A crawl space vapor barrier can prevent moisture, mold, and pests from entering your crawl space. Many homeowners avoid going into crawl spaces whenever possible, and it’s not hard to understand why. These spaces tend to be unpleasantly damp, often due to water vapor rising from the ground and getting trapped there.

If your crawl space has moisture problems, a crawl space vapor barrier can help. Here, we’ll outline the benefits of installing one of these barriers and provide a step-by-step installation guide.

What Is a Vapor Barrier?

A vapor barrier is a material that can stop ground moisture from affecting your home. Aaron Lipman from Two Brother foundation explained it as “A vapor barrier is essentially a shield made of heavy-duty plastic or foil that blocks moisture from seeping into areas like crawl spaces or walls” (Homes and Gardens).

Even the most effective vapor barriers can’t completely seal your crawl space from moisture; therefore, the U.S. Department of Energy refers to these moisture control measures as “vapor diffusion retarders” (U.S. Department of Energy). Still, having one of these barriers in place should help you mitigate moisture problems in your crawl space.

Understanding the Different Types of Vapor Barrier Materials

While many people think of vapor barriers as simply “plastic sheeting,” there are actually several different materials available, each with its own unique properties and ideal use cases. Understanding these differences will help you choose the right vapor barrier for your specific needs and climate. The effectiveness of a vapor barrier is measured by its “perm rating,” which indicates how much water vapor can pass through it. The lower the perm rating, the more effective the vapor barrier.

Here’s a closer look at the most common types of vapor barrier materials:

Polyethylene Sheeting

This is the most common and cost-effective vapor barrier material, and it’s likely what comes to mind when you think of a vapor barrier. It’s a flexible plastic sheeting that comes in various thicknesses, measured in “mils” (one mil is one-thousandth of an inch). For most crawl space applications, a thickness of at least 6 mils is recommended, but it can go up to 20 mils or more for heavy-duty applications. You’ll usually be able to find sheet plastic for vapor barriers in a range of six to 23 mils.

Polyethylene sheeting is a Class I vapor barrier, meaning it has a perm rating of 0.1 or less, making it highly effective at stopping water vapor. While it’s great at stopping moisture, it’s important to note that polyethylene is not a radiant barrier, meaning it doesn’t reflect heat. It’s available in both clear and black, but the color doesn’t significantly impact its performance as a vapor barrier.

Aluminum Foil

Aluminum foil is another excellent Class I vapor barrier, often used in saunas or in combination with other insulation materials. Its key advantage is that it not only has a very low perm rating, but it also acts as a radiant barrier, reflecting heat. This dual functionality makes it a great choice for areas where you want to control both moisture and temperature. However, aluminum foil is more expensive than polyethylene and can be more delicate to work with, so it’s not as common for large crawl space applications.

Asphalt-Coated Kraft Paper

Also known as building paper or tar paper, this material has been used in construction for a long time. It consists of one or two layers of kraft paper bonded together with asphalt. This material offers moderate resistance to water vapor, typically classified as a Class II or Class III vapor retarder. While it resists liquid water, it is more permeable to water vapor than polyethylene or aluminum foil. This makes it a suitable choice for wall assemblies in certain climates where a complete vapor barrier might trap moisture, as it provides a balance between slowing vapor movement and allowing the wall to dry.

Extruded Polystyrene (XPS)

This is a rigid foam insulation board, easily recognizable by its blue or pink color. XPS has a closed-cell structure, making it highly resistant to water absorption and providing a moderate to high resistance to water vapor. It’s often used in applications where it will be in contact with the ground, such as on foundation walls, under concrete slabs, and in basements. While unfaced XPS is typically a Class II vapor retarder, some faced versions can achieve a Class I rating. The great thing about XPS is that it provides both insulation and vapor control in one product.

Foil-Faced Insulation

This type of insulation combines a core insulation material (like fiberglass, polyisocyanurate, or mineral wool) with a reflective foil facing on one or both sides. This creates a product that provides both thermal insulation and a vapor barrier. The foil facing, often made of aluminum or aluminized polyester, acts as a vapor retarder or barrier (depending on the type of foil and how it’s sealed) and a radiant barrier. This makes it particularly effective in climates with significant temperature variations, helping to keep your home warmer in the winter and cooler in the summer.

Why Having a Vapor Barrier in Your Crawl Space Matters

If your home has a crawl space, you’ll need a vapor barrier to prevent water from entering this space. A moisture problem in your crawl space can lead to problems such as:

  • Mold/mildew. If your crawl space is damp, it can easily develop issues like mold and mildew. Due to the lack of light and ventilation in the average crawl space, these problems won’t go away on their own—instead, you’ll need to invest in mold remediation.
  • Rodents and other pests. Vermin are naturally drawn to water, so moisture in your crawl space could make it more appealing to them. A moisture problem could leave you dealing with anything from rats and mice to termites and snakes (among many other pests).
  • Rot. Many structural elements of your home (including joists, posts, and beams) are made of wood. Unfortunately, wood can easily rot when it comes into contact with water.

The Benefits of Vapor Barrier Installation

When you install a vapor barrier, you’ll enjoy advantages like:

  • Mold and mildew prevention
  • A less attractive environment for pests
  • Added insulation, improving your home’s energy efficiency
  • A lack of moisture seeping up through your flooring
  • Less risk of structural wood rot
  • Moisture protection for wiring in your crawl space
  • Easier, cleaner crawl space access

Best of all, you won’t have to spend a ton on your vapor barrier. Although pricing will vary based on the thickness of the sheet you use, you should only pay about $0.15 to $1.50 per square foot for this material.

Installing Crawl Space Vapor Barriers: A Step-by-Step Guide

When it’s time to put a vapor barrier in your crawl space, you’ll need to:

1. Inspect the Situation

At the start of this process, take a close look at your crawl space to identify any moisture-related issues it may have. Check whether groundwater is entering from the floor or walls, whether a plumbing leak is affecting your crawl space, and whether or not vents are letting water in. By taking these issues into account, you can make sure you’re giving your crawl space the type of attention it needs.

2. Clean Your Crawl Space

Next, start cleaning your crawl space. As part of this, you’ll need to dispose of any insulation and building material that has sustained water damage. Additionally, be sure to remove any water in your crawl space with a dehumidifier, fans, or a submersible pump.

3. Prepare and Grade the Floor

Once your crawl space is otherwise clean, remove any debris, large rocks, or pests remaining on its floor. After completing this work, rake the soil in your crawl space to create a rough grade.

4. Seal Open Vents

If you have any open crawl space vents, you’ll need to seal and conceal them at this point. By taking care of this, you’ll stop water, pests, and air from getting into your crawl space. Custom-fitted insulation panel blocks can block the interior vent opening, while exterior vent covers can block vents from the outside.

5. Waterproof Your Crawl Space

Does your crawl space get a substantial amount of water? If so, you may want to consider other waterproofing measures in addition to your vapor barrier. For example, a crawl space sump pump can help you manage any water that remains under your vapor barrier.

6. Insulate Your Walls

According to the U.S. Department of Energy, insulating your crawl space walls can prevent problems related to ventilation and provide other advantages. If you’re already working in your uninsulated crawl space, it’s a good idea to take care of this work as part of your vapor barrier installation process.

7. Install Your Vapor Barrier

Once you’ve handled the preparatory work, you’ll be ready to measure and cut your vapor barrier liner. To create a tight fit, overlap the seams in your vapor barrier and secure them with double-sided tape.

8. Create Positive Airflow

Finally, you’ll need to establish positive airflow in your newly sealed crawl space. You can do that by installing a crawl space dehumidifier.

Crawl Space Vapor Barrier
Crawl Space Vapor Barrier illustration

Why You Should Hire Professional Installers

Compared to some home improvement projects, installing a crawl space vapor barrier isn’t overly costly and doesn’t require in-depth technical knowledge. However, completing this process will require considerable effort on your part. Making matters worse, you’ll probably have to do this work in a moist, dirty, and cramped environment.

Fortunately, you don’t need to handle this job on your own. When you hire local crawl space experts to put in a vapor barrier, you’ll get all the benefits that come with this process with zero hassle on your part.

Having your vapor barrier Installed by a professional is more affordable than people think. Nationwide, the cost will range from .50 to $2.00 per sq foot according to homes and gardens (Homes and Gardens). If your crawl space is 1500-2000 sq feet, you should expect to pay $750 – $4000 for your product which includes labor and materials.