How to Install a Plastic Vapor Barrier in Your Crawl Space

vapor barrier
  • 4-6 hours
  • Beginner
  • 80-120
What You'll Need
6ml polyethylene plastic (or thicker)
Moisture-proof insulation
Caulk gun
Tape measure
Utility knife
What You'll Need
6ml polyethylene plastic (or thicker)
Moisture-proof insulation
Caulk gun
Tape measure
Utility knife

Crawl space vapor barriers should be part of a home's moisture control strategy in most U.S. climates, thanks to how saturated the ground is in most areas. A vapor barrier for a crawl space slows the passage of moisture, preventing it from soaking into structures and causing water damage or allowing mold and mildew to grow. Improper vapor barrier installation can also lead to moisture issues, since the material can prevent surfaces from drying if not installed correctly.

Cleaning and repairing a crawl space or damaged structures from moisture leakage can be expensive, depending on the degree of the damage. Taking preventative measures like constructing a vapor barrier is the best option.

So, what are the benefits and requirements of employing a crawl space water vapor barrier, and how do you install one? Read on to find out more.

What Is a Vapor Barrier

Depending on the perm level, a vapor barrier (or retarder) is a thick plastic sheet that can be installed to keep moisture and potentially dangerous chemicals out of your home by blocking them from entering through crawl spaces, roofs, walls, ceilings, and foundations.

Installing a vapor barrier in your walls, ceiling, floor, or crawl space has numerous advantages, including preventing moisture damage, increasing energy efficiency, and even lowering the risk of mold and bacteria-related health problems. The necessity, however, can vary based on factors such as climate, building type, and plan.

Why Is a Vapor Barrier Important in a Crawl Space?

plastic barrier in crawl space

Moisture is one of the most common enemies of a home's structure, with the ability to cause structural issues. Water that is not regulated can cause rot and decay in wood framing and other materials and mold and mildew. For example, the quantity of moisture present impacts whether or not you can finish a basement.

The type of below-grade flooring you can install is also determined by moisture. Just think about it; if your home is in a wetter area and the ground is more saturated, you will have to put in extra work to ensure that moisture does not affect your home.

This installation of vapor barriers into walls, ceilings, and floors can help prevent the effects of water where there is a known issue.

Furthermore, even though it is not visible and many of us would hardly think about how saturated our ground is, moisture is most destructive in a crawl space, often in direct contact with the land and frequently less protected. It's difficult to emphasize the complications that might arise due to dampness in the crawl space.

Signs You Need a Vapor Barrier in Your Crawl Space

Here are four basic questions to ask yourself that can help tell you whether or not you need a vapor barrier in your crawl space.

Are You Located in a High-Risk Zone?

If you live in high-risk moisture or flooding area, you should seriously consider installing a vapor barrier to keep water and moisture out of your property. Remember that allowing water into your structure can lead to the growth of hazardous microorganisms such as mold and mildew.

Do You Live in an Older Home or Have Older Ductwork in Your Crawl Space?

Did you know that ducting has a 20-year lifespan and that it can begin breaking down and playing havoc on your energy efficiency much sooner? If your crawl space has any ductwork, a moisture barrier can help keep it cool while preventing leaking, corrosion, and premature deterioration.

Do You Have an Issue with Mold or Mildew Growing in Your Crawl Space?

If you're having trouble breathing because of mold in your home, it's possible that your crawl space is to blame. Air can condense if it isn't adequately insulated or protected with a vapor barrier, resulting in dampness inside the area.

If you have mold in your walls or ceilings and think it's coming from your crawl space, a vapor barrier is a must—once you've gotten rid of the source of the problem.

Are Pests Getting in through Your Crawl Space?

It's essential to seal up your crawl area if rodents, bugs, or other pests have forced their way into your home or if your floorboards are rotting.

Moisture damage creates an inviting environment for unwanted visitors, but it can also harm your home's subfloor—and the foundation—if left unattended.

How Much Does It Cost to Install a Vapor Barrier?

Depending on the thickness of the plastic, which ranges from 6 to 20 millimeters, a crawl space vapor barrier costs $0.15 to $0.50 per square foot on average. The cost of a vapor barrier installation ranges from $2 to $4 per square foot or $1,200 to $4,000 in supplies and labor.

Luckily you can save a lot of money by simply buying the plastic and necessary tape and doing the work yourself, rather than depending on an installer. While time-consuming, the steps may be worth the money saved, depending on your budget.

This DIY method of putting up vapor barriers in your crawl space does not take long. With the requisite materials, and depending upon the size of your crawl space, you can likely get the job done in a day or less.

Vapor Barriers Materials To Use

blue plastic moisture barrier on roll

Polyethylene plastic sheeting (poly) is available in many thicknesses and strengths. A six mil thick poly is widely employed as a vapor barrier, and it saves the homeowner money in the short term.

However, if someone walks or crawls on the poly, generating minute holes in the vapor barrier, those savings may be lost in the long term. The vapor barrier will no longer work as well once holes appear in it, no matter how small they are.

As a result, a ten mils or higher thickness is recommended for long-term vapor protection, depending on soil conditions.

Another thing to keep in mind is where you are sourcing your vapor barrier materials. Often polys sold in local hardware stores are thinner, non-reinforced, and may be made up of recycled plastics, which may contain impurities like dirt or moisture.

While thinner plastic or recycled plastic may be acceptable for short-term use, it won’t work as well for long-term applications such as crawlspace vapor barriers. Properly built virgin reinforced poly should last 20 years or more in a crawlspace.

The thickness of the poly you select will be mostly determined by the crawlspace's intended use (storage, maintenance, etc.) and the soil or material on the floor. It's good to use a thicker poly, such as twelve mils, if you will be entering the crawl space regularly for maintenance or if you wish to store anything within.

If the crawlspace floor contains pebbles, roots, or concrete bits, the minimal six mils will be insufficient, and you must choose a thicker plastic. Keep in mind that a single rip or hole in your vapor barrier will allow moisture to pass through, reducing the effectiveness of the barrier.

You will also need fasteners on top of the tape to keep the vapor barrier fastened to the foundation wall. Most fasteners can be easily put into place using a drill bit every 18-24 inches through the butyl tape to create an airtight seal around the hole.

Steps to Installing a Vapor Barrier in Your Crawl Space

hand with drill installing vapor barrier

Installing a vapor barrier is not a difficult or expensive task, but it can be strenuous since you will likely spend much time crawling on your hands and knees in cramped spaces.

While doing this job alone is possible, having at least one person transfer goods through the crawl space door and assist with rolling out the plastic and taping the seams is beneficial.

Working securely and productively in the restricted environment beneath the house requires a good work light.

Step 1 - Find the Entrance to the Crawl Space and Measure the Area

A crawl space's entry hatch isn't always obvious. It can sometimes be found outside, along the crawl space's outer wall, or an indoor entry hatch might be found in a closet, pantry, or utility area.

Look for a square seam in the carpeting that signals a hatch entrance if your floor has wall-to-wall carpeting. Once completed, measure the space in preparation for the next steps and make sure you have enough vapor barrier plastic ready to be installed.

Step 2 - Dry Out and Clean the Crawl Space

It's ideal to start with as dry a crawl space as possible. Utilizing a dehumidifier, setting up fans, removing wet things, and using a pump to discharge any standing water are all options for drying out crawl space.

This drying may take several days to accomplish but is critical to the success of laying down the vapor barrier. Remove all debris, especially anything sharp that could puncture the plastic once it has dried.

Step 3 - Lay Out, Cut, and Tape the Plastic Sheets along the Floor

Using the measurements from before, begin laying down 6-mil or thicker polyethylene plastic over the entire crawl space, starting on one side. Cut the sheets to size, leaving at least six inches around the walls and overlapping the seams by 12 inches. Use the poly PVC tape to bind them together securely together.

Step 4 - Cover-up Exposed Foundation

The next step is to use one to one and a half inches of moisture-resistant insulation to cover any exposed sections of your home's foundation. This insulation increases the overall efficacy of the basement waterproofing system and ensures the safety of your home and belongings.

These barriers also prohibit the rising wetness in the concrete from seeping into the crawl area of your home. Caulk up any areas you think might be particularly vulnerable to moisture or might come apart through general wear.

Step 5 - Attach Vapor Barrier Plastic to Walls, Posts, and Piers

wall plastic in room with window

You are likely to find both support walls or columns and inner support piers and posts in every house, especially if it has more than one story. Cut the plastic sheeting to fit the internal supports and use double-sided butyl tape to seal the material against them.

You can cut additional pieces of plastic to plug any gaps and make a complete barrier if necessary.

Make sure to extend the plastic sheeting at least six inches up the walls around the crawlspace perimeter and fasten it with double-sided butyl tape. This step will give you a fully resistant barrier to moisture coming up from below that might still be going up the base of the crawl space walls themselves.

Extra Step - Introduce Proper Ventilation into the Space

Plastic vapor barriers will keep the majority of water vapor from leaking into your crawlspace, but increasing cross ventilation will keep any remaining moisture from becoming trapped in the space.

Most crawl areas are under-ventilated, and putting the vents up to code can significantly reduce moisture problems. Another option is introducing an electric vent fan system, which might help keep a crawl area dry in more critical circumstances.


Most homeowners have to deal with wetness regularly, some more so than others depending upon where you live. Most people are familiar with moisture problems caused by damaged roofs or rainwater runoff that seeps into foundations or up through the floor.

However, homes built over crawl spaces must also deal with moisture from a different source: water vapor rising naturally from the exposed soil and becoming trapped in the enclosed crawl space.

Moisture in crawl spaces is a severe problem, but installing a vapor barrier is a relatively simple DIY project that can be completed in less than a day in most cases. Are you looking for other vapor barrier or crawl space tips?

For more info on crawl space projects, check out our article on How To Treat Crawlspace Mold or Whether To Leave Your Crawl Space Insulation Faced or Unfaced.