Concrete Vapor Barriers – Understanding Concrete Moisture Problems

Concrete Vapor Barrier before slab installation

Anyone that has had to face a moisture problem with a concrete slab understands the damage that excess moisture can cause. Moisture in concrete can occur from a variety of sources: ground moisture that contacts the slab through either capillary action or as water vapor, high air humidity or drastic changes in relative humidity in its environment, leaking plumbing that passes through the slab, and more.

Excess moisture can cause pH changes in concrete that adversely affect adhesives. Even an excess of moisture that was retained from the original concrete mixture will cause problems if the slab was sealed prematurely.

It should be obvious – concrete starts wet. The water added to the cement, sand and aggregate mixture is necessary to form a good bond in the concrete, but it is also necessary that the moisture evaporates away from the concrete in order to let the concrete dry and to prevent flooring failures.

Adding more water may make concrete more workable but it also means the drying time can increase to unreasonable levels as the rate of evaporation is dependent on a number of variables.

While moisture can be added to the slab through a number of different sources, it can only evaporate away via the slab’s surface. It often can’t escape the slab without remediating treatment. Simply put, a concrete slab in contact with moisture cannot be brought to constant “dry” conditions. Something has to stop the migration of moisture into the slab.

The preventative measure most often used to try to circumvent these contact moisture-related challenges is to install a vapor barrier. But what type and where it should be installed has been a subject of much debate. Some feel that vapor barriers contribute to curling in the slab and that simply casting on a granular slab should be sufficient. Others feel a vapor barrier is a critical element to protecting from flooring or adhesive failures, and even some environmental issues, as vapor barriers can prevent moisture-related mold and mildew growth or even block certain gasses that can pose a health risk if they accumulate in a home or business.

Technically, any material that resists moisture passage is a vapor barrier, a term often used interchangeably with a vapor retarder. But very few are true barriers as they still have a low level of permeance, or the ability to let water vapor pass through. This is often expressed as “perms” that allow for categories of permeability. The degree of permeability that is acceptable may depend on the application – for example, a water vapor permeance less than 0.3 perms is generally recommended but for residential use, a higher permeance rate is considered acceptable. What seems to be crucial is that the vapor barrier or retarder under the slab must have a lower degree of permeance than the flooring or floor covering above the slab. Otherwise, the potential for a moisture imbalance can still cause flooring failures over time. ASTM International gives specific guidelines in ASTM E1745-09 and ASTM E1643 for the use, installation, and inspection of vapor barriers used under concrete slabs.

Vapor Retarder Illustration

Currently, a vapor barrier is typically applied over a layer of granular fill to try to minimize the wicking effect of ground moisture. Of course, if the vapor retarder is penetrated, it will not be effective in preventing moisture migration. Seams must be properly sealed, heavy grade materials must be used where traffic may cause a penetration, and the lowest possible grade of permeance should be used. The concrete slab is then poured directly over the vapor barrier. (This was not always the case. Prior to the 2000s, a “blotter” layer was recommended between the vapor barrier and the slab. But this practice changed as the difficulties of keeping the blotter layer moisture-free became evident.)

It must be noted that this practice, though, causes slower drying times because moisture cannot leave a drying slab through an impermeable sub-layer. With the vapor barrier directly under the slab, moisture then has to move to the surface of the slab and evaporate from there. Placing the vapor barrier under the granular layer let contractors float the concrete sooner and appeared to dry the concrete more quickly because the base accepted some of the water from the original concrete mixture (and, some argue, create more potential for water reserves that could then absorb back into the slab to create adverse conditions).

Because there are pros and cons to both situations, accurate relative humidity testing is also necessary to be sure slab moisture conditions are ready for the flooring or finish application of choice. Moisture-sensitive adhesives and applications can be just as adversely affected by a moisture level too high in the slab as they can by moisture-wicking problems due to an inadequate vapor barrier. Relative humidity testing can give an accurate picture of what the final result of a sealed concrete slab will be when it is fully equilibrated.

There still may be situations where a vapor barrier is necessary over a concrete slab. Even when a slab is dry enough for the recommended use, it is still possible for moisture to migrate from the slab into porous or absorbent flooring materials like wood. The danger is that the wood will swell and crack as moisture moves in and out, and that adhesives will also fail from the changing moisture levels. In these situations, a second vapor barrier is often recommended between the flooring and the concrete subfloor.

There is only one chance to make the best vapor barrier choice, and obviously, that is before the slab is poured. For each concrete slab, it is imperative to understand the best vapor barrier choice based on local conditions, recommended guidelines and careful application. With the correct vapor barrier under the slab, and a knowledge of slab moisture conditions before sealing the slab, moisture conditions shouldn’t be anything to give you the vapors.

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Jason Spangler

Jason has 20+ years' experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.


  1. Bob says:

    I need advice on a product that I can paint (or apply) to a cement floor on a porch that is being converted to a sunroom. The building inspector for my community says I need to apply a vapor barrier to the cement (it has none underneath) to avoid future mold problems. I need a product that will allow me to put down tile, wood or laminate products. Thanks!

  2. lisa says:

    I have a living room that used to be a garage. We put down laminate flooring that is now warped. We haven’t had any leaks so we are assuming it is coming up from the concrete. We put down the recommended barrier with the plastic backing before laying the floor. Now we don’t know what to put down. We had thought about the vinyl laminate flooring but we are afraid it will mold underneath. My son has severe lung disease and cannot have any mold or mildew. We can’t afford to keep trying different things. Please help!!

  3. Jason Spangler says:


    Sorry you are having these issues. Unfortunately, the re-purposing of areas, whether commercial or residential, can lead to problems like this. In your case, it would not surprise me if there wasn’t a vapor retarder below that slab because it was never intended to “live inside” or have a floor installed on top of it. Now that being said, people re-purpose all the time. So, in my opinion, you have two options:

    1) They make VERY good products, intended to be put on top of the concrete slab, that restrict the moisture movement out of the concrete to a level that will not effect most flooring. These usually aren’t inexpensive, but they are better that some alternatives like replacing flooring or replacing the concrete slab. Google “epoxy moisture mitigation”.
    2) Try to find a flooring product that is more “breathable” and/or less moisture sensitive. Say 12″ X 12″ tile or a breathable carpet.

    Obviously, these are just a couple of suggestion.

    Good Luck.

    Jason Spangler

  4. Jason Spangler says:

    Hi Will,

    I’m not sure I understand your question. Can you please provide further clarification? Thanks.

  5. Gary says:

    I have a concrete slab on grade with a plastic vapor barrier below. Should I put a 2nd vapor barrier above the concrete floor before putting down floating laminate floor?

    • Jason Spangler says:

      Thank you for the question Gary. I would refer to the laminate manufacture’s recommendations. If you are unable to find the answer in their written documentation, I would call their technical center.

  6. Andy B says:

    I’m curious we purchased an older cabin that’s on a cement slab when we opened the doors after the winter we found that the floor sweats ALOT! I’m assuming whoever laid the concrete slab did not put a vapor barrier below it 🙁

    We thought well we’ll put on on top and throw some laminate over it, but I worry would that trap the moisture below and create a mold issue on the concrete, or would a vapor barrier and an insulation product on top stop it from sweating so much anyhow? Thanks for any time and insight you may have it is appreciated.


    • Jason Spangler says:


      Thanks for the comment/question. You state the floor “sweats”. This could actually be a dew point issue, instead of a concrete moisture issue. You can measure the temperature and relative humidity of the air in the cabin, then plug those numbers into this calculator: Once you have done this, it will show you what the dew point temperature is, based on those inputs. You can then measure the surface temperature of the concrete. There should be more than a 5 degree temperature variation between actual and calculated temperature to avoid condensation (sweating).

      As far as installing the laminate, I would contact the manufacturer of whatever product you chose and have them give you some recommended solutions. My fear would be more about the floor being ruined than mold. Just my opinion.



  7. Kathryn S. says:

    Hi Jason,
    I live in Las Vegas where it is hot and dry. I’m wanting to lay plywood, cut and stained to look like hardwood floors over our concrete foundation. The “planks” of plywood will use the width of a penny for spacers and using a combination of adhesive and concrete nails to secure. Do I need to seal the concrete before I install? If so, what would I use?

    • Jason Spangler says:


      Thank you for the question. I think you may be best served contacting Although this is not specific to them, they have a recommended installation process for hardwood flooring that utilizes installing plywood on the concrete, prior to the finished product. This may help you in your process.


  8. Arrie Colca, CASE says:

    Great information. I have a client that owns older apartment communities ranging from 400-275 units each. Due to expenses of resident turnover, they decided to remove carpet/pad/tack and replace with sheet vinyl everywhere excluding Bedrooms, on the first floor for each of the properties. Would vinyl planking be a better choice due to fact it would allow for vapor permeability. There are huge failure issues with bottom up staining, i.e., mold and mildew, and they are determined to stick with vinyl. Oh and we use moisture/vapor kits, and a good sheet vinyl with a warranty against this type of problem…doesn’t seem to make a difference. Your thoughts would greatly be appreciated.

    • Jason Spangler says:


      I can’t say whether vinyl planking would be better, but I would at least investigate it. I would also look to see if potentially doing one of the planking systems that allows for a floating installation may not bring something positive to the table. Good luck!

  9. Daniel Reffo says:

    Hi Jason,

    I’m considering building a new home and the new home builder does not install a vapor barrier underneath the slab in the basement. They have stated that a pea gravel underlay is enough to take care of any possible moisture issues. However, the build location is next to the retention area for the neighborhood and was making squishy noises when I walked on the ground today. (we’ve had copious amounts of rain over the last two weeks) They promise that once we get grass down on the ground and the ground is graded we should have no issues with moisture. Should I be concerned?

  10. paul says:

    Hi Jason
    We are laying a new basement floor for bedrooms and new lounge downstairs we are doing the footings 1st – would you recommend laying the plastic barrier in the footings trench also before we do the entire floor? obviously we will lay plastic for the entire floor but thought would be double protection under the footing trenches and posts to protect from moisture.

    • Jason Spangler says:


      Thanks for the comment/question. I would always have to defer back to your local building code on this. Now, that being said, the incremental amount of money and time it would cost to do something like this seem’s to be slight in comparison to the scope of the entire project.



  11. JAS says:

    We currently have hardwood flooring (12mm) installed on our slab foundation. We do not know if a moisture barrier was utilized or not. We have a very long run of approximately 34 ft.
    Several areas including the center of the 34 ft run have become separated and have quite a lot of play in them. Two shorter run areas have screws that we have discovered that are supposedly to keep the floor from buckling and the screws are in about 24 ft runs.
    We are wanting to replace the flooring. We know we will need a moisture barrier, but because of the issues feel we need to use engineered hardwood vs. natural hardwood.
    Do you have any recommendations? What barrier type is best? We were looking at a lumber liquidator barrier with pre-applied adhesive and would possibly glue the boards in the area where the runs are greater than 27 feet…
    And is natural hardwood not an option? And is engineered hardwood any better?
    AND finally does it matter what we choose if we place a high performing barrier?
    Thank you for your time.

    • Jason Spangler says:


      Thanks for the questions. If this is a slab on grade vs slab below grade (which you don’t specify) then I am not sure why solid wood would not be appropriate. I would consult the National Wood Flooring Association at No matter what material you chose, make sure you consult the manufacturer to ensure you are addressing the “long runs” appropriately. As far as barriers, especially if you are using one to compensate for the possibility that you don’t have one below the slab, you get what you pay for, in my opinion. Always read the warranties.


  12. D Speelman says:

    Building a Lake house and we are concerned that our builder forgot to place the vapor barrier down prior to the pour of the slab. The house is framed, but before we go any further would like to know how we can go about getting objective proof of having a barrier. Should we cut a hole (core) in order to verify? If we do this, is there a specific location that is best?

    • Jason Spangler says:

      D Speelman:

      Thanks for the question. Usually performing a core test is the best way to confirm the presence of a vapor retarder. I would consult a local geotechnical company for additional information regarding performing the service.



  13. Terri says:

    Hi Jason,
    I live in Miami, FL bought a two story 1974 townhouse last August 2015. The downstairs had 12 x 12 tile, I replaced with 30 x 30 porcelain tile in November. In May I noticed small droplets of water seeping out of the grout lines in several areas, by August 2016 there was grout discoloration in several areas as well as water seepage. The seepage continues. Could this be caused by moisture in the slab or no vapor barrier? I ruled out plumbing by a pressure test, and ruled out AC by diverting the drain. I’ve also had the roof and windows inspected. What should my next move be, don’t know who to reach out to. Have already spent $5,000 trying to solve this issue. I’m desperate for resolution.

    Thank you,

    • Jason Spangler says:


      Thanks for the comment. You could be having issues with the moisture in the concrete or some type of dew point/condensation issues. Obviously, there could be other culprits, but these are the two that come to my head immediately. I would reach out to an organization like

      They are a network of flooring inspectors. Have one of them evaluate and give you some input. I hope this helps.



  14. Terri says:

    Jason, thank you so much. I will try them. Best regards,

  15. Joseph says:

    Hi Jason. We had a slab poured in the crawl space below our house this summer. The contractor removed the visqueen, poured the cement on the dirt in the crawl space, and then put the plastic back on top of the cement after it cured. I believe we’re now getting condensation between the plastic and the cement. The floor is not even and has lots of peaks and valleys and rough surfaces (since it’s a crawl space that no one walks on, we did not deem it necessary to even out). Water is puddling up in the low spots (less than an inch) and is appearing as a thin layer everywhere else. I don’t believe it’s capillary action from the water table because when I pull back an area of the plastic, that area dries out in a day or so. This leads me to believe it’s condensation. I spoke with the contractor and he said that as long as the water is not touching wood, there’s no concern for black mold on the wood, but I’m afraid we’ll get mold growing on the cement and visqueen and am searching for a solution. I see your mention of “epoxy moisture mitigation” in a previous post here, but because the surface is so uneven, I’m not clear on whether that’s an option or not. My understanding is that we need some kind of vapor barrier to protect against radon (even though from my research it seems the Pacific NW has a low radon hazard), as well as needing some means of keeping the cement dry. Do you have any suggestions? Thanks much for your help. – Joe

    • Jason Spangler says:


      Instead of going back and forth on this (I have questions), why don’t you just give me a call: Monday-Friday 7:30am-4:00pm at 800-634-9961 X235.



  16. linda levy says:

    my mother had a slab leak inMay and it flooded her whole house. we had a plumber come out and fix it by replacing the piping that had a hole in it. He then closed up the hole with concrete and she put down bamboo flooring thoughout thehouse. abt 2 wks ago we noticed that some of the wood around where the hole was was turning black. We called in a leak detection co and they preformed all their tests and there were no leaks.Then we called the flooring people in and they pulled up the wood where the black was showing and there was moisture between the moisture barrier that was put down and the wood.the wood was showing mildew and the moisture was between the barrier and the wood. We had a moisture test done on the concrete with calcium chloride (72 hrs) and that showed no moisture in the concrete, it was just between the moisture barrier and wood. Any ideas? Thank you

    • Jason Spangler says:


      Thanks for the question. Based on how you outlined the scenario of events, I am assuming that the new bamboo was laid shortly after the floor was patched with fresh concrete? This being said, I have a hard time believing that the concrete was dry enough for flooring installation. I would want to test in the concrete section (in situ relative humidity testing), not just the surface of the concrete.

      Another possibility, or it could be some combination of both, you may be dealing with is some type of condensation issue, but I would think there would be signs on the rest of the floor if that were the case. I hope this helps.


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