Installing a Crawlspace Vapor Barrier

A roll of blue plastic sheeting used for a crawlspace vapor barrier.
  • 48-96 hours
  • Advanced
  • 80-250
What You'll Need
Dehumidifier or fans
Measuring tape
Utility knife
6-mil plastic
Poly PVC tape
Double-sided tape

Ventilation openings around the perimeter of a crawlspace are supposed to eliminate unwanted moisture from accumulating around pipes and walls—but they often fail to accomplish this, especially in wet climates. When the warm, humid months of summer arrive in full force, these openings often lead to even more moisture buildup in the crawlspace. This moisture, which also permeates through the soil, can lead to a lot of problems down the road, including the growth of hazardous mold. Installing a crawlspace vapor barrier is a great way to help eliminate unwanted moisture under the house and can be done by following these simple steps.

Step 1 - Find the Crawlspace Entrance

The entrance to your crawlspace will be located on the inside or outside of the home. On the inside, look for seams in the carpets or a hatch door inside a closet. Typical locations include guest bedroom closets, the kitchen pantry, and the utility room. You may also find the crawlspace entrance on the outside of the home near one of the exterior walls.

Step 2 - Remove Moisture

A basement sump pump.

Before installing the vapor barrier, you want to ensure the crawlspace is completely dry. Remove unwanted moisture with a dehumidifier, a sump pump (for large puddles of standing water), or a series of fans. This process may take a few days depending on how much moisture is in the crawlspace.

Step 3 - Prepare the Crawlspace

Once the crawlspace is dry, remove any debris such as discarded tools or building materials. You want to pay careful attention to anything that is sharp and could potentially pierce the plastic. The vapor barrier will only work if it's sealed correctly. Keep an eye out for sharp surfaces that should be addressed before laying down the plastic.

Step 4 - Install the Plastic

Roll out a 6-mil layer of plastic onto the crawlspace floor. Start at one end of the crawlspace and work your way to other side. Leave at least 12 inches of overlap between sheets and attach them securely with the poly PVC tape. This will help ensure that no moisture gets between the overlaps.

Step 5 - Cover Columns

When you enter the crawlspace, you will notice a series of columns or piers that help support the weight of the house. You need to cover these vertical supports up to 12 inches from the ground for a proper vapor barrier. Lay out the plastic against the support pieces and attach with the double-sided tape. Use the PVC tape to cover any overlaps between the plastic pieces.

If your crawlspace contains larger obstacles, like a pressure tank or a water heater, the best method is to install the plastic under the object. You will likely need an extra hand or two to help lift the obstacle and place the plastic underneath.

Step 6 - Cover Walls

A basement crawlspace with concrete and moisture damage.

Just like the support columns, the walls of the crawlspace need to be covered up to 12 inches from the ground. Start on one corner of the crawlspace and attach a layer of plastic with the double-sided tape. Continue this process until all of the walls inside the crawlspace have been adequately covered.

Step 7 - Seal Vents

Once the vapor barrier has been properly installed, you want to seal the ventilation vents to prevent moisture from coming inside the crawlspace during summer months. Seal the vents using the 6-mil plastic and double-sided tape. You can also caulk around the edges of the vents for added protection.


Although a vapor barrier is a great way to eliminate unwanted moisture coming up from the soil, it will not alleviate moisture issues that come from above the crawlspace. This includes a water leak from pipes or sloping ground around the house that allows rainwater to enter the crawlspace. Any plumbing issues should be addressed before you install the vapor barrier.