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What You Need to Know About Concrete Vapor Barrier

By June 3, 2021Foundation
concrete water vapor

When building or buying a home, we always think that the builder will make sure everything is done properly. The builder will take care and make the home like it was their own. However, sometimes that doesn’t happen as in the case of concrete vapor barriers.

What is a concrete vapor barrier? It goes between the slab and the soil and keeps ground moisture away from the concrete. The problem is, sometimes this is left out of the building process. Have you ever heard of the word, “hygroscopic?” It means to absorb moisture from the air and that is what concrete does so the concrete vapor barrier is necessary.

Let’s take a look at why concrete vapor barriers are important, the polyethylene concrete vapor barrier, vapor retarder, keeping radon out, why moisture is bad, other maintenance, and when to call a professional.

Why Concrete Vapor Barriers Are Important

As mentioned above, concrete is hygroscopic so having the vapor barrier is necessary. Why? If there is no vapor barrier, the floor can absorb the moisture, and then it affects the crawlspace, the basement, or the living area and can cause more problems. Problems such as mold, problems with finished flooring, and other health issues. Having the concrete vapor barrier will help keep this under control.

Here is a video about why a plastic vapor barrier under the concrete slab is important:

Polyethylene Concrete Vapor Barrier

Ployethylene is the material that makes up the concrete vapor barrier. This is laid on top of concrete in the building process so that moisture does not get through.

Concrete vapor barriers should not be less than 10 mils thick and the thicker the better. This will help keep it from puncturing. The American Concrete Institute says that it is important to use a heavy grade, vapor barrier that is non-penetrable and low permeance and put this over a layer of granular fill. Then, pour the concrete on top of it.

The American Society for Testing and Materials (ASTM) says the best way to protect concrete slab and components is : ASTM E 1745 Class A vapor barriers (products with less than 0.01 perms) .

Vapor Retarder

Vapor retarders are often confused with the vapor barrier. A vapor retarder has been used since the 1950s but is a thinner material, usually about 6 mil. It is prone to damage when concrete and reinforcements are added. Unfortunately, holes begin and then the water vapor comes through.

Three Classes of Vapor Barriers

Class I Vapor Retarders: 

  • This type is impermeable, which does not allow fluid to pass through
  • The strongest vapor barrier
  • Examples are glass, sheet metal, plastic sheets, and rubber membranes

Class II Vapor Retarders:

  • Semi-permeable which is some fluid can pass through.
  • Examples are unfaced expanded or extruded polystyrene, 30-pound asphalt-coated paper, or plywood

Class III Vapor Retarders:

  • This type is permeable which is water vapor can pass through
  • Examples are gypsum board, fiberglass insulation, board lumber, concrete block, or brick
Radon gas

Helps Keep Radon Out

This material also helps in keeping radon out of the home. This is because the concrete vapor barrier covers the exposed dirt. Then, a vent pipe or radon fan can be installed to carry the gas out and away from the home.

In addition, some homes that are slab-on-grade foundations are also able to get the radon out. Unfortunately, people think just opening the windows works to get the radon out, but sadly it does not and requires mitigation.

Why Moisture is Bad in Concrete

Moisture is bad in concrete because it raises the surface ph. Then the surface ph breaks down the adhesives on the back of flooring and causes issues like bulging, cupping, or swelling. This then causes a mess in your nice floors. In order to avoid this, a concrete vapor barrier is needed. Also, there is always water near concrete. It may be under the building site, or may even be in the soil and come up through the soil to the surface.

Concrete Vapor Barrier is Missing

What if the concrete vapor barrier is missing? Unfortunately, this will be an expensive repair. There are some people who will tear up the flooring and add an epoxy vapor barrier to the concrete, called CrownCote. On the other hand, others may cut out the floors, install a moisture barrier, and then re-pour the floors. This of course requires the involvement of a professional.

Torn vapor barrier
Photo courtesy of Waypoint Inspection

New Construction of a Home

In the new construction of a home, there are three phases of a home inspection you can have done: Pre-pour, pre-drywall, and final inspection. The concrete vapor barrier is looked at in the pre-pour stage. This is when the home inspector will catch the vapor barrier not applied correctly or it is torn. You don’t want the builder to continue building until that is fixed. This is an extra service to have done so contact a professional before the building process.

Wet carpet

Other Maintenance

Wet Carpet

You may notice that one of your rooms has wet carpet. You may be thinking that is odd. It could be no vapor barrier was placed in between the soil and the slab and the water has risen to the carpet. The other possibility is there is a water leak somewhere in the home. Either way, having a professional check it out is a good idea.

Puddle of Water Near Water Heater

If you find while doing maintenance around the house that there is water by the water heater, there may be an issue. When this happens, the water heater will probably turn itself off as it senses there is a problem. If it does, you may need to find the water heater reset button. If there is no response to pushing this button, you may need to call a professional.

water leak on ceiling

Look Up and Not Always Down

Take a look at the ceilings also in the home. Do you see any stains? If you do, there could be an old leak or a current one. On the top floor, go into the attic and investigate where it could be coming from. Perhaps it is the ceiling below the upstairs, so it could be the bathroom. Homes over 10 years old may have caulking that has worn out and needs replacing. There may also be black mold in the shower on the silicone because of a water leak too.

Check the Water Meter

The water meter is also a good place to check to see if there are any issues. Here is a link on how to check it. This will be a good idea, especially if you have taller patches of grass, leaks from the faucet, wet spots on the floor, or even a running toilet.

When to Call a Professional

Call a professional if you are finding water issues in your home or if you think there may not be a concrete vapor barrier. A professional can help you come up with a plan as to how to tackle this issue. Water damage can wreck havoc in your home so consulting a professional is important.


Concrete vapor barriers are a very important part of our home. Unfortunately, we have to trust the builder and the workers when a home is under construction that the concrete vapor barrier is added. Alpha Building Inspections will look to see if there is a concrete vapor barrier issue during your home inspection in New Hampshire and Southern Maine.