How to Use a Vapor Barrier in a Crawl Space to Control Moisture

Crawl Space with Vapor Barrier
Installing a plastic vapor barrier in a crawl space helps control moisture and protect your home. By 2.0

Moisture is part of the life when it comes to houses. Except for the driest homes in the driest areas, moisture of some type cannot be entirely avoided. Water infiltrating your home at ground level is one way moisture can adversely impact the structure. When you have a crawl space, moisture is visible and highly present.

One method of controlling crawl space moisture is with a very simple and inexpensive project: laying out and connecting rolls of ordinary sheet plastic. Traditionally, this plastic has been called a vapor barrier. More recently, the U.S. Department of Energy has revised this popular term and now calls it a "vapor diffusion barrier." In a sense, this is more accurate since you cannot seal off 100 percent of the moisture migration. Rather, the plastic slows down the process.

Crawl Space Moisture Problems

Moisture and houses do not mix. The presence of moisture determines if you can finish your basement. Moisture, too, determines what type of below-grade flooring you are able to install. It affects kitchen and bathroom walls, floors, and especially ceilings. Insulation is installed with the vapor-barrier facing inward in response to the huge build-up of internal moisture. At its most extreme, moisture will even determine if you can build that house or put on that addition in the first place.

But in no other place is this more apparent than in the crawl space. Because the crawl space has contact with the ground, it should be no surprise that moisture will develop here. It is difficult to overstate the problems associated with crawl space moisture.

  • Mold, Mildew: The main problem is mold, fungi, and mildew. Remediating mold-related problems is expensive. While in some cases mold problems might be blown out of proportion, as a direct byproduct of crawl space moisture it will begin blackening floor cavity insulation and structural elements. Because there is no light and only minimal ventilation, the problem never improves on its own.
  • Rot: Structural elements of your house such as joists, sills, posts, beams are made of wood. Being an organic material, wood will begin to rot when it comes into contact with water.
  • Vermin and Rodents: Animals of many types are drawn to water and can infest your home. This includes rats, carpenter ants, termites, and brown marmorated stink bugs.
  • Resale: Even if moisture did not create those problems, its presence may turn away home buyers and erode resale value. When you try to sell your house, the property inspector will explore the crawl space and note the presence of water on the report. It does not matter if you have contentedly lived in your home like this for the last thirty years, the buyer will want the problem fixed or ask for a credit.

    What a Vapor Barrier in a Crawl Space Will Do

    Laying down a sheet plastic vapor barrier may help with normal soil moisture. In this case, normal means water that comes in the form of invisible vapor and does not include excessive pooling of water from below or from above.

    • Below: When large amounts of water pool up in the crawl space, you may have a groundwater problem or a problem with rainwater intrusion. In this case, you should hire a crawl space water remediation company. In most cases, they will dig a trench around the perimeter, add drain pipe, cover the pipe with drain gravel, and add a sump pump.
    • Above: Water leaking from above must be fixed before you put down a vapor barrier. Water may be coming from pipes leading to the bathroom and kitchen. When water from above comes into contact with a crawl space vapor barrier, the water will only pool up and remain.

    How to Install a Vapor Barrier in a Crawl Space

    Installing a vapor barrier is not a complicated or expensive project. However, it can be extremely physically taxing because of the amount of crawling you need to do. What would be a simple task at ground-level is multiplied in complexity by the crawl space's low ceiling and by the small entrance to that area. While it is possible to do this job by yourself, it is beneficial to have at least one helper to pass items through the crawl space door and then to help with rolling out the plastic in the crawl space.

    Tools and Materials

    • 6-mil minimum thickness clear polyethylene plastic
    • Butyl double-sided tape
    • Poly PVC tape intended for crawl space sheet installation, such as Dow Weathermate Construction Tape
    • Utility knife
    • Measuring tape

    Locate Your Crawl Space Entrance

    Crawl spaces often have entry hatches located in closet floors. If your floor has wall-to-wall carpeting, look for a square seam in the carpeting that indicates a hatch door. Look for it in little-used areas, such as inside of a kitchen pantry, coat closet, or guest bedroom closet. Or you may find the entrance outside, on the side of the crawl space wall.

    Dry Out the Crawl Space

    Begin with a crawl space that is as dry as possible. Methods for drying out a crawl space include: using a dehumidifier; setting up fans; removing wet materials that prevent lower sections from drying out; and installing a sump pump to discharge excess water.

    Clean the Crawl Space

    Crawl spaces are notoriously messy, as construction workers often toss discarded building materials in this space. Remove all debris, especially anything sharp, as it will puncture the plastic.

    Roll Out the Plastic

    Beginning at one side of the crawl space, lay down 6-mil or thicker polyethylene plastic over the entire crawl space. Overlap the seams by no less than 12 inches and attach with the poly PVC tape.

    Lap Up Against Columns

    With every house, you will encounter internal vertical supports such as columns and piers. Cut the plastic as needed and lay it about 12 inches up each column or pier and attach with the double-sided butyl tape.

    Lay the Plastic Against Walls

    Bring the plastic about 12 inches up the crawl space walls and tape it up with the double-sided butyl tape.