Ground Plastic, Liners and Seams Under Raleigh Homes

Fixr's 2025 guide puts a replacement vapor barrier in a 1,500 sq ft crawl space at $1,500–$4,000. North Carolina's code minimum for a closed crawl space is 6-mil polyethylene covering the exposed earth, with seams lapped at least 12 inches. Many full liners are thicker and carry up the foundation walls.

Ground cover or full liner?

A ground vapor retarder covers the dirt floor. A liner goes further: its edges are sealed to the foundation walls and support columns and its seams are sealed, so the whole space is wrapped. Builder-grade sheets are often the 6-mil minimum, which tears easily under knees and tools. Thicker, reinforced material costs more but survives the next plumber's visit. Ask for the thickness in mil and whether the sheet is reinforced.

The details North Carolina spells out

• The floor is graded so it drains to one or more low spots, with a drain to daylight or a sump pump at each.
• Those crawl space drains stay separate from roof gutter drains and foundation perimeter drains.
• A full liner is sealed to the walls and beam columns, and runs at least 4 inches up each column.
• On masonry walls the liner stops 3 inches below the top of the wall, leaving a 3- to 4-inch gap under the wood sill so termite tubes can be spotted.

In an April 2026 interpretation, the NC Office of State Fire Marshal said running the plastic up the walls is allowed but not required. Wall moisture can also be handled with liquid-applied block sealants, spray foam or rigid foam board.

How can you tell a crawl space vapor barrier has failed?

• Bare soil showing between sheets, or sheets bunched against one wall.
• Puddles sitting on top of the plastic after rain.
• Seams that were never taped, or tape that has let go.
• Dark growth on the underside of the sheet or on nearby wood.

Before new plastic goes down

Old sheets, wood scraps and fallen insulation come out first. Standing water needs a drain or sump, because plastic laid over a wet floor holds the water against the house. Check the joists and sill for rot while they are easy to reach, and consider a radon test for a baseline. For how the liner fits into the whole bill, see what each part of encapsulation typically costs.