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How Do I Maintain Appropriate Crawl Space Humidity?

By Anna B. Smith
Updated May 16, 2024
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To maintain appropriate crawl space humidity, close ventilators to outside air, install a vapor barrier, and redirect water away from the home. These measures will help ensure that excess water does not collect beneath the home and cause mold damage to the foundation and floorboards. Dry air in a crawl space is necessary to maintain good condition of the timbers and concrete blocks generally used in construction.

A crawl space is the area located beneath a home. It is usually comprised of the outer walls of the structure's foundation, and is typically made of concrete, wood, and the dirt upon which the home was originally built. Crawl space humidity will generally be dependent on the type of ventilation used within the foundation of the home and the local climate.

This area is commonly ventilated through a series of holes built at ground level, into the lower concrete portion of the foundation. These openings are frequently covered with screens or netting to prevent animals from entering the crawl space and making nests. This ventilation allows air to flow evenly beneath the home.

Many homes built in hot, humid climates have a large number of ventilation openings within their crawl space areas. This is attributable to the theory that humidity in the air can create rot and mold within the timbers. Allowing a crawl space to remain open to the outside climate should, in theory, prevent the foundational wood from growing mold.

It is generally accepted among builders and home inspectors that wood will be in danger of growing mold if it is allowed to reach a humidity saturation point of 65-70%. To prevent wood from reaching this humidity point, the crawl space air should ideally remain more dry than the air outside the home. This air should not be too dry, however, as this can also damage the timbers of the foundation. Crawl space humidity should generally remain at or below 55%.

Humidity can enter the crawl space when saturated outside air is allowed to flow into a dry crawl space, or when water is allowed to stand in the area. It may also occur when the air inside the home is cooler than the air outside the home, or that in the crawl space. Homes that are highly air conditioned during the summer may create moisture along the undersides of the floor boards and the duct work within a crawl space.

To prevent unnecessary moisture in this area, begin by closing the crawl space vents to the outside air. This can prevent moisture in humid climates from entering the space year-round. It will also help a home maintain its cooled or heated temperature and reduce energy costs.

The dirt floor of the area should also be covered with a vapor barrier. This can help maintain appropriate crawl space humidity by stopping water that seeps naturally into the ground during rainfalls from evaporating into the air beneath the home. Vapor barriers are usually constructed of plastic, and sometimes aluminum, to add insulation to the foundation.

Water should generally be directed away from the foundation of the home to ensure crawl space humidity does not exceed the levels at which mold can begin to grow. This process may be accomplished through gutters, downspouts, and regrading the land around the home to slope away from its foundation. Rainfall on the roof of the home will then be directed into lower levels of the land where runoff will not damage the property.

HomeQuestionsAnswered is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.

Discussion Comments

By SanSlayer — On Dec 09, 2013

In the case of a sudden increase of moisture that can't be avoided, such as after a big rain or a flooding event, a crawl space dehumidifier or ventilation fan can be used to draw out excess moisture before mold becomes an issue.

Construction guys I've talked to, however, warn against using them on a constant basis, because an overly dry crawl space can also present problems, as the article above points out. So use them when needed, but don't leave them on constantly.

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