Green Building Library
Weatherization Tips

How to Install a Ground Cover

  • All crawlspaces, basements, and garages with exposed soil must have a ground cover moisture barrier before insulation is installed.
  • The purpose of a ground cover is to keep moisture from moving out of the soil and coming into contact with wood framing and insulation.
  • Even if the ground feels dry to the touch, there is probably moisture in it.  When the moisture evaporates, the humidity level in the space increases.  If the temperature drops below a certain point, the humid air condenses to liquid water on the wood framing.  The result could be wood decay.
  • Never install a ground cover over standing water.
how to install a ground cover


  • Use 6 mil black polyethylene
  • 8 ft. x 100 ft. rolls- best for crawlspaces under joisted floor
  • 12 ft. x 100 ft. rolls- best for crawlspaces under post and beam type floors

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© Copyright 1993 Iris Communications, Inc.


More Installation Tips:

Where to Start

Your goal is to roll out large sections of polyethylene and make as few cuts as possible. Look around the crawlspace.  Start from the edge of the foundation in an area that gives you the most unobstructed ground. Then work your way around the rest of the crawlspace making sure you overlap all pieces and cover all bare ground.

Some installers try to have the polyethylene overlap away from the access so that when they drag insulation or other supplies through the crawlspace, they don't catch an end of the ground cover and pull it out of position. To do this, you'll have to start laying polyethylene at the point furthest from the access and work back toward the access.

How To Roll It Out

Roll out a few feet of plastic but don't unfold it.  Crawl forward, pushing the roll ahead of you.  When you reach the wall, cut the plastic with enough extra to extend 6 inches up the foundation.  Then turn around and move back over the piece you just laid.  As you go back, unfold the plastic and spread it out in both directions.

Repeat this process as you work your way across the crawlspace.

Use your utility knife to cut slots so the plastic fits neatly around utility pipes or other obstacles in the space.

If you come to an obstacle and still have a couple of thicknesses of polyethylene to unfold, use your knife to cut a slit through all thicknesses at once.  Then when you unfold the roll you will have a neat, straight cut.