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Crawl spaces

Crawl space vapor barriers and liners

By Robert Stulpt Last reviewed

Short answer

A vapor barrier is plastic sheeting placed over the dirt in your crawl space to reduce the moisture coming up from the ground. You can use 6 mil for a simple ground cover in a vented crawl space. If you’re looking to encapsulate your crawl space, I recommend a reinforced liner between 12 and 20 mil thick. Whatever you use, make sure it works as one continuous sheet: seams overlapped and taped, snug against the piers and run up the walls.

On this page
  1. Vapor barrier or full encapsulation?
  2. How thick should it be?
  3. What the code says
  4. How much material you need
  5. How to install a crawl space vapor barrier
  6. Common mistakes
  7. When to replace it

Vapor barrier or full encapsulation?

A vapor barrier is just the plastic over the ground. When you encapsulate your crawl space, the liner is also sealed to the walls and piers, the vents are closed off, and a dehumidifier or conditioned air keeps the space dry. See crawl space encapsulation.

How thick should it be?

I typically use a 12- to 20-mil reinforced liner when encapsulating a crawl space. Twelve or 15 mil will work for most crawl spaces. I use 20 mil if there’s a lot of gravel, the ground is very uneven, I know there will be more foot traffic and technicians coming and going, or I just want a more puncture-resistant liner. Not every house needs 20 mil. But I wouldn’t use 6 mil for an encapsulation that people will be walking on for years.

Six mil works as a basic ground cover if your crawl space is vented, doesn’t get much foot traffic and has clean ground, and you’re on a budget and just want to reduce the vapor coming from the soil. The issue with 6 mil isn’t that it lets moisture through. It’s that it gets punctured and torn more easily when people work on it.

LinerWhere it fits
6-mil polyethyleneBasic ground cover in a vented crawl space with little foot traffic and clean ground
12–15 mil, reinforcedMost encapsulations
20 mil, reinforcedGravel, very uneven ground, heavy foot traffic or frequent service visits

Based on Robert Stulpt’s field experience, not lab testing.

What the code says

In a closed crawl space, the code requires a Class I vapor retarder, the most vapor-resistant class, over all exposed dirt. Class I means a perm rating of 0.1 or less: very little water vapor gets through. In a vented crawl space, covering the ground with a Class I vapor retarder also lets you reduce the required vent area. In the 2021 Virginia Residential Code, for example, vents normally need 1 square foot of opening for every 150 square feet of crawl space; with the ground covered, that drops to 1 for every 1,500 (R408.2). For a closed crawl space, seams must overlap by 6 inches and be sealed or taped, and the liner must run at least 6 inches up the wall and be sealed to it (R408.3). Your state may use a different edition, so check with your local building department.

How much material you need

Measure your floor area and add the wall area you’ll run the liner up. Then add the overlap at every seam and extra for each pier you’ll wrap and seal. If your crawl space has many piers or an odd shape, you’ll need noticeably more than the floor area alone.

Laying a crawl space vapor barrier Schematic section of a crawl space liner. 1: strip of wall left visible below the sill plate for termite inspection, about 1 to 2 inches. 2: liner run up the foundation wall and fastened and sealed to it. 3: seam overlapped and taped. 4: liner continuous over the ground. 5: liner wrapped up the pier and sealed. 6: sill plate. Schematic, not to scale. 123456
  1. Wall left visible below the sill plate for termite inspection (about 1–2 in)
  2. Liner run up the foundation wall, fastened and sealed
  3. Seam overlapped and taped
  4. Liner continuous over the ground
  5. Liner wrapped up the pier and sealed
  6. Sill plate
SoilConcrete / masonryWoodInsulationWater
Schematic, not to scale.

How to install a crawl space vapor barrier

If you’re doing this yourself and water is coming into your crawl space, fix that before you install the vapor barrier. See water in the crawl space. Then follow these steps:

  1. Clean the ground and remove rocks and anything sharp.
  2. Wear protective gear: gloves, eye protection and a respirator if there’s dust or possible mold.
  3. Overlap the seams, then seal them.
  4. Fit the liner tightly around each pier.
  5. Run the liner up the foundation walls.
  6. In areas where termite inspections apply, leave about 1 to 2 inches of wall visible below the sill plate. In an unvented crawl space, Virginia’s residential code requires it (R408.3.1).

And a few things not to do:

  • Don’t cover drains or the sump pit.
  • Don’t let a dehumidifier drain onto the plastic.
  • Don’t close all the vents until you’ve decided how you’ll control the humidity.

Common mistakes

These are the problems I see most often around piers, walls and seams:

  • Cutting too much around a pier, and not sealing it afterward
  • Joining pieces with a strip of tape and nothing else
  • Dust under the tape
  • Taping to damp walls or masonry that wasn’t prepped
  • Stopping the liner short of the wall
  • Gaps behind pipes and ducts
  • Covering the sill plate completely, or wood that needs to be inspected
  • Not sealing around the sump pit

When to replace it

A good reinforced vapor barrier can last 15 years or more. I’ve seen the liner outlast the tape and some of the fasteners. I don’t replace a vapor barrier because it’s 10 years old. I replace it when it’s no longer doing its job. Here’s what I look for:

  • Tears or holes
  • Seams that have opened up
  • Material that’s gone brittle
  • Liner pulling away from the wall
  • Water sitting on top of it
  • Areas torn up by rodents or by people working down there

Sources