Crawl space vapor barriers
Plastic over the dirt is one of the most basic crawl space improvements, and one of the most often done poorly. Here's how to tell a good one from a bad one.
Bare soil under a house releases water vapor constantly. On the low, wet flood plain ground found along the creeks and rivers of Sumter County, that soil can stay saturated for months. A vapor barrier, sometimes called a vapor retarder or ground cover, is a plastic sheet spread over the soil to slow that moisture before it reaches the joists and subfloor.
Does a vented crawl space need one?
It isn't always required, but it makes a big difference to the code math. Under the IRC, a vented crawl space needs 1 square foot of net vent area for every 150 square feet of floor. When the ground is covered with a Class I vapor retarder, that drops to 1 square foot per 1,500 square feet. That tenfold change shows how much moisture comes out of the ground.
Why do old barriers fail?
- Thin plastic rips when plumbers, HVAC techs and pest inspectors crawl over it
- Pieces were laid side by side without overlapping, so soil shows at every seam
- Water collected on top, and the sheet now holds it in place
- It covers only part of the floor, or stops short of the walls and piers
- Rodents or other animals have chewed it or nested in it
What does a proper install look like?
- Every bit of exposed soil is covered, including around each pier
- Seams overlap and are taped, with 6 inches as the IRC minimum in a sealed crawl space
- The edges run up the foundation wall and are fastened there, not left loose
- The material is heavy enough to survive the next service call
- It goes in after standing water and grading problems are fixed
Is a vapor barrier all you need?
Sometimes. If the space stays mostly dry and the main issue is moisture coming up from the soil, a good ground cover can be enough. If humid air is coming in through the vents, the ducts are sweating, or water runs underneath after heavy rain, a barrier is just one part of the fix. Ask the company where your moisture is coming from before deciding between a ground cover and full encapsulation.
We don't quote prices. No two crawl spaces are alike, so the company you're connected with prices the work after someone has gone under the house, measured it and found where the moisture or damage is coming from. A figure offered over the phone before that visit is a guess.
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Common questions
How thick should a crawl space vapor barrier be?
Thicker liners tear less, especially along the paths people crawl. The right choice depends on whether it's a simple ground cover or part of a sealed crawl space, so ask what the company recommends and why.
Does the plastic need to go up the foundation walls?
In a sealed crawl space, the IRC requires it to run at least 6 inches up the wall and be attached. Many installers take it higher.
Related services
More background: Sumter crawl space guide.
Musty air, sweating ducts or a floor that sinks near the bathroom?
Call and we'll connect you with a crawl space company that works in Sumter County.