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Crawl Space Vapor Barriers: What You Need to Know

Home experts estimate that in homes with a crawl space, 50 percent of the indoor air comes from the crawl space itself. Ever seen the inside of a crawl space? They aren’t the cleanest places—and you’re breathing a lot of it in.

But vapor barriers are designed to keep your crawl space fresh. By creating a shield against moisture and the outdoor environment, a vapor barrier turns the crawl space into a controlled environment.

Should you get one? You’ll be able to make the choice for yourself and know everything you need to know about vapor barriers after reading this page. Let’s jump in!

Fully encapsulated crawl space.

What is a Vapor Barrier?

Man installing a vapor barrier inside a crawl space.

A vapor barrier (sometimes called a “moisture barrier”) is exactly what it sounds like: a tool designed to stop water from getting into an area. When used inside a crawl space, vapor barriers are attached to as much surface area as possible.

This includes support posts, foundation walls, and the floor itself. Once installed, the vapor barrier creates an enclosed environment protected from various sources of moisture like rain or humidity.

How a Vapor Barrier Protects Your Crawl Space

Most crawl spaces are designed with partial or complete exposure to the outdoors. That means every time the weather gets humid or rainy, the same conditions are affecting the crawl space. Vapor barriers are an effective way to block this water and humidity from getting into your crawl space. 

When installed properly, the plastic sheeting prevents moisture from entering the crawl space and causing issues like mold and wood rot. Plus, a vapor barrier covers up dirt floors in a crawl space, making it harder for groundwater to pool below your home and for pests to dig their way into the crawl space. 

Note: While a vapor barrier is one of the most effective ways to seal out moisture, it can’t stop all forms of moisture by itself. Protecting your crawl space with other encapsulation solutions (such as a dehumidifier and drainage system) helps create even more protection against moisture in your crawl space. Find more of our crawl space solutions here.  

5 Benefits of a Crawl Space Vapor Barrier

Deciding to install a vapor barrier can protect the value of your home and also protect the health of your family. Some of the top benefits include: 

1. Lowers Energy Bills

Sealing your crawl space using a vapor barrier keeps the area warmer in the winter and cooler in the summer. This means your furnace doesn’t have to work as hard, saving energy and cash. A fully encapsulated crawl space can save you up to 10 percent on your energy bills. 

Energy gauges on the side of a home.

2. Deters Pests

A wet, dark crawl space is the perfect breeding ground for pests. If you have pests below your floors, they can easily make their way into your living space. Installing a vapor barrier inside the crawl space makes the area less attractive to rodents and creepy crawlers. Even better, the vapor barrier makes it harder for pests to get inside the crawl space in the first place!

Spiders crawling along a crawl space wall and ceiling.

3. Stops Mold Growth

Mold thrives in a crawl space without a vapor barrier. Without protection, mold can move from the crawl space and into your home! Sealing out water and humidity with a crawl space vapor barrier stops mold growth in its tracks. Stopping mold means no more musty smells or mold spores in the air you and your loved ones breathe.  

Mold on a crawl space support beam.

4. Protects Structural Integrity

Too much moisture in a crawl space leads to wood rot. Since most crawl spaces are built with wooden support beams, your home’s structural integrity is at risk if moisture is not addressed quickly. Vapor barriers help keep the crawl space—and wooden structures—dry! 

Floor joist in a crawl space dipping under heavy weight.

5. Increases Home Air Quality

Because of a process known as the stack effect, the air in your crawl space flows through your home into your living areas. That means dust, mold spores, and even pest droppings can be in the air you breathe. Installing a vapor barrier reduces the number of these unhealthy particles below your home.

Man in home, holding his nose like something smells bad.

How to Install a Crawl Space Vapor Barrier

While installing a vapor barrier in your crawl space may sound easy, working in a tight space, often on your knees or stomach, gets old fast. Not to mention getting in and out of the crawl space typically involves squeezing through a tight door and into unsafe, dirty conditions.

Avoid the hassle; work with a professional who has the tools and experience to properly install a vapor barrier that will transform your crawl space. Since each crawl space is unique, so are the solutions. However, below are the general steps to installing a crawl space encapsulation system with a vapor barrier.  

  • Step 1: Cleaning and Grading – Any debris, like old building materials or trash, is removed. Existing damaged insulation is bagged and pulled out. A rough grade is established.  
  • Step 2: Drainage – If your crawl is wet, some sort of drainage will be recommended. Drainage matting, a crawl space drain, or a sump pump are installed next. 
  • Step 3: Wall Insulation – Locking outside air out means sealing walls using a closed-cell foam board. This is attached to the walls using mechanical fasteners. This material is also used to block any open vents from inside the crawl space. 
  • Step 4: Vapor Barrier – The high-quality 20-mil thick liner is installed on the ground and attaches to the wallboard. All seams are taped to ensure the crawl space remains as airtight as possible.  
  • Step 5. Air System – Most crawl spaces need some type of air filtration system. A high-efficiency, self-draining dehumidification system is installed to circulate clean air throughout your home.

Work With a Professional Team, Work With JES Foundation Repair

If avoiding the backbreaking crawl space work isn’t enough reason to call the professionals, it’s important to remember without the proper products and tools, your results will be limited.   You could invest a weekend or two of your time and hundreds of dollars only to see nominal results because a step was forgotten or a seam wasn’t secured properly. 

At JES, our installers do this type of work every day. We’ve been helping homeowners for over two decades, and we’re ready to help you too. With our financing options and free inspections, we’ll get your crawl space protected quickly and professionally. Contact us today to get started!

JES Can Help Encapsulate
Your Crawl Space

Our team at JES Foundation Repair is proud to serve homeowners in locations throughout Virginia, including: 

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Crawl Space Vapor Barrier


Securing the vapor barrier is one of the most important installation steps. If your vapor barrier isn’t properly secured, meaning that gaps are left unsealed, outside air will continue to flow into the crawl space, making the system nearly pointless.  

Creating a virtually airtight seal is the only way to experience the benefits of a vapor barrier. This is done with a combination of mechanical fasteners and specialty tape. 

Yes. If you want to protect your family’s health from mold and dust spores and the structural integrity of your home, investing in a vapor barrier in your crawl space is essential.  

A vapor barrier’s job is to seal out moisture, so making sure it’s thick enough to do just that is essential. Big box store liners are typically 6-mil thick. These liners aren’t able to handle the area’s conditions and often rip before installation is even completed.  

We recommend a 20-mil, zero perm liner. Our liner is American-made and comes with a nationally backed, long-term warranty.  

Phillip Mazzotta

Phillip Mazzotta

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