a vapor barrier installed on the ground outside a building as the concrete slab foundation is readied to be poured

If you’re thinking about pouring a concrete slab, it’s essential to understand the basics of vapor barriers. What are they, if you need one, and how you install them. This information will help you make the best decision for your project.

A vapor barrier is a material that prevents moisture from passing through it. It is important in construction because concrete is a porous material that can absorb ground water. This moisture transfer can cause cracking, spalling, and other expensive problems.

This article will discuss everything you need to know about vapor retarders on concrete. It will give you an overview of what they are and how they work. We will also answer some common questions about their installation and use.

And if you’re looking for a calculator to see what installing a vapor barrier might cost you, try out our Vapor Barrier Cost Calculator here.

Do You Need a Vapor Barrier Under Concrete?

The short answer is a resounding yes. You need a vapor barrier under your concrete slab to protect it from moisture damage. This application can be the difference between a durable, long-lasting concrete patio and one that cracks and crumbles in a few years.

Vapor barriers are most effective when installed on top of a solid foundation, such as gravel or compacted soil. However, it would be best if you also placed them under most fill materials, such as sand or dirt.

The most common type of vapor barrier is a sheet of polyethylene plastic. This material is available in various thicknesses, but a 10-mil (0.25 mm) thick sheet is typically used for concrete applications. With proper installation, a polyethylene vapor barrier can last for many years.

However, this is not the only type of vapor barrier available. There are also asphalt-coated paper, foil-faced bubble wrap, and other materials that homeowners can use. While these products may be less expensive, they’re not as durable or effective as polyethylene sheets. 

Then there are also moisture barriers, which are another effective way to keep moisture away. Check out our article on the difference between moisture barriers and vapor barriers for more info.

What Does a Vapor Barrier Do for Concrete?

Here’s the thing: Concrete is a porous material that can absorb water from the ground and other sources. Unfortunately, this can lead to severe problems like cracking, spalling, and efflorescence. And that’s where vapor barriers comes in. 

A vapor barrier prevents moisture from finding its way to the concrete. Waterproofing your concrete is especially important in regions with high water tables or where the ground freezes and thaws. 

The freeze-thaw cycle can cause the concrete to expand and contract, leading to cracking.

a crack in a concrete foundation running up to a vertical beam

Also, a vapor barrier will help prevent mold and mildew from growing on your concrete if you live in an area with high humidity. This protection is vital for the concrete’s structural integrity and your family’s health.

You don’t want to take any chances when protecting your investment. 

Can You Pour Concrete on Vapor Barrier?

You can pour concrete on a vapor barrier so long as it is properly installed. Make sure to use a concrete mix designed explicitly for vapor barriers, and don’t forget to cure the concrete.

A few things you should keep in mind when pouring concrete on a vapor barrier include:

  • Ensuring the vapor barrier is properly installed – you should place it on top of a solid foundation and top of the filler material. The edges of the barrier should be lapped and sealed with tape or another waterproof material.
  • You need to use a concrete mix designed for use over a vapor barrier – these mixes usually have a lower water content, which helps prevent moisture from being absorbed by the concrete.
  • You must cure the concrete properly – keep it moist for at least seven days after pouring. You can do this by covering the concrete with a wet burlap or plastic sheet.

Failure to follow these instructions can result in expensive problems, so you might want to take the necessary precautions.

What Is the Best Vapor Barrier for Concrete Floor?

Polyethylene is the best vapor barrier for concrete floors. It’s durable, effective, and available in various thicknesses. Concrete slabs can last well over 20 years without problems when homeowners use a polyethylene barrier.

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

You can also use asphalt-coated paper, foil-faced bubble wrap, and other materials. However, they’re not as durable or practical as polyethylene sheets. They can degrade over time, allowing moisture to pass through.

We know the world is trying to reduce its reliance on plastic. But when it comes to vapor barriers, polyethylene is still the best option.

Is It Good To Put Plastic Under Concrete?

Yes, it’s perfectly fine to put plastic under concrete. Actually, it’s beneficial—you should put plastic under concrete to prevent moisture from reaching it. As long as the plastic is installed correctly, you don’t have to worry about it leaching chemicals into your home.

The slightest damage to the plastic can allow moisture to seep through. That’s why it’s crucial to ensure that the plastic is appropriately lapped and sealed at the edges.

Besides that, there’s no need to worry about putting plastic under your concrete slab. It’s an effective way to prevent moisture problems.

What Can I Use for a Vapor Barrier Under Concrete?

Polyethylene is the best material to use for under-concrete vapor barriers. However, you can also use tar paper, foil-faced bubble wrap, and even old plastic carpets.

Anything you can put down that will act as a moisture barrier will work. Just remember that some materials are more durable than others. Tar paper, for example, will degrade over time and allow moisture to pass through.

Under-concrete protection is one part of the build you don’t want to skimp on. If you’re going to do it, do it right. Use a material that will last as long as your concrete slab.

How Do You Install a Vapor Barrier on Concrete?

The first step to installing a vapor barrier on concrete is laying down a polyethylene sheeting layer. Then, pour your concrete slab and cure it once it’s in place.

A concrete foundation being poured on top of a vapor barrier

For the first layer, you can use 6-mil or 10-mil (0.15 – 0.25 mm) polyethylene. The sheets should be lapped and sealed at the edges with tape or another waterproof material. Once the polyethylene is in place, you can pour your concrete slab.

If you’re pouring a new slab, make sure to use a mix designed for use over a vapor barrier. These mixes usually have a lower water content, which helps prevent moisture from being absorbed by the concrete.

Once the concrete is in place, you need to cure it properly, meaning keeping it moist for at least seven days after pouring. You can do this by covering the concrete with a wet burlap or plastic sheet.

If you are wondering, you can install a vapor barrier over an existing concrete slab. The process is the same as installing one on bare ground.

What Thickness of Vapor Barrier Should I Place Under a Slab?

You should use a 6-mil or 10-mil (0.15–0.25 mm) polyethylene sheet as a vapor barrier under a concrete slab. These thicknesses are durable and effective at preventing moisture problems.

However, 10-mil (0.15 – 0.25 mm) is the industry standard. You will only use a thinner sheet if you’re putting down a vapor barrier over an existing concrete slab. In that case, you would use a 4-mil or 6-mil polyethylene sheet.

When it comes to concrete slabs, the thicker, the better—a thicker slab is more resistant to cracking and will last longer.

The industry standard for concrete slabs is four inches (101.6 mm). However, we have seen 6-inch (152.4 mm) thick slabs. These are usually used in commercial applications or for driveways and patios that will see a lot of traffic.

How Do I Stop Moisture on My Concrete Floor?

Ok, so the vapor barrier under your concrete floor is not working as it should. What are your options?

To stop moisture on your concrete floor, you should identify where it is coming from. Once you’ve found the source, see if you can patch the leak or remove the water source. If you cannot eliminate the source of moisture, you may have to replace your concrete slab. 

The first step is to find the source of the moisture. This can be done with a moisture meter or by simply looking for efflorescence on your floors and walls.

If the floor feels damp and has white stuff growing on it, there’s a good chance that water is getting into the concrete from somewhere. The most likely culprit is a crack in the slab or around the edges where it meets the wall.

Efflorescence and a crack on a concrete wall
Courtesy of Dalinghaus

If you can’t find the source of the moisture, you may need to remove the slab and install a new one. This is not a complex process, but it should be done by a professional.

Once the old floor is out of the way, you can install a new vapor barrier and concrete slab. This time, use a thicker polyethylene sheet and seal all the edges properly. You may also want to consider using a concrete mix that will work better with a barrier. 

What Are Acceptable Levels of Vapor Barrier Permeability?

The best way to measure the permeability of a vapor barrier is by looking at the permeance rating. This number ranges from 0-100 and represents the amount of water vapor that can pass through the material per minute.

For most applications, you want to use a vapor barrier with a perm rating of 60 or less. This certification will prevent most humidity from passing through the material and causing problems. Still, it is essential to remember that even the best barrier will not stop all moisture.

Sometimes, you may need to use a combination of vapor barriers to achieve the level of protection required. For example, you might use a sheet of polyethylene with a perm rating of 60 under your slab and then install another barrier layer between the concrete and flooring.

Can You DIY Vapor Barrier Installation?

Installing a vapor barrier is not a difficult task. However, it should be left to the professionals.

The reason for this is that there are a lot of things that can go wrong if the installation is not done correctly. For example, if the vapor barrier is not sealed properly, moisture can still get through and cause problems.

Additionally, if the barrier is not installed correctly, it can void the warranty on your flooring. This is why it is always best to hire a professional to install a vapor barrier.

What Is the Cost of Vapor Barrier Installation?

The cost of installing a vapor barrier will vary depending on the size of your space and the type you choose.

Generally, you can expect to pay between $0.50 and $1 per square foot for the material. This figure does not include the cost of labor. Expect to pay between $1.50 and $2 per square foot for the work.

Several workers pouring a foundation over a vapor barrier
Courtesy of W.R. Meadows

This means the total cost of installation will be between $2-$3 per square foot. Compare this to the cost of replacing a damaged floor, which can be upwards of $10 per square foot, and it is easy to see why vapor barriers are such a good investment.

Are There Any Drawbacks to Using a Vapor Barrier?

There are a few potential downsides to using a vapor barrier. First, it can cause more harm than good in the wrong situation. For example, a barrier can trap moisture and promote mold growth if you live in an area with high humidity.

In addition, some can make it difficult for your floor to breathe, causing problems like cupping and warping. Finally, vapor barriers can be challenging to repair if damaged, which is why hiring a professional to install them is so important.

Final Thoughts

Despite a few potential drawbacks, vapor barriers effectively protect your floors from moisture damage. They can add years of life to your floor and save you a lot of money in the long run.

If you are thinking of installing one in your home, hire a professional to do the job. This will ensure that it is installed correctly and will not cause any problems.

Now that you know everything there is to know about vapor barriers, you can make an informed decision about whether or not they are suitable for you. 

Thanks for reading, and good luck!

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