How Ground Moisture Gets Into Your Crawl Space

Rain and irrigation soak into the soil around your home. That water travels sideways and downward, following gravity and the path of least resistance. Your crawl space floor is often the lowest point, so groundwater naturally collects there.

The soil itself holds moisture like a sponge. Even after the surface dries, deeper layers stay damp for days or weeks. That persistent dampness keeps pushing water vapor upward into the space below your house.

• Clay soils drain slowly and hold water longer
• Sandy soils drain fast but can still wick moisture upward
• High water tables push groundwater close to the surface
• Sloped yards direct runoff toward the foundation
• Downspouts that discharge too close add extra water

A vapor barrier is a plastic sheet laid over the crawl space floor to block soil moisture from evaporating upward. Without one, or with a barrier that is torn or poorly sealed, ground moisture moves freely into the air.

Even a small gap at the seams or around piers lets a lot of humidity through. The barrier also needs to cover the floor completely and be taped at the edges. If it is missing or damaged, the crawl space stays damp no matter how well the rest of the home is sealed.

Even without standing water, high humidity in the crawl space can cause condensation on cool surfaces. In the mild marine climate with wet winters and dry summers, outdoor air changes temperature and humidity through the year, and that affects the space below your home.

Warm, damp air hitting a cool foundation wall or ductwork forms water droplets. That condensation adds moisture to the space and can soak insulation and wood framing. Ventilation and dehumidification help, but the source of the ground moisture still needs to be addressed first.