How to Seal and Manage Crawlspace Moisture

Crawlspaces are the lungs of a home, yet they are often the most neglected zones. When moisture builds up, you aren't just dealing with damp floor joists; you are creating a prime sanctuary for termites, spiders, and rodents. A dry crawlspace is naturally resistant to pests, as most unwanted visitors require consistent humidity to thrive. Getting this right requires a shift from passive observation to active management. You want to encapsulate the ground to stop earth-based evaporation and ensure that the outside air isn't doing more harm than good during humid summer months. When done well, your crawlspace will be clean, odor-free, and inhospitable to anything that crawls.

1. Sweep the Foundation Clean. Remove all debris, old wood scraps, and trash from the crawlspace floor. These items act as moisture wicks and provide immediate food or shelter for termites.
2. Plug Every Air Leak. Inspect the rim joists and where utilities enter the foundation. Use expanding spray foam or caulk to seal any gaps that allow damp air or insects to migrate into the home.
3. Grade for Drainage. Use a rake to smooth out high spots in the dirt floor. This prevents tears in your plastic liner and promotes better drainage if water does enter.
4. Lay the Moisture Shield. Lay down 12-mil or 20-mil polyethylene plastic over the entire floor. Extend the plastic six inches up the foundation walls and secure it with masonry pins.
5. Control the Air Exchange. If you live in a humid climate, install automatic vent covers that close during high humidity. If the space is unconditioned, ensure cross-ventilation is open only during dry months.
6. Dry Out the Space. Place a commercial-grade crawlspace dehumidifier on a concrete block or platform. Route the drainage line directly to the exterior or a sump pump.