Soil Moisture & Ground Vapor
Soil moisture plays a role in crawl space humidity and odors, but it is not the main cause of the severe moisture damage seen in Southeastern homes.
Soil moisture and ground vapor are often blamed for crawl space problems, but their role is frequently overstated. Moisture does evaporate from exposed soil, especially in crawl spaces without ground cover. This evaporation can raise humidity slightly and contribute to damp conditions, particularly during warm weather. However, in humid climates, the amount of moisture introduced by soil evaporation is small compared to the moisture carried in by outdoor air.
Ground vapor barriers help reduce evaporation from the soil and provide a cleaner, more stable surface within the crawl space. They also protect equipment like dehumidifiers from dust and debris and make future access easier. However, ground vapor control does not stop condensation from forming on surfaces above the barrier. Water that appears on top of a vapor barrier is typically dripping from joists, ducts, pipes, or insulation rather than rising from below, and a vapor barrier can actually make moisture and mold worse in vented crawl spaces.
Misunderstanding the role of ground vapor often leads homeowners to believe thicker plastic or wall liners will solve moisture issues. In reality, condensation forms from the top down, not from the soil upward. Ground vapor barriers are a supporting component, not a primary moisture solution. Without air sealing and humidity control, plastic alone cannot protect structural systems.
When used as part of a complete moisture strategy, ground vapor control plays an important role. It reduces secondary humidity sources, keeps the space clean, and supports long-term crawl space maintenance. The key is understanding its limitations and integrating it with systems that address the real drivers of moisture.
