If you've ever peeked into a crawl space, you've probably seen a sheet of plastic on the ground. That's the vapor barrier, also called a ground cover, and it's one of the most important parts of a house most people never look at.
What it does
Soil is always giving off moisture. In Western Washington, where the ground stays wet for months, that's a lot of water vapor. A ground cover stops most of it before it reaches the floor framing, the insulation and the air you breathe upstairs.
What good looks like
- At least 6-mil polyethylene, which is what Washington's code calls for in vented crawl spaces. Thicker, reinforced material is more durable.
- Full coverage: wall to wall, around every pier.
- Seams overlapped generously and taped.
- Laid on smooth ground with sharp rocks and debris removed.
- Run a short way up the foundation walls where needed.
What we usually find instead
Thin plastic that tore years ago. Sheets that don't reach the walls. Gaps around piers. Plastic bunched up in a corner after a plumber or cable installer crawled through. Sometimes nothing at all.
Any of those lets the ground breathe moisture straight into the crawl space.
Signs yours needs replacing
- Musty smell in the house, worse in wet weather
- Condensation or mold on joists
- Sagging, damp floor insulation
- Bare dirt visible anywhere
Replacing a ground cover is usually a modest job, and it protects everything above it. Learn more about vapor and moisture barriers, or crawl space cleaning if yours needs clearing out first.