An illuminated encapsulated crawl space with a vapor barrier installed on the ground

If you are in the process of constructing or renovating a home, you might be wondering about the essential components to include in it. You have probably already considered important foundational and structural items.

However, you might have overlooked some critical components designed to protect your home from specific environmental elements such as moisture.

Moisture can become a nuisance if it ends up in the wrong places around your home. Likewise, water or vapor entering your crawl space can lead to many problems impacting its livability. 

The good news is that it is possible to keep such moisture and vapor out of your crawl space using a vapor barrier.

This guide will discuss many issues that moisture and vapor can cause in crawl spaces and how vapor barriers help prevent such problems.

What Problems Do Moisture and Vapor Cause in Crawl Spaces?

Moisture and vapor naturally exist in the soil under your crawl space. This soil moisture level typically gets higher in the hot and humid summer months when the seasonal precipitation levels are high. This process increases the rate at which moisture and vapor move upwards and into your crawl space.

Crawl spaces tend to be tight areas with poor ventilation. As a result, any moisture that ends up in this space is likely to stay there. 

If such water and vapor remain in the crawl space for an extended period, it can cause problems such as:

Wood Rot

Moisture may help living plants and trees grow, but it can be detrimental to dead wood used around your home. Wooden posts, beams, joists, and sills can start to rot over time with continual exposure to high moisture levels. This rot will impact their integrity and could potentially cause structural failures.

Mold growth and wood rot behind the wall in our second Net-Zero home
Mold growth and wood rot behind the wall in our second Net-Zero home

So if you have any wooden structural components around your crawl space, a moisture buildup could jeopardize its overall stability.

Mold and Mildew

Mold and mildew tend to develop in areas that have high moisture. These growths can be challenging to get rid of. To make matters worse, they can harm the health of your home’s residents. 

Even short-term exposure to mold and mildew can cause problems such as:

  • Wheezing
  • Itchy skin
  • Itchy eyes
  • Stuffy nose

Long-term exposure to mold and mildew can cause:

  • Migraines
  • Muscle cramps
  • Increased exhaustion


Moist crawl spaces also tend to attract different types of pests. 

These critters include:

  • Rats
  • Mice
  • Snakes
  • Termites
  • Ants

Some mice can squeeze through tiny gaps in your crawl space and enter your home. Termites and ants can also eat through your crawl space wood and spread to other parts of your home. 

Nobody wants to deal with such problems, so it is best to ensure the moisture in your crawl space is minimized.

So what can you do to keep unwanted pests, moisture and vapors from coming up through the soil and entering your crawl space? One common solution is to install a vapor barrier.

What is a Crawl Space Vapor Barrier? 

As its name implies, a vapor barrier is a barrier that keeps vapor out. It generally consists of a plastic sheet or multiple layers of plastic sheets. The barrier can be rolled out and affixed to the floor and walls of your crawl space to prevent moisture from getting through.

A vapor barrier installed on a basement floor in a home

The US Department of Energy officially refers to vapor barriers as “vapor diffusion barriers” to accurately describe what they actually do. Vapor barriers have been around for many decades and have undergone many innovations since their initial inception.

The truth is that vapor barriers can’t block 100% of moisture. This shortcoming is because a small percentage of it will still manage to diffuse through the barrier surface. For this reason, the term “vapor diffusion barrier” is more fitting than “vapor barrier.”

How Do Vapor Barriers Work?

As mentioned, vapor barriers don’t block moisture completely. Instead, they slow down the rate at which water vapor diffuses through the soil and into your crawl space. 

This means your crawl space will still receive some amount of moisture. However, the amount will be much smaller than if the crawl space had no barrier.

Reduced moisture concentration is a lot less likely to create issues. That means getting a moisture barrier can effectively keep your crawl space and the rest of your home safe from the problems described above.

What are the Limitations of Vapor Barriers?

Vapor barriers effectively slow the migration of vapor through the soil and into your crawl space. However, they don’t offer protection against water that collects and pools up in your crawl space. Such scenarios can occur for several reasons. 

The most common include:

Leaky Plumbing

Leaky plumbing in drain and water supply pipes can cause water to enter your crawl space and collect in puddles. In this situation, simply removing the water and drying up the crawl space won’t offer any benefit as more will come through the leaky plumbing.

A puddle of standing water in a crawl space
Courtesy of JES Foundation Repair

Adding a vapor barrier also won’t do anything, as the water will just collect between the crawl space floor and the vapor barrier and create mold and rot issues.

The best solution in this damp situation is to fix the leaky pipe, dry the crawl space, and apply the vapor barrier.

Rainwater Runoff

If your property is located downslope to an adjacent area, rainwater runoff may collect in its crawlspace. But, again, a vapor barrier won’t be able to stop the water from entering the crawl space and causing issues.

In this damp scenario, you must create a trench around your property perimeter and add a sump pump. Alternatively, you may need to change part of the earth grading directing the water runoff to your property.

Are Crawl Space Vapor Barriers Worth It?

Crawl space vapor barriers will set you back anywhere from $1,200-$4,000, depending on the material and size of your crawlspace. Some might believe this is a large amount of money to spend on a plastic sheet. However, they may not realize that vapor barriers can protect your home from the damage that would cost you far more.

Once installed, vapor barriers can last anywhere from 5-10 years, making their cost a relatively minor expense in the long run.

Choosing a Quality Vapor Barrier Brand

You might wonder what to look for before choosing a vapor barrier brand. 

Some things to consider before making your purchase include:

No Reprocessed Fillers or Resins

Some vapor barrier brands add fillers or reprocessed resins to their primary material, which can compromise the barrier’s integrity and cause it to be weaker. As a result, such barriers are more likely to deteriorate, tear, or let vapor through. 

The best vapor barriers are typically made from pure resin compounds. They offer high strength and low permeance and are resistant to degradation and punctures.

Low Permeance

You generally want your vapor barrier to be low permeance because this characteristic indicates it is less likely to let vapor through. 

A vapor barrier roll, staple gun, and some staples resting atop a piece of plywood

Permeance is measured using a unit called “perms.” Under current American regulations, vapor barriers tested under ASTM 1249 or ASTM E96 standards must have a permeance of 0.01 perms or less.

Great Puncture Resistance

A vapor barrier with low permeance may still let vapor and moisture through if it gets punctured. Such punctures may occur from numerous sources, like rocks, equipment, foot traffic, or falling debris. For this reason, you should opt for a vapor barrier with excellent puncture resistance.

Environmental Degradation Resistance

Plastics may have a reputation for withstanding environmental degradation. However, vapor barriers do degrade relatively easily because they are thin. 

Processes such as oxidation will degrade low-quality barriers within a few years. However, high-quality vapor barriers withstanding testing under ASTM E154 standards fare much better against natural elements.

Correctly Installing a Vapor Barrier in Your Crawl Space

Installing a crawl space vapor barrier is a relatively straightforward process. However, it may be challenging to do if you’re unfamiliar with the materials involved and working in a crawlspace. Therefore, leaving this task to a professional in this scenario may be better.

If you plan to install your crawl space vapor barrier yourself, the following instructions should be helpful.

Find Your Crawl Space Entrance

Every crawl space has a dedicated entrance, even if it doesn’t look obvious. You might already know where this entrance is if you have seen your home’s blueprints.

You could find the crawl space entrance outside your home and along the space’s perimeter walls. In other cases, the entry may be accessible from inside your home. Such entrances are usually accessible through a small door or hatch.

Once you have found your crawl space entrance, you can enter your crawl space and get to work.

Ensure Your Crawl Space is Dry

Be sure to look over your crawl space with a flashlight once you are inside. Also, keep an eye out for any pools of water that may have accumulated over time. If possible, dry these out using a cloth or a pump. 

A homeowner laying on his side inspects his crawl space with a flashlight

You can then set up a dehumidifier to eliminate the humidity built up inside the crawl space. 

This whole drying process may take several days to complete. However, it is necessary to ensure the crawl space vapor barrier installation process goes properly.

Clear the Crawl Space

Crawl spaces can be dirty areas due to their position below your home and because they are poorly sealed. They may also contain loose debris or items from your property’s initial construction.

It is vital to ensure the crawl space is clean before starting the installation process. 

An immaculate crawl space in a home

If possible, you should attempt to level the soil surface and ensure it is flat. Any sharp objects sticking out of the soil could puncture the vapor barrier, so you must remove them in preparation.

Lay Down the Vapor Barrier

Now you can start the process of actually applying the vapor barrier. First, unroll the vapor barrier beginning from the far end of your crawl space. You can cut the sheets and continue the unrolling until you have reached the other end of the area.

When cutting the sheets, you should ensure enough length to cover six inches along the walls, and 12 inches overlap with the next sheet. You can then attach the sheets using strips of PVC tape.

Attach The Vapor Barrier to the Walls and Supports

You must also apply your vapor barrier to the crawl space walls and supports. To apply it to the walls, press the six inches of sheet you reserved earlier against the corners and up the walls. You can then use double-sided tape to ensure it stays in place.

Next, cut and wrap sheets around the crawl space supports and apply the tape. You may need to cut and tape several smaller sheet strips to ensure all the gaps are covered.

Optimizing Performance of Your Crawl Space Vapor Barrier

As you can see, getting a vapor barrier for crawl spaces is essential to prevent many serious problems. 

Most home developers and renovation experts will recommend you set up a vapor barrier the first chance you get because it will save you from having to perform significant repairs in the future. It will also protect your home’s residents from pests and structural risks.

The above guide should prove helpful when selecting the perfect vapor barrier for your crawl space. The installation instructions will also be useful if you plan to install the vapor barrier yourself. However, there is no shame in asking a professional to install it for you.

A vapor barrier will perform as it should only once it has been installed correctly. So consult the above guide before purchasing and setting up your crawl space vapor barrier.

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