Crawl space moisture typically comes from three different sources:
The Earth constantly releases moisture into the air which results in higher relative humidity. This is why, at a minimum, you should have a 6-mil or 12-mil poly vapor barrier covering 100% of the ground. This can prevent the need for remedial measures in the future.
Crawl space vents are designed to allow Earth’s moisture to escape. The problem occurs in the summer when the hot, humid air enters the crawl space through these vents. Since warm air can hold more moisture than cool air, the moisture in the air is basically squeezed like a sponge when it is cooled. When this hot, humid air meets the cool air in the crawl space, the relative humidity skyrockets leaving condensation to form on HVAC, fiber-glass insulation, and floor framing.
Standing water underneath a house is a big problem. It typically comes from groundwater after heavy rain. It is common in this area because of the composition of the soils (mostly clay!). This water is constantly evaporating into the air which maintains a very wet and humid environment. Standing water should be dealt with immediately either through additional sand or with a drainage system and sump pump. Sand can be used when the standing water is minimal in a low spot. Drainage is appropriate when standing water is moderate to severe. If crawl space flooding isn’t dealt with, it can result in foundation settlement, other foundation problems, wood rot, crawl space fungus, mold, sloping floors, cracked walls, cracked bricks, and more.
*This crawl space had no vapor barrier. 31 joists had to be repaired, 6×6′ sections of the main beam had to be replaced, and the wood had to be treated with a fungicide. It cost them over several thousand dollars… Would that have happened if they had taken the minimum precaution and had a vapor barrier installed? The damage would not have occurred or it would have been minimized.
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