This moisture may not only lead to insect infestation, but may also cause damage to the roofing of the structure. Therefore, moisture control in the attic is as important as in the crawl space. The primary cause of moisture problems in the attic is inadequate ventilation. The rule of thumb is the more ventilation, the better. This can be achieved by one or more of the following, depending on the specifics of your structure: gable vents (preferably with fans), ridge vents and soffet vents.
The exterior of the structure should also be looked at with respect to moisture control. If there is vegetation growing up against the structure, or trees overhanging the roof, then the sun is unable to help dry the inside of the exterior walls or roof. These things should be cut away so that there is at least a gap between vegetation and the siding (at least 2 -3 feet) to allow ventilation and that there are no trees overhanging the roof. Drainage should also be evaluated. Water running towards the structure is a definite prblem and can usually be fixed with regrading the yard and, if needed, proper drain installation. Clogged or broken gutters can also allow moisture penetration, especially along the edge of the roof. They should be cleaned and kept in good repair. The bottom ends of the gutters should also lead away from the structure. It does no good if the gutter runoff end up in the basement or crawl space.

If you have any questions, just ask Pestdude!