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Why Are Vapor Barriers So Important in Concrete Construction?

  • March 27, 2024

Whenever there is a process that can ensure the longevity and quality of a structure, it is well worth the time and effort.

Installing a vapor barrier under your concrete slab is one of those processes. 

This article will cover:

  • Just what is a vapor barrier
  • Why they are needed
  • Where and how to install one

…and a few more other good tidbits of information about vapor barriers. 

Table of Contents

1. What Are Concrete Vapor Barriers?
2. Why Vapor Barriers Are Needed
3. Vapor Barrier Permeability and Thickness
4. How to Install a Vapor Barrier

What Are Concrete Vapor Barriers?

A vapor barrier is a material made from a special plastic that prevents moisture from entering a concrete slab. It is not an underlayment, but some underlayments can function as vapor barriers.

Vapor Barrier vs. Vapor Retarder

Years ago, vapor retarders were used to protect concrete floors from moisture. As time went by, the need for greater waterproofing was evident.

A vapor retarder is made from a more flimsy material, a 6-mil polyethylene plastic sheet, that prevents only part of the moisture from entering the concrete slab. 

Also, because they’re so delicate, vapor retarders often get torn during placement. These holes allow even more moisture through.

Vapor barriers, on the other hand, stop all moisture vapor from entering the concrete slab and are highly puncture-resistant. 

Why Vapor Barriers Are Needed 

According to ACI PRC-302.2-06: “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials,” vapor barriers are crucial in certain circumstances.

Generally, it is better to overprotect than underprotect. Adding a vapor barrier cannot do any harm, even in an area that doesn’t present a high moisture content. 

Vapor barriers should be installed for the following reasons:

Moisture is Destructive

No matter where you build, there is water somewhere in the soil under the slab. The capillary break from the sub-base stops water from rising but not water vapor.

When this water vapor enters the slab, it can cause a lot of damage. 

This moisture can weaken aggregates in the concrete, reducing the structural integrity of the slab. It can even make the slab curl. 

Moisture could also break down adhesives such as epoxy or on flooring above the slab. This can cause many flooring failures, such as curling, swelling, or cupping of the floor coverings. 

Even if no flooring is laid over the slab, moisture can delaminate the concrete sealer and discolor acid stains.

Coating failure is often caused by the flow of this moisture vapor, known as vapor transmission. This overlay bond failure occurs over time, resulting in flooring problems months or even years down the road. 

Metals Can Corrode

When moisture is present, rust or corrosion can compromise metal ductwork or pipes above or in the slab.

Even coated, galvanized, or stainless steel can eventually be affected. When corroded, rebar, which can match any preceding steel type, reduces the concrete’s tensile strength. 

A vapor barrier protects this metal by removing moisture from ever reaching it from the ground.

Electrical Components Are at Risk

Conduit or plastic sheathing won’t fully protect wiring from damage caused by water vapor.

Vapor can cause major electrical issues and unsafe conditions. When moisture reaches the electrical components, they could short. 

Furthermore, the minerals and salts in the water vapor can be deposited onto the wires. They are considered corrosive agents and eat away at any metals in the circuitry. 

It Also Stops Gases and Chemicals 

Toxic gases can be present under the surface of the ground, including

  • Radon
  • Methane
  • Hydrocarbons 

These gases are hazardous to our health. Radon is the second largest cause of lung cancer, and methane can result in asphyxiation.

A vapor barrier can successfully block these gases from reaching the inside of the building envelope. 

Destructive chemicals, such as solvents and oils, seep up from the ground and damage concrete if they come in contact with it. 

Vapor barriers protect from these chemicals as well. 

It Protects The Indoor Environment

When water vapor seeps through the concrete and into the indoor air, it can raise this environment’s relative humidity. This excess humidity will be absorbed by the porous materials in the building, such as walls and furnishings.

These damp materials allow mold and mildew to grow. The spores can float off the materials, causing unhealthy air quality. Frequent exposure can result in allergies and other health issues (not to mention an unpleasant and musty smell). 

Installing a vapor barrier ensures inhabitants are protected from these dangers and discomforts.

It Saves Time and Money

Granted, adding a vapor barrier to your slab construction will increase the project’s overall cost. However, the price of a vapor barrier is much less than an expensive repair that could result from opting out of one. 

As we’ve discussed, a lack of a vapor barrier can result in the need to replace flooring and repair concrete cracks and imperfections. It also provides extra insulation, reducing energy costs for heating or cooling the building. 

Vapor Barrier Permeability and Thickness

Two factors determine the level of protection a vapor barrier provides: the water vapor permeance and the thickness of the material. 


The permeability of a vapor barrier is the measurement of how much vapor can pass through the material. 

According to ASTM E-1745: “Standard Specification for Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs,” a vapor barrier must have a .03 perm rating or lower.

Any more permeability of the material means it can no longer be considered a vapor barrier. It can only act as a vapor retarder, which provides less protection. 

There are three levels of permeability: 

Class I

This is the highest class of vapor permeability. It has a .01 perm rating or less and is considered impermeable.

Some materials that meet this classification include glass, sheet metal, polyethylene film, aluminum foil, and insulated sheathing with a foil facing.

Class II

The second class is considered semi-permeable. It still has a low permeance but isn’t as protective as Class I.

Class II has a rating higher than .01 but lower than 1.0 perm. Some of these materials can work as vapor barriers, while others are only vapor retarders. 

Examples of Class II permeance materials include bitumen-coated kraft paper, extruded polystyrene, and plywood.

Class III

The last class has a 1.0-10 perm rating. These materials are not useable as a vapor barrier but could work as a vapor retarder.

These include latex-painted gypsum board, #30 building paper, and cellulose insulation.


For vapor barriers to provide the level of protection necessary, they must be resistant to tears and punctures. In order for it to be considered puncture and tear-resistant, it must have at least a 10 mil vapor barrier, but thicker is even better. For commercial projects, a 15-mil thickness is advised to ensure the vapor barrier remains intact. 

How to Install a Vapor Barrier

Before we discuss the steps to install a vapor barrier, let’s identify where to install it. 

There are three places where a vapor barrier can be installed

For residential projects, the vapor barrier can be placed in a crawl space under the floor or below grade under the basement floor. 

However, this article is meant for commercial construction, which requires installing the vapor barrier over the crushed gravel sub-base. If necessary, you can add a thin layer of sand over the gravel to minimize the tearing of the under-slab barrier. 

1. Unroll Barrier 

Unfold the barrier and roll it out to completely cover where the slab will be poured. Overlap all barrier seams at least 6 inches to ensure no gaps remain. 

Make sure your barrier extends past the edges of the slab perimeter. The excess barrier should be able to terminate past the height of the top of the slab to be poured.

2. Clean and Tape

Use a cloth to clean off any moisture, dust, or debris on the surface of the barrier. Even a tiny spec of dirt or a little bit of moisture could break the seal. 

Once you are sure the surface of both seams is clean, tape the seams together to form a sealed barrier. Seal around pipes that penetrate the vapor barrier with tape or mastic.

Vapor barrier tape has a strong adhesive on its underside to grip the barrier tightly, while it has a textured topside that will bond with the concrete poured over the barrier. 

3. Seal Perimeter

The edges of the vapor barrier must be sealed against the foundation in some way. This can be done against a wall, on the footing, or against the side of the slab itself. 

Each project may have different requirements for terminating edges. The designer or engineer on site should make the instructions clear. 

Use barrier sealing tape, mastic, or a combination of both to ensure a strong and complete seal around the perimeter of the foundation. Tape can be used for long, flat surfaces, while mastic can be utilized on rough surfaces and where precise application is needed. 

A common mistake to avoid is sealing the barrier edges to a temporary concrete form. At some point, this form will be removed, and so will the sealed edge of the barrier. 

This would require you to remove the tape and find another area to seal the barrier to, which may or may not be reached with the amount of barrier material you have. 

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4. Check For and Repair Any Damage

Carefully look for any weakness or tears in the vapor barrier after you’ve completed installation and sealing. If any are present, cover or repair them with tape. Make sure you clean the area well before applying the tape. 

Larger tears can be repaired by cutting another piece of vapor barrier at least 6 inches bigger than the tear in all directions. Lay this piece over the tear and seal it with tape around the perimeter.

5. Pour Concrete 

Now that your barrier has been installed and sealed, you can pour your concrete directly onto it.

As the concrete is poured, check that no seals or tape come loose. If this occurs, stop the pour and reseal the area, then complete the pour.


Installing a vapor barrier in concrete construction is smart, especially for commercial projects. It just doesn’t make sense not to protect your investment from the damaging effects of moisture.

For a vapor barrier to fulfill its purpose, it must be correctly installed. Hiring an experienced and professional commercial concrete contractor is your best bet. 

Our team at FMP Construction has over 70 years of combined experience in building sales and construction, and we pride ourselves on delivering the highest-quality commercial construction in the area. 

Contact us today to discuss your next commercial project.

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