How to Make a Crawl Space Vapor Barrier

An unfinished crawlspace.
  • 4-6 hours
  • Intermediate
  • 150-250
What You'll Need
Six mil or greater plastic sheeting
Water-resistant sealing tape
Landscaping stakes
1 1/2-inch rigid foam insulation
Expanding foam
Siliconized caulk
Galvanized metal flashing (optional)

Making a crawl space vapor barrier is a great way to keep harmful water from collecting on pipes and foundation walls beneath a house. Water that is allowed to collect there can mold, rot wood, rust pipes, and freeze during the winter, which causes cracks in the foundation. To keep water out, all you need is a few materials and the willingness to crawl through the small area under your house.

Step 1 - Keep Water Out

First of all, be sure that as little water as possible is collecting near the foundation of the house. Proper gutter systems installed on the eaves should route water away. This simple step will ensure that the vapor barrier will only have to deal with the most minimal of moisture issues.

Step 2 - Line the Ground with Plastic

Crawl into the space below the house and lay six mil plastic sheeting along the ground inside. Of course, you could use a plastic sheeting with a thickness greater than six mil; however, as plastic sheeting gets thicker it tends to get more expensive as well. Six mil is a good thickness because it is thick enough to be vapor proof and tear-resistant without being too terribly expensive.

Overlap any seams in the plastic and tape them with a water-resistant sealing tape. This type of tape is usually plastic-like and has an adhesive that will not weaken in a moisture-rich atmosphere. Make sure that the plastic sheeting covers the ground and runs about six inches up the side walls of the crawl space.

Finally, push thin landscaping stakes through the plastic every couple of feet. The tiny holes won’t compromise the integrity of the seal enough to hurt anything and will instead allow any water that does get in to drain away. Once the landscaping stakes are in place, it will be much easier to maneuver within the crawl space without bunching or tearing the plastic.

Step 3 - Line the Walls with Insulation

Carefully attach 1 1/2-inch rigid foam insulation with foil on one side to the walls and ceiling of the crawl space. If foil-faced insulation is not available, extruded polystyrene will work. This will not only insulate the house better, but it will also keep dampness off the concrete foundation by preventing condensation.

Make sure to insulate the rim joists as well, by cutting insulation to fit between them. Fill any large gaps with expanding foam. Seam the insulation together wherever possible with siliconized caulk to prevent any cracks that air could seep through.

Step 4 - Cover the Insulation with Plastic

To finish the vapor barrier seal, cover over all the insulation that was placed along the walls and ceiling with a six mil plastic vapor barrier. This plastic should overlap the plastic that was run up along the walls in step one. This final barrier should seal off the crawl space from any moisture. Some people will install galvanized metal flashing at this point to protect against termites, but that is not an absolutely necessary step.