Concrete Vapor Retarders: Everything You Need to Know

What is a Concrete Vapor Retarder?

A concrete vapor retarder is any material that prevents moisture from entering a concrete slab. Vapor retarders are used because while fresh concrete is poured wet, it’s not supposed to stay that way. It needs to dry and then stay dry to avoid flooring problems.

Now that you know what a concrete vapor retarder is, in the article, we’ll discuss the following:

If you’ve ever had a problem with a basement floor (or any concrete floor), you know the kind of damage that too much moisture can cause. Moisture enters concrete in various ways, including via the ground, from humidity in the air, and through leaky plumbing that passes through a slab.

Of course, the moisture was also in the original concrete mixture.

concrete vapor barrier underlayment

There are only one-way moisture leaves concrete, though, and that’s via its surface. If you have a concrete floor in continuous contact with a source of moisture, you will have problems.

This is why a vapor retarder under concrete is essential. Vapor retarders are a way to keep moisture from getting into the concrete.

Note: A vapor retarder is not the same as an underlayment. However, some underlayments act as vapor retarder.

Vapor Retarder Permeability is Expressed in Perms.

Vapor retarders have varying degrees of permeability, expressed in perms. The higher the number, the more permeable the material.

You’ll hear people interchangeably using the terms ‘vapor barrier’ and ‘vapor retarder.’ In this article, we will use the term ‘vapor retarder.’

What’s an Acceptable Degree of Vapor Retarder Permeability?

The acceptable degree of vapor retarder permeability depends on the application. While a water vapor permeance of less than 0.3 perms is recommended, a higher permeance rate is usually considered acceptable for residential use.

However, the vapor retarder under the slab must have a lower degree of permeance than the flooring (or floor covering) above the slab. A moisture imbalance could eventually cause a flooring failure if it doesn’t.

ASTM International gives specific guidelines in ASTM E1745-17 and ASTM E1643 for the use, installation, and inspection of vapor retarders used under concrete slabs.

concrete vapor retarder

Watch are video to find out what you need to know about concrete vapor retarders.

Why is Too Much Moisture in Concrete a Problem?

One word: adhesives. Too much moisture in concrete is a problem because it can cause pH changes that destroy adhesives. Here’s what happens.

Excessive moisture in the concrete allows soluble alkalies in the concrete to move to the flooring adhesives causing the causes the adhesives to break down. After this happens, the adhesive can no longer hold down the floor covering. This can result in flooring failures such as swelling, bulging, or other problems.

Shop Rapid RH L6

Do You Need a Vapor Retarder Under a Concrete Slab?

In a word, yes. Here’s why.

There’s almost always water underneath a building site. It may not be near the surface, but that doesn’t mean it’s not there. This water can move up through the soil and come into contact with the bottom of a concrete floor via capillary action.

Capillary action can be stopped by installing a capillary break, a layer of crushed rock between the subgrade and the slab.

Capillary breaks do a good job of stopping water in its liquid state from reaching a slab. However, they can’t stop water in vapor form from reaching and entering a concrete slab. Therefore, there needs to be something underneath the slab that prevents vapor moisture from entering.

You might also need a vapor retarder for liability reasons because most manufacturers of flooring include vapor retarders in their installation guidelines.

How Thick Should a Plastic Concrete Vapor Barrier Be?

According to the Guide to Concrete Floor and Slab Construction published by the American Concrete Institute, a vapor retarder should not be less than 10 mils thick. (A mil is one-thousandth of an inch.) You might need an even thicker barrier if you cover the material with sharp angles.

Bottom line: Vapor retarders need to be strong enough so they don’t easily puncture. If they do, moisture will get in, and that’s what you’re trying to keep out.

What Can I Use for a Vapor Retarder Under Concrete?

Most vapor retarders are created using polyethylene or polyolefin sheets that are strong enough (at least 10 mils thick) to tolerate the kind of heavy construction activity that goes on over concrete subfloors.

Free Download – 7 Things You May Not Know about Concrete Slabs

Where Should a Vapor Barrier Be Installed?

What type of moisture retarder should be used and where it should be installed is debatable. Some think vapor retarders can cause slabs to curl and that simply pouring concrete directly onto a granular base (gravel, crushed rock, etc.) should be enough.

Others see vapor retarders as essential and argue that they prevent adhesive failures, retard the growth of mold and mildew, and even prevent certain noxious gasses from entering a building.

However, the current practice recommended by the American Concrete Institute is to apply a heavy-grade, non-penetrable vapor retarder with the lowest possible permeance for the application over a layer of granular fill (crushed rock, gravel, etc.). The concrete slab is then poured on top of it.

Note: An earlier practice for vapor retarder involved placing a “blotter” layer between the vapor retarder and the concrete slab. This eventually fell out of use because keeping the ‘’blotter’’ layer dry was hard.

Generally, you’ll want to use a low-permeance vapor barrier when protecting a slab that moisture-sensitive materials like adhesives and floor coverings will cover.

Shop Rapid RH L6

Last updated on April 16th, 2024


  1. freedomcrawlspace says:

    Understanding concrete vapor retarders is crucial for maintaining a durable and moisture-resistant foundation. By preventing moisture infiltration, these barriers protect against flooring damage and adhesive failures caused by excessive moisture. Whether residential or commercial, ensuring the correct type and thickness of vapor retarder under concrete slabs is essential for long-term structural integrity and to meet flooring manufacturer guidelines. This proactive approach guarantees peace of mind for property owners.

  2. RICK DUNN says:

    I come at this from abuildr who does a lot of new work. I insist that a vapro barrier be installed. Typically 15 mil and fully taped and sealed regaurdless of teh ground conditions. There are several reasons for this. Among these include an effort to stop moisture intrusion into the slab from below damaging applied floor finishes and promoting slab curl. Vapor barrier also helps you stabilize the ground moisture to reduce movement from building forces applied to the soil. From a slab quality point of view, vapor barrier helps you control slab shrinkage cracks and hydration during slab finishing by forcing all the moisture in the concrete to exit up to the surface, rather than being stored in the soil. This results in a higher quality slab.
    Slab curl of a piece of concrete is a result of moisture staying in the slab near the center the piece while the edges (whether control joints or construction joints) dry out. The moisture in the center of the piece occupies space while the drying near the edges opens space and allow the edges yo shrink, and consequnetly curl.

  3. rubber driveway says:

    This article provides valuable insights into concrete moisture testing. The comprehensive information on preventing moisture-related issues in concrete is enlightening. The writer’s expertise shines through, making it an excellent resource for professionals and DIY enthusiasts alike.

  4. Steve Deuchars says:

    Does any of this apply if there is never going to be heating/cooling in the structure? Planning a shed on a concrete slab and don’t know if the same requirement for vapor barrier applies.

  5. Alan Franklin says:

    Hello, and thank you in advance for answering my question. I live in South Florida where it is very wet and the ground water level is very close to the ground surface. I hired a contractor who is not completing the work in a timely manner and as a result, the vapor barrier has been left exposed to rain, sun, humidity and the elements all summer (which is the brutal season down here). I also have lost all confidence in this contractor, and I am concerned that she does shoddy work. How can I check that the vapor barrier is installed correctly and not ruined, or should I just pay to have someone redo it?

    Thank you very much.

  6. Sofia says:

    I had a small room addition built as an office, or even to put plants in like a sunroom in the winter. I was quoted and the contract was for interior grade concrete floor with a moisture barrier. I came home from work and the concrete was only partially poured with no barrier, which I didn’t think about until later. It is maybe 6″ or more thick and it’s over a decomposed granite/sand type area in a relatively dry area of California. So I am going to install teak interlocking deck tiles to finish it as it seems my options are limited without the moisture barrier. Is there anything I can or should do to seal the concrete to keep moisture out and to keep the concrete unstained? When I asked the foreman, he said it didn’t call for interior grade. When I showed him the contract he said they wouldn’t redo it.

  7. Steve Simon says:

    Hi Jason! Great article and very informative, and timely for me! I have an attached garage (in NY) and in the process of redoing my concrete floor. Long story short, I had an epoxy coating placed over the 70 year old concrete floor and things went very bad! Horrible smell that wouldn’t go away in 2 years, and discoloration and slippery. So last month had a contractor demolish the floor and plan to repour with 4 inches next month. So it’s just a clay / dirt like foundation today.
    My garage always has been VERY humid in the summer, and can tell even more so now visually looking at the clay / dirt. My concrete guy was planning to put down a vapor barrier prior to pouring the concrete. From your article sounds like I should ask for 10 mils. Would this vapor barrier help the humidity issue in the garage once the concrete is poured? Or would it seal in the moisture and cause other issue? THANK YOU for any thoughts!!!! Steve

  8. Lucian says:

    Home inspector in Texas, I had a builder tell me that holes and rips in the polyethylene membrane under a post tensioned monolithic slab are no problem(around plumbing penetrations and were pieces overlap as well) and that the concrete crew poking holes in the membrane as they re pouring the concrete is common practice. Is that true?…

  9. Anabell Odisho says:

    Hello Jason,

    Thank you for taking the time to respond to all of these questions! We are currently at our wits’ end.

    Our house in Dallas was built in 1965 with a slab foundation. We purchased the home in 2019.

    In September 2021, we noticed buckling and moisture in the living room (hardwood floors) under the couch. We conducted a plumbing test, and the test indicated a sewer leak only, ruling it out as the cause of the moisture in the living room. The plumbing test didn’t indicate a clean water leak.

    Repairs for the sewer leaks were scheduled for February 21, 2022. On February 9, 2022, the floors in the living room completely buckled, and water was present – the extreme buckling occurred in another area of the living room, not where we first noticed moisture under the couch. We were told to wait until February 21 when the Leak Company would come out to fix the sewer leaks, and on February 21, the company detected a cold water line leak.

    Repairs were made to the cold water line leak by rerouting through the walls, so there was never a hole in our floors. All repairs were completed in March 2022.

    In July 2022, we had our floors replaced with engineered wood and confirmed through the flooring company and the invoice that a vapor barrier was installed, and they did conduct a moisture reading.

    In February 2023, we started to notice discoloration and moisture blistering on our floors in the living room, in the exact same area. We conducted a plumbing test in March 2023, but no leaks were detected – sewer or clean water.

    Since then, we’ve conducted several more tests, and no leaks were found. We had our chimney inspected and underwent a water test where water was placed in the chimney, and then through thermal imaging, it was determined that there were no leaks.

    I do want to note that our neighbors told us that the previous owner in 2017 was a do-it-yourself person. There was a clean water leak in the living room (in the same exact area), and he fixed the leak himself by creating a hole in the floor.

    That being said, the building engineer who came out said that it is possible the vapor barrier under the slab has been compromised, and that may be the reason for the problems in the living room. We’ve contacted the flooring company, and they have the wood to replace the damaged pieces, but what do we do if the issue is a compromised vapor barrier under the slab? Can they put additional vapor barrier?

    • Jason Spangler says:


      Thanks for the question. If it happens to be that one area that has an issue and it is due to a compromised vapor retarder, you may be able to find a topically applied moisture mitigation product to use in the affected area. You will need to contact that manufacturer to see if their products would be viable as a solution. I would recommend contacting Ardex, Uzin, or Mapei. Good luck.

  10. charlotte ann freeman says:

    I live in louisiana and a mitigation company pulled up oak floors to find a 1 x 3 and black tar think called mastic. Is it safe to live with all this exposed.

    • Jason Spangler says:


      Thanks for the question, but this is really one you should be asking the mitigation company or even better yet an air quality testing service/environmental engineer. Good luck.

  11. Angelica Garcia says:

    Would a concrete foundation wall need vapor barriers? I know certainly waterproofing is important, like a water proof painted fluid or plastic lining…

    • Jason Spangler says:


      Thanks for the question. This would depend on the placement of the wall and whether or not the wall elevation was below the soil/water level. If it is, then there would be a strong case for having one.

  12. Cristian D Forgaciu says:

    In Naples Florida. New house with vapor barrier under the entire monolithic slab.
    However, I had to bust the concrete for adding a drain to a pantry sink. The vapor barrier was ripped. How can I repair or what profits can I use to lay over the new pipe and under the new concrete uet to be poured into this 6 inch wide ditch or groove ?

  13. Expanded Metal says:

    I’ve always wondered about the difference between vapor barriers and vapor retarders, and this article clarified it for me. It’s good to know that vapor barriers are less permeable than vapor retarders. The guidelines provided by ASTM International are also helpful in understanding the recommended degree of permeability for different applications. This article is a great resource for anyone looking to learn more about concrete vapor barriers.

  14. Brad Foster says:

    I have some free tubes. Can I use it as a barrier?

  15. Jim Kolb says:

    We live in North Carolina and have a brick home built over a crawl space. The crawl space has a moisture barrier.
    The rear porch, concrete over dirt , was closed in and made a sunroom.
    What system do you recommend over this slab ? The room stays damp and cooler than the rest of the house.
    Thank you.

    • Jason Spangler says:


      Thanks for the question. You might want to look at some type of tile or stone, making sure that you or your installer takes the situation into account and researches appropriate products for the application. It may also be prudent to look at some type of dehumidifier to run in the area. Good luck.

  16. Linda says:

    I have a raised house with an open crawl space that has a dirt foundation in South Louisiana. My lot slopes and I get significant rain water running through my lot when there is a heavy rainfall, with some flowing under the house and making puddles. While the area eventually dries, I recently had my kitchen renovated and found that the sub-flooring was rotten. I will be removing the batt insulation under the subflooring because it falls and attracts rodents.

    My house was built in 1982. I need to address the moisture coming from the ground under the house. Some contractors are suggesting I put in french drains but I am leary of a maintenance issue with them.

    One contractor is suggesting that the dirt/mud under my house be graded so that the rainwater flows quickly through the crawl space, a vapor barrier (6 ml plastic) be placed on top of the graded dirt/mud and a 2″ cement reinforced slab by poured over the vapor barrier.

    I am interested in your recommendations. Thank you, Linda

    • Jason Spangler says:


      Thanks for the questions. It actually sounds like both of these options could work in theory. I haven’t heard about excess maintenance issues from French drains, so I don’t think I would negate that as an option. Either way, having the vapor retarder under the house should be part of the solution in my opinion. Good luck.

  17. Scott Fraser says:

    I have a question about adding a vapor barrier to a slap the will be used as patio for a year or so prior to the addition being built. Do I still install a vapor barrier? Will it have the opposite effect out in the weather? Same for foundation wall for a dug out portion of the addition. I feel it should be built as if it will be interior slab and wall. Then run dehumidification once built to draw moisture out of the slab as soon as it is dried in from its year or two out in the rain. Thanks for you wisdom.

    • Jason Spangler says:


      Thanks for the question. In my opinion, if the slab is planned to have a finished interior floor on it at any time now or in the future, you should install a vapor retarder directly under the slab.

  18. Joshua Broussard says:

    Hi Jason,

    Thank you for all of this! I am about to build a concrete “prefablock” house in Costa Rica with concrete slab. Building materials aren’t always easy to find here and builders aren’t always the most knowledgeable. We also have a rainy season here for about 6 months where the ground is almost surely never dry. I am very worried about moisture and mold in the house as humidity also rarely gets under 80 or 90%. If I cant find an actual vapor barrier for below the slab, do you think any type of plastic liner would be worth a try? If its in a roll would you tape the seams or overlaps? If no barrier is put underneath, would it be sufficient to just have a roll on sealant barrier on the top surface of the slab? We plan to just have bare polished concrete floors.

    If I could also ask about the walls? I plan on putting some type of roll on sealant/barrier on the inside and outside of my walls as well as the surface of the slab. Do you think I should put put this sealant over the entire slab before the block walls to prevent moisture from entering from the slab into the walls? Should the top of the walls and window openings all be sealed as well?

    Thanks a lot for your time!!

    • Jason Spangler says:


      Thanks for the questions. I would want a vapor retarder under the slab if it were my house. That said, if you are going with just polished concrete, you may not “need” it. I would personally do it in case I wanted to change to an actual flooring product down the road. I would also do it to try and mitigate against the potential for efflorescence. In my opinion, I would seal everything. Good luck.

  19. Julia says:

    I live in Florida near the inter coastal on the west coast. My home was built in the 70s and I have recently learned that my foundation / slab has a moisture reading of 91. I laid new tile 2 years ago and noticed water and evervesence growing at the grout lines. I took up all the tile and I am waiting on the slab to dry. Hired a leak detection company and there are no leaks. I have been told the original vapor barrier (if there was one) is breaking down causing the slab to have a ton of moisture.

    I am told I need a vapor / moisture barrier on top of the slab before putting tile back down however the moisture reading is still at a 91. I am told this is still too high to put the vapor sealer on top of.

    Any suggestions? I am frazzled after this two year ordeal.

    • Jason Spangler says:


      Thanks for the questions, and I am sorry for the problems you are having. If the thought is that the vapor retarder below the slab is not doing its job (if it ever had a retarder) and it is allowing moisture in through the bottom of the slab, then you need to make decisions with the assumption that the slab COULD get to 100% RH. So pick products accordingly. They make plenty of moisture remediation products that will work to 100% RH. Look at Ardex, Uzin, Mapei, Koster. There are others, but this gives you a starting point. Good luck.

  20. Joe says:

    Thanks for the info. Just a quick one my builder have put a vapour barrier at the bottom of the insulation on a sand base. 100mm of kingspan insulation has been installed and then concrete base on top. no secondary barrier has been insulated on top of the insulation ive been informed from my building inspector it isn’t a building requirement just a manufacture requirement. Would you have and idea what the best practices would be to remedy? Thank you!

    • Jason Spangler says:


      Thanks for the question. Obviously, fixing the slab construction the way it is now, would be difficult. If you are trying to minimize the amount of moisture that may come in contact with a flooring finish, there are topically applied products that have the same classification (perm rating) as an under slab retarder. First thing to do though is a moisture test to see what the situation is.

  21. Jerry M says:

    Thanks for the info. Exactly what i thought. I have seen endless builders pour slabs for houses with no vapor barrier. They leave the owner with future major problems but by God they saved that $200. Exactly why I have zero trust in them.

  22. Bryan jackson says:

    My front porch is brick over concrete. The mortar in the brick is breaking down and is becoming unsightly. Can I install tile over the brick with a moisture barrier in between? What kind of barrier would be best?

  23. Christopher Haeska says:

    In Prescott, AZ I have noticed many slab homes being build and to date I have not witnessed one with a thermal or vapor barrier. I get it’s AZ but am I missing something? Shouldn’t a vapor barrier be installed for moisture and radon? Thanks in advance!

    • Jason Spangler says:

      Hi Christopher,

      AZ or not, the American Concrete Institute recommends that a vapor retarder be placed directly under the concrete slab if there is going to be a finished floor product installed on the top. Of course, ACI isn’t necessarily the end all, be all in the construction specification world. Different areas have different thoughts. If it where me and I was building in AZ, I would most definitely want a retarder installed.

  24. Grace says:

    Hello, thanks for this article!
    I live in East TN and have a pole barn that I will be pouring concrete into soon and building into a residential space. It was installed directly on the ground – some blocks underneath to maintain level, but there is no pre-existing slab.
    How would you suggest going about the vapor barrier? They also say they usually use 6mil…which is too thin based on the standard 10mil or more, correct?

    • Jason Spangler says:


      Thanks for the question. Especially with this being converted to residential living and presumably will have a finished floor on top of the slab, I would install the vapor retarder directly under the concrete and I would use something 10mil or greater. Again, this is a personal choice. You can look at a company like Stego for options Stego Industries | Moisture Protection Solutions. (

      Good luck.

  25. Brandon says:

    I’m confused. I’m seeing that vapor barriers on top of concrete are not ideal because they can trap moisture. However, they are required by laminate flooring manufacturers to prevent moisture from getting to the floor. As long as the concrete is relatively dry, there should be no problem, right? We had a slab raised a few months ago, with a vapor barrier underneath the new portion. I plan to do a calcium chloride test to determine the moisture of the concrete. I was debating on putting a quiet walk underlayment down, as it is supposed to be more breathable for the concrete, but the manufacturer said it might be best to put a layer of 6-mil poly underneath the underlayment, as well. Just trying to figure out the best approach, although it seems some form of vapor barrier will be necessary to maintain the warranty of the floor.

    Thanks in advance.

    • Jason Spangler says:


      Thanks for the comment. As long as the slab has a good quality vapor retarder directly under the slab, the concrete has sufficiently dried with respect to what is acceptable for flooring manufacturers, and a reputable type of moisture test is conducted, then I would agree, in theory, there shouldn’t be a problem. Breathability for the concrete is fine, but the issue is where the quiet walk has too much breathability for the flooring product. In the end, follow the manufacturer’s guidelines to ensure the highest potential for a successful installation.

  26. Toby K says:

    Hi Jason, recently had a leak under the slab of our 7 year old home in Atlanta. In order to access the leak, the plumber had to cut the hardwood flooring, jackhammer through the concrete, cut the rebar and cut the plastic vapor barrier. Which left us with two holes, one roughly 2 ft X 1 ft, the other 3 ft X 1.5 ft also multiple exploratory holes from the leak detection company. Now we have a contractor who states he will just pour regular concrete into the holes, prior to replacing the hardwood flooring, which concerns me. So, my question… Is it okay to just pour concrete into the larger holes without support? And what, if anything should be done concerning the vapor barrier?

    • Jason Spangler says:

      Thanks for the questions. I would consult local building codes to get the answers in your area. That being said, in most cases, I have seen albeit larger holes/trenches, there is a desire to attempt to tie back into the existing vapor retarder and slab to get as much stability and moisture migration protection as possible. Good luck.

  27. George says:

    Hi, I have a 20 year old house with a “mud” slab crawl space floor. I don’t know if there’s a vapor barrier underneath the floor. Near a lake but never have had any water of any sort on the floor. But, I’ve been having some musty smells in the upper floor. Would it be advisable to put a vapor barrier on the floor to reduce any smells? Thank you.

    • Jason Spangler says:


      Thanks for the question. Having a vapor barrier on the ground of your crawl space is a recommended practice. I would also look to make sure you have proper ventilation and circulation in the crawl space. All of these things help with minimizing moisture issues from below.

  28. Kelly Knight says:

    If there is a vapor barrier under the concrete slab in my house, and the laminate installers put a vapor barrier on top of the slab, before installing the laminate, will this not trap moisture in the concrete? And if so, what affect will this have on the concrete long term if it never dries completely?

    • Jason Spangler says:


      Great question. Keep in mind, concrete never really dries completely and concrete, for the most part, loves water. It’s the finish we put on top that usually has issues with the level of moisture in the slab. Having that vapor retarder on top tends to lessen any issues associated with moisture and the finish.

  29. Amie says:

    Thank you for your page Jason!
    I am installing flooring in a basement with remedied moisture issues however, I want to add an additional layer of vapor barrier on the concrete floor before I place carpet or vinyl flooring planks. I will paint it with Drylok first, however would a plastic sheet placed under the carpet padding or vinyl planks be appropriate? Thank you!

    • Jason Spangler says:


      You are welcome. Thank you for the question. Although this may be a prudent option (I haven’t heard of it under carpet padding), I would consult the manufacturer of the products you will be using to get their feedback and guidance based on their specific product characteristics. Good luck.

  30. bilal says:

    hello, how are you hope you will be fine. I have one observation and question I seen here in one of the project where I am working now they are using a vapor barrier under concrete paving with a hole in 300mm spacing in the vapor barrier what is the purpose could you please define

    • Jason Spangler says:


      Thanks for the email. Well, if this is intended to be a true vapor barrier or retarder, I am not sure why they would have designed holes every 300mm because that really defeats the purpose. Now, if those holes were done after the fact (vapor barrier not manufactured that way) by the concrete finisher, I have seen this. This is usually done so the water in the concrete will dissipate and finishing can de done more quickly. This is great for finishing, but not good for the overall health of the finish that will be installed on the surface of the concrete.

  31. Branden says:

    Thanks for all your help to this community, it is truly appreciated!

    I have a question on floating concrete slab (interior). Existing space was exterior carport with concrete columns. Previous owner infilled between the columns with stud wall, however never raised the floor.

    As the carport was exterior space, the slab is lower (approx. 4″) than the interior floor to the rest of the house. My question is how would you form up against these existing stud walls when pouring/screeding the new concrete floor?

    I plan to drill into existing slab and place #5 rebar 24″ on center, however I am confused when it comes to pouring up to the stud walls.

    Would you place a vapor barrier against stud wall? Would you also put on expansion joint in?

    Thanks Jason, much appreciated 🙂

    • Jason Spangler says:


      Thanks for the question. There are a million ways to skin a cat and I would definitely start with checking your local building codes for acceptability of anything you do. A vapor retarder across the entire floor and wrapped up the wall seems like a viable idea or pouring and reframing the wall on top of the second slab as intended. Doing this would also alleviate the question of an expansion joint. Good luck.

  32. David Johnson says:

    Jason, thank you for all the work you’ve put in to assemble this information! Question for you. Our rental home had a slab with laminate flooring glued directly to the slab. We decided to get LVP in the kitchen. The contractor said he could rip up all the laminate and put down a vapor barrier, but he said it would be a waste since the laminate floor would act as the barrier. Eight months later the floor is buckling. The contractor says that we must have a cracked slab that’s letting in moisture and that his installation technique was fine. He feels we need to consult with a foundation expert. I’m inclined to trust him, but what do you think? Does a laminate floor serve the same purpose as a vapor barrier that sits on top of a slab?

    • Jason Spangler says:


      Thanks for the questions. First I would contact the manufacturer of the LVP and find out if it is acceptable to install their product over the laminate product you have below. This will allow you rule out at least one potential culprit. After that I think I would be inclined to contact a flooring inspector at to get their unbiased opinion. Good luck.

  33. Ferdinand Longaza says:

    Hello Jason,

    I’m building my own house and I’m at the stage that my main floor has already been poly’ed, styrofoam installed, rebar done and finishing the loop for my radiant heating. Unfortunately, the deck part of my house is not finished yet (still bare wood) and rain have started to seep through the beam and leaked onto my ready floor. I am scheduled for concrete pouring already in the next 2 days but this situation concerns me. I think there is now trapped rainwater in between my poly and styrofoam. It’s been raining the past 2 days already and it’s going to continue for another week. I don’t think I should proceed with the pouring of my radiant concrete slab given this situation. Can you pls give me an advise on what I can do? Do I need to dismantle my rebar and styrofoam to dry the potential puddle sitting on top of the poly? Pls tell me what are my options?

    Thanks in advance for your time.

    • Jason Spangler says:


      Thanks for the question. I don’t claim to be a radiant heat slab construction expert, but I would think you would want things as dry as possible prior to the pour. That being said, I doubt some moisture (you didn’t say how much) would be a huge problem as long as you gave the flooring assembly ample time for the moisture to escape and dry, prior to installing a finished floor.

  34. Russ Kozerski says:

    Jason — Thanks for the informative article. I live in Thailand, where the temperature is never below 70 degrees F. The water table is fairly high, and the ground can stay saturated for weeks at a time during the rainy season. I’m planning to build a shed, approx. 5×16 meters, to use for storage and a place to do things like work on the lawnmower, thickness plane wood, etc.. It will have a concrete slab, a sheet metal roof, and two concrete block walls. The front and one side of the shed will not have walls. Given that the “interior” will be so open to the elements, is there any benefit to installing a vapor barrier under the concrete slab? Thanks for your advice.

    • Jason Spangler says:


      Thanks for the question. Based on the fact that two sides will be open and that the finished floor will be the actual concrete, I see no real reason to have the vapor retarder. Good luck.

  35. Mike Janouski says:

    I just bought a raised ranch modular home that sits on a poured concrete foundation on 3.25 walls. After moving in I discovered, on the stick built 4th exterior wall, a spot of mold on the interior drywall covering the stick built section. Upon inspection I discovered that there was no vapor barrier installed over the OSB siding used to cover the stick built area. After removing the drywall section where the mold was located I found that the OSB was rotted entirely out in several sections. Upon further investigation I found that the concrete walled sections of the foundation were covered with 1/2″ plywood over top of some type of other material and the plywood was covered on the outside with a latice wood material, purpose unknown to me. I am trying to figure out if I should remove this plywood and latice material and recover the foundation, externally, with something else. The foundation walls were constructed in 1990, there does not appear to be any moisture leakage through any of the sections of concrete. I have no manufacturing information about the construction of the modular home back in 1990 and am not sure that any of the exterior walls of the modular have vapor barrier material on any of the exterior walls. My question is simple for the stick built issues, what is the best and most economical way to replace the failing OSB and what is the best vapor barrier to use over it when it is replaced.

  36. Nick says:

    Hey Jason. Thanks so much for this article and sharing your knowledge! This page is a great resource.
    I have a home in West Florida with a detached garage that is a slab on grade. I doubt the slab has a vapor barrier. I am planning on convert it into a living space and adding flooring. What options do I have for vapor barriers/vapor mitigation? Would pouring a new slab over the existing one be something to consider? Do you have any other recommendations?

    • Jason Spangler says:


      Thanks for the email. I would probably want to verify if there is an intact vapor retarder, prior to planning. If there is, the cost to verify would probably be small in comparison to doing a good mitigation system. You should be able to find some type of geotechnical service in your area that can perform a core sample test of the garage floor. As far as mitigation products, epoxies have been the gold standard in the industry for years. You should research them and look at warranties. Make sure the one you chose covers slabs that DON’T have a vapor retarder below. Good luck.

  37. Tracy says:

    Hi Jason, I have a problem. Basement installed by reputable contractor 20 years ago. Ground is heavy clay, no pump out – he utilized natural draining with weeping tile.
    He did not put vapour barrier under cement!
    Plagued with efforescents. Tried epoxy paint to seal – blistered in many areas. Efforescent and moisture more to middle of floor than near walls.
    I cleared efforescent and dried floor. Installed drop down vinyl plank as per manufacturer – directly in cement. Within 2 months have orange coloured crystals growing through seams.
    How would you recommend mediation to resolve issue. If I pull up floor, lay plastic and put new flooring down is there a risk of mold growing between concrete and vapour barrier? Thanks Tracy

  38. Rick says:

    Thank you in advance for your help.

    I am thinking of installing radiant heating in our basement. Should I use a membrane between before pouring self levelling concrete over the pipes?

    • Jason Spangler says:


      Thanks for the question. Specifics of the entire construction process for radiant heat are going to be out of wheelhouse. I would probably search construction practices or construction details of radiant floor heat on Google. Good luck.

  39. Joanna says:

    Hello Jason,

    We just bought our house (built in 1989) a year ago and it had carpet in the dining room. Livingroom and Hallway have hardwood floors.
    We removed the carpet and had a contractor from Floor & Decor install hardwood floors that would match the rest of the house. When he started working I asked him if he measured the moisture in the concrete and he said no he hasn’t but it doesn’t feel moist. I asked him to get his moisture reader and just measure it. He did and the moisture level was at 100%. He continued to install and said his device must be broke.
    Well…. not even 2 weeks later the entire floor started to buckle. All the hardwood was ruined.
    So now Floor & Decor told us to get the issue fixed before they come out and install another floor. They told us they could only install Vinyl since our concrete floor moisture levels are too high.

    I had a leak detection done and there was no plumbing issues detected. The plumbing in the entire house was done in 2018.

    Then we had different landscaping companies come out and all of them agreed it is the rainwater causing this. We had a new draining system installed, Drylock was painted on the outside slab and gravel was installed alongside the house wall. Yet, we are still reading 100% on the concrete slab in the dining room. All the rooms in the back of the house are reading below 40%.

    What could be causing this? Any help would be greatly appreciated.


  40. Angie says:


    I hope you can help me.
    I want to pull up the carpet and padding in the main areas of my home. I have a slab foundation (on grade), and one room seems to have a moisture issue when it rains. The carpet feels slightly gritty at all times (regardless of steam cleaning), but when it rains, the carpet almost feels damp and gritty. While I cannot soak up any excess moisture with a towel, a Damp-Rid container fills up in about 3-4 months time, so there is definitely excess moisture in this one room. The floor level is techinically 15″ above grade the way our yard slopes. It’s a 15 year old house, but we’ve only been here 4 years, so I don’t know how long this has been an issue. We’re finally ready to deal with it though.

    I plan to install Luxury Vinyl planks with cork backing everywhere, but do not want to install a crinkly underlayment/moisture barrier. What can I paint onto the subfloor to eliminate moisture coming through when it rains? I live in the St. Louis area, and it’s humid in the summer and can feel quite dry in the winter.

    Any reccomended products that I can use to eliminate the need for an extra underlayment?

    • Jason Spangler says:

      Thanks for the questions(s). The first thing you need to do is identify if you have a moisture issue and quantify how bad it is. A good place to start would be look for a technician in your state. They specialize in concrete moisture testing. Once you have this information then a solution would be easier to identify. I look forward to your shared results.

  41. Dan says:


    We are in Florida and would like to know the best method for installing 6 mil poly vapor barrier over our concrete slab, prior to putting down laminate flooring. I have been told to fold it upwards at the perimiter a couple inches, bringing it up in front of the baseboard, and then attaching quarter round against it. However, it was not my intention to use quarter round which means that I would, instead, bring it up behind the baseboard (which of course gets sealed with caulk at the top). My concern is two-fold: 1) Will using this “air tight” method cause moisture to get trapped under the plastic sheet with no way to escape? 2). If it does find a way to escape, will it get trapped between my walls and cause a problem?

    Finally, is bringing it up IN FRONT of the baseboard and behind the quarter round a smarter alternative? Or are there problems with both of these methods?


    • Jason Spangler says:

      Thanks for the questions. First and foremost, follow the manufacturer’s recommendations for a proper, warrantable installation…including installing the 6 mil. That being said, yes moisture vapor can condense under the 6 mil, but the odds are it will be a constant flux of condense and reabsorption back into the concrete. Regarding moving to the walls, I have seen some discussions about this, but I have seen nothing that would lead me to believe, in most cases, this would actually transpire to a level that would cause issues. Good luck.

  42. Patrick says:


    I’m remodeling a home in the Florida Keys. In one room on the first floor we wanted to remove the rather low drywall ceiling, metal studs and framework then apply nickel gap shadow box ship lap on the ceiling to create more headroom. The ceiling we’re wanting to clad is the bottom of the concrete second floor slab which is 4-years old and has had no moisture issues in the past.

    If I wanted to apply the shiplap directly to the slab would I need to install a paint on or plastic vapor barrier, or must I use furring strips?


    • Jason Spangler says:


      Thanks for the question. Not sure there is a text book answer to this, but I think I would probably look to do some type of sheet vapor barrier down.

  43. Susan Diane Smith says:

    I’m going to start building, and vapor barrier doesn’t come in sheets as wide as the house, so I’m getting the idea that the sections/layers should be sealed together with some sort of exterior silicone stuff?

  44. Matt says:

    I am working on a project and the exiting mat slab is maintained and will have a new foundation next to it. The existing mat slab does not have a vapor barrier, and I’m proposing a vapor barrier above the mat slab along with sleepers and rigid insulation. My question is the waterproof with the new slab next to it. I am planning on a vapor barrier underneath the new slab and running it 12″ beyond the existing mat slab. And adding epoxy resin between the new and old slab. Is this okay? Do I run the vapor barrier that’s on top of the mat slab over the new foundation as well?

    Thanks for your help.

    • Jason Spangler says:


      Thanks for the question. I would recommend talking about your specific project and application with the epoxy manufacturer to ensure the process is to their specifications. Good luck.

  45. Philip Trunk says:


    Much of what you said I’ll agree with but not the part that water will hurt the concrete. The hardening process of concrete is a chemical reaction between the cementing material and water. As long as there is moisture present in the concrete it will continue to get stronger and improve at an ever decreasing rate. The second it dries out this reaction stops.

    While some building codes may require it, a slab on grade does not necessarily need a vapour barrier underneath. The quality of the concrete used (Eg 15 MPa vs 20 MPa / 2175 psi vs 2900 psi) has a large bearing in it. Under some building codes the lower strength mix requires the use of a vapour barrier where as the higher strength mix does not. Vapour barriers are a good idea to prevent radon gas from coming up through any cracks in the floor.

    Phil Trunk

    • Jason Spangler says:


      Thanks for the comments. I think if you were to read my other articles and/or view my video’s you would see my overarching opinion that concrete loves water, but it’s the flooring finishes that we use that don’t. So I think we can both agree on that. As far as concrete every drying completely, even with a vapor retarder that’s not going to happen. Lastly, regarding vapor retarders, there is a need to balance the entire building envelope “system” which would include the slab and finish as one unit, not the slab and finish separately. Although not using a vapor retarder may be debatable for the improved strength and health of the concrete, not using one is catastrophic for most flooring finishes. So we have to balance the overall system makeup to ensure all parts perform well together. This is one reason why I always refer to ACI302.1 when discussing vapor retarder usage and proper placement. Take care.

  46. Wm. Shane Ralph says:

    The contractor I hired to form and pour the slab for my free standing garage has poked holes in plastic I had him put down as a vapor barrier. isn’t worth a crap now. I’m I right or am wrong this guy has really screwed me on this the form he built wasn’t strong enough to hold so the top of the form is push out 3 inches from the bottom there a spot in the middle of the slab that holds water this guy thinks he’s going to get paid for this he got the first draw he’s not getting anything else. I paid the concrete company my buddy owns it and dont want him to get screwed. Be careful of who you hire dont trust what you read on angie’s list

  47. Bruce says:

    Hi Jason
    I have a 2yr old concrete slab with Spray Polyurethane Foam sprayed underneath to act as a vapour barrier. How effective is this as a vapour barrier?
    Also, I’ve seen no evidence of moisture, even after weeks of leaving a taped 6mil poly section over it. I want to glue cork tiles down.Should I be concerned? My home is in southern Michigan.

    • Jason Spangler says:


      Thanks for the message. You really need to look at the permeability specs on the Spray Polyurethane Foam to determine if it is an effective vapor retarder. As far as the “results” from the 6mil poly on the surface of the concrete, there is a reason this is not a flooring industry-recognized concrete moisture testing standard. That reason is that it isn’t always indicating the moisture in the slab and if it does, you don’t know how much moisture there is in relation to a potential flooring failure. I would test via a Calcium Chloride test or a relative humidity test like the Rapid RH test. Both of these are flooring industry-accepted methods for concrete moisture testing. Good luck.


  48. John Rutter says:


    I have a 40 X 20 pole-barn type building with a concrete floor, it was probably built in the mid 70’s.

    A couple years ago I put up another building adjacent to the old one with the floor being 4″ higher than the old building, with the intent on someday pouring a new floor in the old building to match floor levels.

    I was wondering if it would be a reasonable plan to spread a layer of fibered roof coating over the existing concrete to make a seamless vapor barrier before pouring the 4″ slab over it?

    Also, would you see a need for wire mesh in this situation?


    • Jason Spangler says:


      Thanks for the questions. Regarding the roof coating, it would just depend on the actual perm rating of the coating as to whether it would be an effective vapor retarder. Here are the parameters:

      Materials can be separated into four general classes based on their permeance:
      • Vapor impermeable: 0.1 perms or less
      • Vapor semi-impermeable: 1.0 perms or less and greater than 0.1 perm
      • Vapor semi-permeable: 10 perms or less and greater than 1.0 perm
      • Vapor permeable: greater than 10 perms

      Here is also a link for the construction of a slab on slab Good luck.

  49. Angel says:

    Should you continue living in a home where no moisture barrier was used when you have already experienced mildew smell?

    • Jason Spangler says:


      Thanks for the question. If you have concerns I would have an environmental specialist come out and evaluate the air quality in your living environment. Good luck.

  50. Zachary Driver says:

    Good afternoon,

    I recently had some water in my basement due to heavy rain. I installed carpet two years ago when I moved in. I used Healthier Choice Spillmaster carpet padding under my carpet. The concrete that it is setting on is kind of rough. was not leveled at all. After my recent rain damage, I pulled up the carpet and there was already mold and smaller mold spores forming. It appeared the mold had been accumulating far before the water damage took place (I had a minor flood the year before but THOUGHT i remedied the drying and cleaning of the carpet myself). Granted i did let the carpet sit for about a week before pulling it up entirely. I did pull back parts of it and fan it and used a carpet shampooer to suck the water out of the carpet within the same hour it flooded. I want to put carpet back in my basement as I work on my music projects down there. any people are insisting i get rid of the carpet because it is a basement and they say it will always have moisture even with a dehumidifier running. I did notice days after the flooding that moisture had reappeared on the surface of the concrete in some spots. I really really really want carpet, but i really really really don’t want to be replacing my carpet every two years and develop respiratory issues. my biggest question is CAN I SUCCESSFULLY INSTALL CARPET IN MY BASEMENT WITHOUT THE OCCURRENCE OF MOISTURE AND/OR MOLD? I have no tiling system. Just a sump-pump tied to a single floor drain. the foundation blocks around the floor drain (the sump almost in the corner of the basement) have holes drilled in them where they meet the floor to drain any block absorbed water. Block foundation, built 1952. basement window will corrected before flooring installation FYI lol. Would drylock on the floor first prevent the moisture capillary affect or whatever it is that is happening? Would a vapor barrier padding prevent this or would it just keep the mold on one side of the barrier? the Spillmaster stuff was supposed to prevent mold from coming through yet it seem mold formed right at the seem of the foam and plastic/vapor barrier which is on the living space side of the upper ( the side i walk on) All answers are appreciated. thank you.

    • Jason Spangler says:


      Thanks for the question. I would expect that you can successfully install carpet in this situation (or any other flooring product), but it is going to require that you find someone knowledgeable to work with that acknowledges the challenges you face in this application. Some of these challenges include potential hydrostatic pressure (basement below water table) and the lack of a vapor retarder below the slab due to the age of the house. Based on these, I would guess the slab will require some type of product be applied to the surface to slow down the vapor that is coming through the slab. I would look to multiple, reputable flooring installers for their solutions and don’t just shop based on the least expensive price. It is obviously cheaper to do this right the next time than to replace the floor every 2-3 years.

  51. Kanaan says:

    Hi Jason,
    aren’t we afraid of the water in the concrete to be leaked to the sand. that’s the reason of putting the sheet?

    • Jason Spangler says:


      Thanks for the question. The sheet would keep the construction water from dissipating into the blotter layer and help it to actually dry from the slab, if given enough time. On the other hand, the sheet also helps to ensure there is a break between the blotter layer and the concrete to keep groundwater from entering the slab later in its life.

  52. Steve Hirsch says:

    Hi Jason 🙂

    I have mold issues and am looking for a house. When did it become standard practice in CA to use a water/vapor barrier under a slab? How can I tell if there is one? Looking for a low-mold home isn’t easy or cheap. Thank you!! Steve

    • Jason Spangler says:


      Thanks for the question. ACI 302.1, which is an American Concrete Institute standard for slab construction, was modified in the early 2000’s to reflect the change in the placement of the vapor barrier directly below the concrete if a low permeable membrane (flooring) was to be installed. Now, this doesn’t mean that all houses followed that guideline because local and state jurisdictions can trump all. The only real way to know for sure is to have core samples taken of the slab by a geotechnical type of firm. I hope this helps.


  53. Konstantin says:

    Hi Jason,

    I’m installing 5mm LVP (w/o padding) on the concrete floor in the basement. The floor is generally dry, but I’d like to have some vapor barrier protection and the cushion. I’m concerned that a vapor barrier would trap moisture underneath it. I’m avoiding a simple plastic ones for that reason and looking into underlayments that are designed for LVP (such as FloorMuffler LVT). Would that be sufficient or do I still need to be concerned and look into membranes (Platon, Superseal) that allow airflow?


    • Jason Spangler says:


      Thanks for the questions. I would say that it really depends on the amount of moisture in the concrete as to which one would be best to utilize. I am sure you probably already have, but I would go back to the respective manufacturers’ installation documents and/or contact them directly with your situation and get their guidance. Especially in a basement, I would bet the concrete is fairly wet, even if it looks dry cosmetically. Good luck.

  54. Tina wilkinson says:

    We are pouring a 24×30 patio in central Florida on a sand base . The patio will have an open screen cage on it . We are questioning using a vapor barrier under the concrete and also wire mesh rust bleed through due to using the vapor barrier. What are your thoughts, we are new to Florida and sandy ground conditions.
    Thank you

    • Jason Spangler says:


      Thanks for the question. The answer depends on what type of finish you will be placing on the top of the concrete. If you just going to leave it as concrete, cosmetically, for now and the future, most probably wouldn’t utilize a vapor retarder. If you are going to do some type of finish, most would recommend a vapor retarder. I hope this helps.

  55. Kim Frederick says:

    Hi, I live in Houston and have an outdoor patio that I had stained and sealed. I have had efflorescence issues really bad with white dots coming up all over the patio. I have had it “fixed” several times and each time it all comes back. I have had people suggest diamond grading it and putting stamped overlay down and then I have had people say it will just come back up through that too because there no vapor barrier under my patio. I’ve then had people suggest laying tile on it after it’s been diamond graded but that it could still come up through that but not be as bad because it will only come up through the grout. But then people have said if I have that much moisture, the tiles might not stick. I’m at a loss and have waisted so much money on each “fix.” What is the best way to deal with this?

    • Jason Spangler says:


      Thanks for the question. I am not sure that diamond grinding and an overlay would do any better unless you used some type of pH resistant moisture mitigation product that would help the transmission of the moisture and salts. The tile option might be a good one if you can use something like Ditra or Ardex Flexbone. Putting this down prior to the tile may help alleviate some, if not all, of the issues. Good luck.

  56. Jennifer says:

    Hi Jason,
    My dad and husband just poured two footings in our basement and forgot to put down the 6mil poly specified by the engineer before pouring the concrete. How much difference does the poly make in the strength of the concrete? Is there a chance that the footings could fail at some point in the future?

    • Jason Spangler says:


      Thanks for the question. I have never heard of anyone utilizing poly to increase the strength of the concrete. Typically, the poly directly under the slab is to make a separation between the concrete and the soil to keep moisture from infiltrating the concrete after a flooring finish is installed. That being said, I would consult with the engineer that specified the poly for guidance.


  57. kevin smith says:

    hi jason
    thank you for your response. the slab is mainly structural.
    there are 2 20″ wide and 1 4′ wide 8″ deep food trough that are in the floor.
    this is why i’m doing the slab so i have one solid and flat floor.

  58. kevin smith says:

    hi jason
    i have a workshop in an old barn with a cement floor pre 1030 vintage.
    the floor is not flat or smooth and it has channels in it. no cracks in it tho. i going to put a new slab over it. i know correct thing to do is to replace the pad but i cant. the barn is on the pad. my issue is moisture. can i put the moisture barrier between the slabs?

    • Jason Spangler says:


      Thanks for the question. I’m not sure if you are putting the “slab” over the top for cosmetics or something structural. I will assume cosmetic based on how you described it. There are many products on the market that would allow you to apply a moisture remediation product on the original concrete and then you a self-leveling product for smoothing. Some companies to look at: Ardex, Mapei, Koster, and Uzin just to name a few. Good luck.


  59. Don says:

    I’m curious if I need to put visqueen and support wire down before I pour a 10 x 12 concrete floor on the west coast of Florida? It will have a laminate floor when done.


    • Jason Spangler says:


      Thanks for the questions. Different areas have different building requirements. I would ALWAYS consult your local building code for questions of this nature. Good luck.


  60. Don says:

    Hi, I’m building a wood framed shed 12 x 12, or 10 x 12 with a concrete floor. It will eventually be turned into a music room with climate control, flooring, etc.. I live in Florida on the west coast. Does my slab need to have a vapor barrier and reinforcement pre pour? Just curious if I’d have issues when the shed gets converted with flooring over the concrete down the road if I don’t use a barrier. Thanks!

    • Jason Spangler says:


      Thanks for the question. There are debates about this, with varying national, regional, state, and local regulations. The American Concrete Institute states that a vapor retarder should be placed directly under a concrete slab on grade if now, or in the future, a low permeable membrane (floor finish) will be installed upon the slab. Personally, I would follow their guidance.



  61. raphael says:

    HI I live in brooklyn NY and i have a crawl space of earth (dirt) underneath my den. i recently started a renovation and i decided i wanted to pour cement over the whole crawl space. i put 6 inches of cement however i did not put any tarp or any vapor barrier underneath the slab . Now i started reading that this was important to do. unfortunately i did not . now i am trying to figure out what can i do to have this crawl space safe and not humid I don’t have any access to any drains there so i can’t put a dehumidifier there . should i cover the cement ground using poly tarp ? would that protect the wood beams from mold and mildew? any suggestions would be greatly appreciated.

    • Jason Spangler says:


      Thanks for the question. Based on your situation, it may be best to ensure you have proper ventilation and airflow in the air space. You may want to also look at power vents that may assist with increased airflow when preset levels of relative humidity and/or temperature levels are hit. Good luck.


  62. Akansh says:

    1. If the thickened slabs are poured prior to the top 4″ slab, can the vapor barrier be placed over the thickened areas in lieu of going under the thickened footings? Please advise.

    • Jason Spangler says:


      Thanks for the question. I have heard of people doing this, but I would always check local building codes to confirm acceptability.


  63. Rebecca says:

    I just had a new concrete slab poured in my basement. We excavated the old floor and dirt put in a layer of gravel, French drains and a sump pump and then covered with a 14mm vapor barrier. However I noticed when the vapor barrier was laid down there were some gaps of a couple inches of exposed gravel on a couple of the sides and probably the overlap and sealing was not done very well, so I’m concerned that there is some exposed gravel right beneath the new cement pad in several places. I am sure it would be better if the vapor barrier had been tightly installed and sealed well with good overlaps, but should I still be in better shape or is the floor going to be vulnerable to wicking dampness because the vapor barrier was not installed thoroughly? I noticed what the installer was doing and I questioned him but did not push through asking him to fix the spreading of the vapor barrier before pouting the cement which I regret. He said with all the gravel and the French drains along the edges it was not necessary to have the membrane right up to the edges.

    • Jason Spangler says:


      Thanks for the question. With the French drains, this may not be as critical, but you will usually see uniform coverage in a vapor retarder. Very nice that they used a thicker membrane though, you don’t see that as often in residential applications. Good luck.


  64. CJC says:

    I am looking to build a camp on a slab. No frost wall is required per zoning. Is a vapor barrier below the slab sufficient or would you recommend also treating the slab w/a liquid carrier or lay one down before applying flooring? Also, do you have any recommendations on if you’d suggest laying down a subfloor before installing the flooring? Thanks.

    • Jason Spangler says:


      Thanks for the comment/question. I would always recommend a vapor retarder directly beneath the slab if you are going to install a low permeable finish on the surface. As far as any topical treatment, I would need to moisture test to determine that. Good luck on the project.


  65. Tony H says:


    We are experiencing moisture on our concrete slab. Our luxury vinyl floor was laid directly on top of it. Now we are considering putting some type of barrier between the two, and are toggling between the mvb moisture mitigation product and a moisture barrier underlayment beneath the vinyl. Which would be better?

    Any insight would be helpful.

    Tony H

    • Jason Spangler says:


      Thanks for the comment/question. Based on the information, I really can’t answer the question. Here are some things I can add for you to think about though:

      1. Have you done moisture testing? What were the values? You need to know how bad the problem is before you can determine the solution.
      2. Is this existing construction? If yes, does the slab have an intact vapor retarder? If no, then you need to utilize a mitigation product that will warrant up to 100% RH and will do so even if an intact retarder isn’t present. Sometimes this is in the fine print, so it’s always best to ask specifically.
      3. Is this new construction? If yes, does it have an intact vapor retarder? If yes, then what were the moisture testing results? Based on these results, shop the product that handles your moisture testing results.

      I hope this helps.


  66. says:

    HiJason, I have a concrete slap screened in porch that I have painted several times. Paint just seems to peel and pop up. I am getting ready to strip it for about the fourth time. The paint store said to put on this armor 2 part epoxy paint and it should resist the moisture. I would appreciate any tips from you. Thanks.

    • Jason Spangler says:

      Thanks for the comment. From what I hear, epoxy paint does tend to be a bit more durable in situations like this. If this was my project and I had done it multiple times without success, I would do a small test area first. Pick an area that seems to have the worst issues, prep the area thoroughly, and apply the epoxy paint. Doing this lets you see what to expect if you did the whole job. Good Luck.


  67. Deb says:

    Hi Jason, our garage is approximately 12 years old. We purchased our home 4 years ago. Our garage floor gets damp and slippery when it’s humid out which is often in WI. I don’t believe a moisture barrier was installed under the concrete. Our neighbors don’t get dampness and wetness in their garages when it is humid out. I have opened the garage doors to dry it out and it gets even worse. I did the vapor test with plastic on a dry floor and it the result was very damp. It’s a nice double garage and I want to use it more
    Without slipping. Can you help me please? Thank you!

    • Jason Spangler says:


      Thanks for the comment. If I had to guess, I would say this has something to do with dewpoint and the surface temperature of the concrete. If it is environmental and there isn’t something causing it i.e dryer vent not venting correctly to the outside, etc.) then “correcting” in may be difficult. One option may be to have a coating installed that has some type of slip resistant surface. Another option may be to have someone put a minor surface profile on the slab so if there is surface moisture present, the concrete has some traction, not just completely smooth. I hope this help.


  68. Gloria says:

    Hi Jason

    We just built a condition party barn in Middle TN. It has concrete floors. A neighbor asked if we installed a vapor barrier. I checked the construction photos and do see that a barrier was installed. The concrete slab is 12” thick. We are not putting ang flooring down. Just using concrete as the floor. We do have two rugs on the concrete floor. I had no idea about vapor issues and concrete so I googled, found your site. Is there anything I should be looking for in terms of moisture on a bare concrete floor? Also, in some of the inquiries I read about smells, and radon. Is there something I need to be aware of and watching for in having a concrete floor as my flooring? Thank you. Gloria Broming

    • Jason Spangler says:


      Thanks for the question. Usually, if you aren’t looking at putting a finish on the concrete, you have no issues with the moisture in the concrete. Depending on the breathability of the two rugs you have down on the slab, you may want to keep an eye underneath them to ensure you aren’t trapping any potential moisture. Enjoy the party barn.


  69. Samuel Martinez says:

    Thanks you so much !????

  70. Samuel Martinez says:

    Hey Jason o have new house build on 2017 and around the 2 sides of the house (this meets with exterior) we got some porcelain floors and just notice the tiles haves some moisture and water drops in some areas ,any idea what is the problem and solution ? I need you help or is something I need to do around the Concret slab?? Help me!????

    • Jason Spangler says:


      Thanks for the comment. This could be something as simple as varying temperatures and condensation forming on the tile. Potentially, the area around the perimeter of the floor may not be as well insulated as the rest of the floor and so that variation may be part of the issue. If the problem persists or is bad enough, you may want to reach out to and see if there is a qualified tile inspector in your area. Good luck.


  71. Chip says:

    Jason, Over the Christmas holiday we had a water heater in the 2nd story attic leak causing flooding throughout our first floor which has continuous solid hardwood floors throughout. Our ins. claim is taking care of everything related to reconstruction phase, I think. We have had several hardwood floor contractors out to bid putting in new floors. The house is on a concrete slab. The original hardwood floors 2 1/4″ White Oak were installed on top of 5/8″ plywood which sits on 15# felt as a vapor barrier. A couple of the hardwood floor contractors recommended that a concrete sealer be installed prior to putting down new felt and plywood. What is your thoughts on this? Is it necessary or would you just make sure the slab is completely dry before going back with the felt, plywood and new hardwoods? If you think the concrete sealer is warranted, is there a particular product you recommend?

    • Jason Spangler says:


      Thanks for the comment. Especially due to this being a flooding situation, I think the “sealer” is warranted. Sometimes the drying out process is done well enough by accepted insurance practices, but not always food enough to accepted flooring practices. I think this may help ensure a successful installation. Unfortunately, I don’t get into recommending a specific product, so I always recommend finding a knowledgeable installer that you feel comfortable with and trust. Once you have done this, you need to rely on their expertise to help guide the decision. Good luck.

  72. Michael Archer says:

    Thanks Jason! Your answer helps.

  73. Bashar says:

    Hi Jason,

    For a basement floor (9′ below grade concrete slab – Ottawa, Canada location), are there any bad effects that could occur by putting a 6mil vapour barrier on top of the concrete slab, that a subfloor would be installed on top of? The slab is in excellent condition and is 16 years since curing. Should I be concerned about humidity build up between the vapour barrier and the concrete? Because the concrete slab is floating – that’s how basements are built in Canada – if moisture was to penetrate between the slab and vapour barrier, would that compromise the concrete slab over time? It has not happened in the last 16 years but Mr. Murphy could show up at any time. So, am wondering what would the effect be if it were to happen?

    Best regards,

    • Jason Spangler says:


      Thanks for the comment. My only concern with laying a vapor retarder of that type, on the slab, would be the potential buildup of moisture between the slab and the retarder. It makes me wonder if that may potentially be a perfect environment for mold growth. I have no way to answer that for sure. Good luck.



  74. Michael Archer says:

    Hello Jason,

    I live in Atlanta, Georgia, and I have a house which is 33 years old with a unfinished daylight basement. The slab has plastic sheeting underneath it already. The slab is in fine condition. I would like to put down an impermeable class 1 vapor barrier on top of the slab before I put down any flooring. What risks are there to do so? My two main thoughts are potential mold growing under the vapor barrier, and potential slab degradation.

    Is the mold really an issue if it is not disturbed?
    If slab degradation, then in what form?

    Thank You,

    • Jason Spangler says:


      Thanks for the comments. I would say you are right on track with the thought of mold growth. As far as I am concerned, mold is an issue no matter what. I don’t see much issue with slab degradation. I am guessing that you are trying to ensure there are no moisture related issues with the flooring you are going to install? If this is the case, research some of the various moisture mitigation products available that are topically applied to the slab. Just ensure the product you choose warrants against hydrostatic pressure.



  75. Paul Barnett says:

    We have a partial concrete slab that we are planning to subfloor over to make a level floor throughout the house. The subfloor will consist of 2 X 6 joists about 1 inch off the slab. What type of moisture barrier would you recommend? The house was built in the 70’s so I’m not sure if there was any kind of moisture barrier under the slab.


  76. Raymond Wright says:

    We have a condo in Florida that is on the ground floor. It is an old building built in the early 70’s which purchased 4 years It had mold remediation done but we do not have contact with the previous owner to query where this work was done. We had the carpet replaced in one bedroom soon after we moved in, and the installer recommended that we keep the underlay as it was a good brand and still very new, but it was never lifted. So I do not know if there was a moisture barrier down on the slab

    Now 4 years after the carpet was installed l found 3 wet spots about 6″ diameter which have recently appeared on the carpet which I tested with a moisture sensor at over 35% ( thats the highest the meter reads) I suspect that the slab may have cracked and and any moisture barrier may have been compromised. What would be the best procedure to follow to remedy the issue



    • Jason Spangler says:


      Thanks for the comments. You may want to start with a geotechnical engineer. They should be able to verify the existence of a vapor retarder and potentially help identify the source of the moisture. Good Luck.

      Thank you,


  77. Mike says:

    We have a split home with typical family room downstairs that sits on concrete.
    We installed laminate flooring about 6 years ago and now have a terrible odor. I remember when the floor was installed there was a foam underlayment but I to not remember it having any plastics on the bottom side.
    I did notice that this past summer was extremely hot and humid and there was wet areas on the floor.
    Could this odor be caused from the moisture coming from the cement and trapped in the foam underlayment?



    • Jason Spangler says:


      Thanks for the comment. This could be a distinct possibility. I am not sure about the moisture being trapped in the foam, but there is a distinct possibility that the moisture could be because of some air quality issues. It might be worth investigating yourself or having someone out to evaluate.



  78. Cheri Howard says:

    We have a home in Houston that has been flooded with the hurricane, and all flooring other than the travertine tile has been removed. In a small 5 x 5 toilet room in the master bath, we took up clay tile, In order to tile the whole bathroom Including this separate toilet room with matching tile instead of re-installing carpet. We found that under the clay tile, It wasn’t concrete slab. It seems to be more gravel and sand. Initially we thought we could just pour concrete in the small area but in reading your information we are now worried about moisture and vapor barriers. What do you suggest?

  79. Erica says:

    Hello. I am getting a new home built and the contractor put in a walk in conditioned crawlspace. They put down a plastic Barrier which I discovered the other day had a puncture. The builder says they’re going to put a patch over the puncture and lay another sheet of vapor barrier. I’m just wondering because they are saying it can be used as storage the open space of ways to reinforce the barrier to make sure that the plastic is not punctured. I’m just concerned that if something is stored in there that it could cause problems if the barrier is punctured. Do you have any suggestions on ways to reinforce the vapor barrier to make sure that it’s not punctured and OK for me to use as a storage space? Maybe sometime down the line when I can afford I’ll get a concrete slab but I’m not sure how mandatory that would be. I’m just more concerned with prevention of mold growth and moisture.

    • Jason Spangler says:


      Thanks for the email. Based on what you have outlined, I would say getting the thickest (within reason) vapor retarder available is going to be your best bet. Keep in mind that there are varying thicknesses available and with the varying thicknesses you obviously get better performance. Take a look at this website They aren’t the only game out there, but it is a place to start.



  80. Jason says:

    Hi Jason,
    I have a basement concrete slab on grade that is 30 years old. I would like to install 12×24 tile on it but am wondering if I need to apply a vapor barrier or sealer of sorts prior to install? Or would a poly modified medium bed thinset be sufficient? Also is 1/2 x 1/2 x 1/2 trowel best for this size of tile?

    • Ron Smith says:

      If the basement slab was poured directly on the ground, rather on a true vapor barrier, then the ground underneath the slab will always be ‘in play’, and the moisture in the ground below will always want to move upward to the slab’s surface. There are industry-accepted test methods for determining the moisture condition of the slab, and I suggest you look into this before doing anything. At this point, the most-accepted method commercially is the American Society for Testing and Materials (ASTM) F2170 method.

      Next, I am not an expert on modified or unmodified thin-sets, but I would think the thin-set manufacturers would have specifications regarding any (if any) vapor barrier characteristics. I rather suspect that they would have specifications regarding the acceptable levels of slab moisture before you could actually use their products for an application.

      Lastly, here is a link to a tile size/trowel size guideline table:

  81. Ian Koolik says:


    I have a home that I did a substantial renovation on 3 years ago. The home (19yrs old) has a concrete slab on grade with a visqueen vapor barrier under the slab. In various places around the house we installed a gypcrete leveling course (1/2″ – 5/8″) and then a glued down engineered wood. Prior to installing the wood, the wood was left at the house to acclimate and the slab was checked to ensure that the moisture level was acceptable to the manufacturer’s specifications. The adhesive was supposed to be a “2 in 1” adhesive/vapor barrier product. I am not sure what product was used.

    After about a year, they noticed small “pimple like” defects in the wood, in a few boards in the office. The boards were removed and the slab was found to have a high moisture content. We could find no reason (roof leak, plumbing leak, wall leak, etc.) for this. We left the boards out for over a week until the floor was dry. It was tested and found to be fine. The boards were installed again. About a year later the defects showed up again in other boards in the office. The same process was performed. Now another year later and the defects are showing up in the wood in multiple rooms in the home. The rooms are separated by a marble floor installed in the main parts of the house.

    I had someone come out to test for moisture and they found “pockets” of high moisture throughout the house wherever the wood floor shows the defects. I must also add that we find that most of the floor area other than where the defects are to have very low/no moisture. It is very weird.

    Is it possible that a temperature difference between the slab and room above the wood is causing condensation to build up under the floor? In the pieces of wood that have been removed, sometimes it appears that moisture was between the adhesive and the slab and in some cases it looks like the moisture was between the adhesive and the wood.

    Any insight would be helpful.

    • Ron Smith says:

      If parts of the slab got cold enough to reach its dew point, I suppose condensation could form, but to have it be spotty is indeed strange. What type of moisture testing was done on the slab, and then on the gypcrete?

      Is the slab poured on a true vapor barrier?

  82. Lee says:

    I am pouring a concrete slab under a pavilion. The soil is very sandy and the slab will be subject to occasional flooding both in summer and winter conditions. Should I put a vapor barrier down before pouring???

    • Jason Spangler says:


      Thanks for the comment. This question is usually linked to the type of floor finish being installed, so, what type of finish are you installing on the surface of the concrete i.e. tile, carpet, paint? Is the pavilion fully enclosed with doors and windows?



  83. Joe Torkell says:

    Hi Jason,

    I have a problem with my walk-in-beer cooler located in a convenience store in W. TN. The cooler is sitting on a bare slab appx 6’x16′. The slab is butted right against the outside wall of the building which is an old wooden country store with the wall opened up that has the normal reach in doors to the inside of store. The cooler was installed apparently without any vapor barrier between the slab and the outer wood of the floor structure. The top of the slab is basically even with my wood floors and the only thing in between is maybe a 2″ to 4″ gap at best and some tin or aluminum sheathing in places.

    My uncle had the cooler installed appx 10 to 15 years ago. The wood lasted for a good while (old saw mill type) and we replaced the rotted wood almost 4 years ago. We used treated joists (but not treated sub flooring and cheap pine hardwood tongue and groove, I hate pine). During the replacement, I attempted to reduce the moisture transference into the wood by installing some tar paper to act as a vapor barrier between the wood and the concrete slab. Apparently I may have made worse because we just had to replace the sub flooring again here within the last month. I did use treated sub-flooring this time and covered it with a vinyl plank instead of hardwood.

    It is still too wet on the wood near slab under the crawlspace. I removed any tar paper that I could and placed a crawl space fan under the store to try to help dry it out. I’ve also mounted a remote hygrometer on joist in crawl space near the slab. It has been there almost for a week, and it is not helping hardly any, of course we have had some high humidity since I’ve installed it. But I’m seeing like 74 degrees with 98 to 99% humidity. I do not want to replace this floor again anytime soon!

    I’m thinking maybe I need to insulate the cooler floor(top of slab) to possibly help so it would keep the cold from permeating down through the slab (basically raising due point), or some how or another insulate the exterior of slab that is exposed to the crawlspace to keep that moisture from wanting to stick to the cool wall of slab. The crawlspace is probably only 1.5′, so it’s kind of tough to manuever around, but it is possible.
    I also thought about using a spray foam on the exposed slab and between crevices and on the wood joists in that area, but everything is too wet/ damp.

    I’ve read some of your other posts and I’m about to google the epoxy moisture mitigation, which that may or may not be for me. Please help. Any suggestions?



    • Jason Spangler says:


      Thanks for the comment. You have a lot going on here and many are moving variables. I would say you are on the right track with insulating the break between the cooler slab and the wood. The crawl space fan is good, but I would also look at how much ventilation there is to the outside. I hope this helps.


  84. sir..i need advice does precast earthing pit need vapor barriers…since this considered as non sturctutral concrete dimension is 30cm x30 cm by 20 cm
    our engineer require as to wrap the the precast before backfill is it necessary..??

    thank rey

    • Jason Spangler says:


      Thanks for the question. Honestly, since you have an engineer who has intimate knowledge of your specific project, I think I would follow their advice. I really have no recommendations to add.



  85. Danny says:

    I am replacing a laminate floor with a new laminate floor (it sits on concrete). The installer says that my moisture level is too high, and doesn’t want to be responsible if it ever warps. I think he is just handing me BS and hiding behind the “moisture level” issue. I am on the 4th floor of a condo. The concrete sub-floor has no direct access with the ground. The condo has had laminate flooring in it for years. I am putting down the plastic barrier on top of the sub-floor, and then the new Pergo laminate on top of that. I have even allowed the new laminate flooring to acclimate to my house for over a month. Is there anything else I can do to help the moisture level? For example use Damp Rid for a few days? I also heard that that there is a moisture retarder/barrier that is like a primer that I can just “paint” on top of the concrete (it’s usually colored pink or purple). Does anyone know what that is? (ls that DryLok?) will it help?

    • Jason Spangler says:


      Well, I can say that having issues on concrete slabs, other than the ground floor, is more common than you would think. Before you can try for a solution, you need to know how big the problem is. How high are the moisture levels? Once you know this, then you can try to investigate solutions. I, personally, have never heard of Drylok, but I can tell you that there are numerous products on the market that can help with issues like you have described. If Pergo is the finished product, I would start by calling their technical hotline (after quantifying the moisture levels) and asking them for recommendations or guidance. If that fails, I would track down a flooring sundry distributor in your area and ask them about concrete moisture mitigation options. I hope this helps.


  86. David says:

    I have an existing concrete floor on a very old building (~1920). I need to raise the floor by about 2″ for about 600 sq ft before putting on 6″ x 48″ porcelain tiles. I’m getting two different directions from two different contractors (with very different prices):

    #1 (Cheaper option) Pour new concrete on top of the old concrete
    #2 (3x more expensive option) Concrete must be at least 3″ thick, so dig up some of old concrete, put in new vapor barrier, wire mesh, and concrete

    Don’t know which way to go! And is a vapor barrier under the new to-be-poured concrete needed? I have no idea if one was originally installed, but my guess is, for 1920s construction, probably not. Thanks!

  87. Chuck says:

    Good Blessed afternoon, I have a 50’x40′ heated garage, which is also heated, insulated and drywalled (walls & ceiling), its 92° outside… On top of the 6″ concrete floor w/ wire mesh, I also installed 12″x12″ porcelain tiles. Today I was sitting in a chair, cleaning tools and putting them in the toolbox… I felt somewhat of a movement, and then my chair began to rock, I kept hearing cracking noises and the floor under me began to lift my chair up. As I looked over to the right of me, I noticed a large crown in the middle of the floor(about 16′ wide). I was really scared, I thought I was in a movie and dreaming too… I called the fire dept. They came out and said there were no earthquakes reported here… Under the concrete is 21AA crushed concrete(I did not put a vapor barrier in before concrete was installed… Every blue moon, the floor used to sweat. I popped up 1 of the tiles, it was really moist under there…can you please tell me what has happened here? What are the solutions? Will I have to tear out concrete and start from scratch & be sure to add a vapor barrier, what will be the proper steps to take before pouring the concrete?
    Thank you in advance for your replies
    Didn’t mean to be long winded, I’m still shaking as I type this message

    • Jason Spangler says:


      Wow, what an experience. I hope you are ok. I would call a local building professional to evaluate the situation.



  88. Neil says:

    Hello Jason,
    I’m hoping you can guide me in the right direction because i’m in the middle of a basement flooring nightmare…our house was built in 1977 and we have a walkout basement. We have had carpet with a mold resistant pad for the past 6 years. After a drain backup ruined the rug we decided to upgrade with a porcelain 6×24″ tile so our floor could withstand any future pipe issues. My installer was half way through the job laying the tile when I realized some of the tiles were hollow to the tap…i pulled up some of these tiles to see that the thinset was not adhering to the slab (brittle actually) and in other areas it seemed to adhere fine…under one area of tile we noticed that the thinset adhered but the was some beading water in some of the thinset groves. I had heard of vapor barrier problems and wondered if this could be the reason behind these problems….(or possibly the contractor didn’t prep the floor properly or mix the thinset properly) so today i ripped up all the tile and halted the project until i can find answers…i really want to install tile again but my question is if I should use a painted sealer like redgard or something different to prevent moisture vapor to escape and ruin another attempt at tile!?….all the tile installers i talk to about this seem to have no knowledge of this kind of possible vapor interference with adhering thinset….i want to do this right and feel confident that the tile floor will last for many years! But I worry that sealing my floor will cause more problems with trapped moisture underneath?!……please help? Thanks

  89. Steve says:

    I have a contractor that says half of my garage reads a “5” on moisture gauge and wants to lay down a ristoleum vapor barrier for $1850. Size is 670 so ft. Does that sound right to you? Prepping for polyurea coating….

    Thanks for any feedback

    • Jason Spangler says:


      Thanks for the comment. Unfortunately, a “5” means nothing in most of the finish world. These surface meters are meant to be used to identify suspect areas on a floor, not to make installation decisions. That being said, there are some coatings companies that use these devices in their installation documents. Your best bet is to have the installer provide the installation guidelines for the polyurea that will be installed. In there, they should spell out “concrete moisture” maximums. This may help you understand better.



  90. Dan Marks says:

    Hello Damian & Jason,

    Great questions. I will send you an email shortly with some additional information and attachments.


    Dan Marks
    Technical Director
    (949) 325-2035

  91. Hello Jason
    We are creating a footing for a large piece of equipment that we are installing within our facility. The footing will be created by cutting a 10 foot square section out of the existing slab, digging down 5 feet to make a 10x10x5′ deep hole. Rebar framework and equipment mounting studs will be built into the hole and then the hole filled with cement. The current slab has a vapor barrier already installed. The floor is epoxy coated.
    My question is whether a vapor barrier should be installed under this footing, I assume so, but do we need to bring the barrier up the sides of the hole also?

    Thanks, Damian
    Austin, Texas

    • Jason Spangler says:


      Thanks for the question. Being that this is a very specific, unique question, I am enrolling help from one of my industry counterparts from Stego. I just hit him up on Twitter and I am hoping he will help us out.



  92. 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.


  93. 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.



  94. Terri says:

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

  95. 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.



  96. 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.



  97. 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.


  98. 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.



  99. 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?

  100. 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!

  101. 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.


  102. 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.



  103. 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.

  104. Jason Spangler says:

    Hi Will,

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

  105. 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

  106. 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!!

  107. 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!

Leave a Reply

Your email address will not be published. Required fields are marked *