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Community and Q&A

Can anyone recommend a vapor barrier for use under a cement floor?

user-831496 | Posted in Green Products and Materials on

I live in an area that is close to an EPA superfund site. I am aware of a toxic plume of underground water that is close by. I am concerned that these voc’s can permeate the cement floor and foundation. We are going to pour cement on top of an existing basement floor which shows white deposits on it. Is there some type of vapor barrier that can be placed under the newly poured cement. Our contractor tells us that thinset is non porous. Is that true?


  1. Laurel Standley | | #1

    I don't know of a good barrier but it would also be a good idea to put in ventilation as a backup as is done to remediate radon gas in basements.

  2. GBA Editor
    Martin Holladay | | #2

    Concrete is a good air barrier; if cracks develop, the cracks should be caulked.

    Any concrete slab should include a sub-slab vapor barrier (6 mil polyethylene with taped seams).

  3. Riversong | | #3


    I would never use standard 6 mil polyethylene under a concrete slab. There are a variety of plastic films designed for that application that can take the abuse, such as Tu-Tuf vapor barrier. I would also highly recommend installing a sub-slab soil gas vent, as is typically done in all responsible new construction, with an outlet through the roof for passive relief of both VOCs and the common and carcinogenic radon (which can leak through a crack too small to see).

    If you have an existing concrete slab, it may be best to break it up and remove it, install 4"-6" of crushed stone, a perforated perimeter gas vent pipe with a tee for a stack through the roof, sub-slab vapor barrier (taped or caulked at all seams and around penetrations), and pour a new slab with high-strength concrete and steel mesh reinforcement.

  4. davidmeiland | | #4

    If thinset were non-porous, wouldn't tilesetters be waterproofing their showers with it, rather than all the membrane materials they're using?

    My favorite underslab barrier is Stego Wrap. You probably have to go thru a commercial construction supplier to get it, but it's worth the trouble (and cost).

    Is there insulation under your existing slab?

  5. J99aAMQzYo | | #5

    Arlene - You may also want to look at a product called Slab Shield. We used this recently on a Habitat for Humanity LEED Platinum project and were very happy with it.

    Here is a quick blurb from their site about it: “SlabShield is manufactured using a flame lamination process bonding two layers of polyethylene to a layer of aluminum. The foil will be centered between the layers of polyethylene to protect the foil from the lime in the concrete. SlabShield works as a continuous vapor barrier, thermal break, and radon retarder all in one.”

    It is much stronger than typical 6 mil, so it is designed for that type of rugged application as Robert alluded to. We had volunteers walking on it and it never missed a beat. The ship-lap edge design also gave us a high comfort level that the taped seams will perform well.

    As for Radon, a properly designed system really is the way to go if it’s in your budget. But if not, then as their specs mention, this may provide you with an affordable “Plan B.” *Note- they do say radon “retarder,” and not “barrier” so it is definitely as secondary option based on budget.

    Pulling everything existing and starting from scratch is never a bad strategy, but that will at least double your cost (if not more) so you need to be sure that the cost/benefit ratio makes sense for you. If your current ceiling clearance is seven-feet or higher, then your basement meets current code for a living space. So you may also want to be sure that an overlay slab won’t decrease that height, if that’s an issue for you. If it will put you below seven-feet, then it may be worth the complete removal cost so that you don’t lose that resale value at a later time.

    As for the thin-set strategy: There is no such creature as a true VAPOR barrier cementitious material that I’ve ever come across. AIR barrier yes, as Martin points out, but NOT vapor. And vapor is what you are concerned about with the superfund off gassing.

  6. J99aAMQzYo | | #6

    David - Thanks for the tip on Stego Wrap. I hadn't come across it before so I just jumped on their site and read some more about it: It doesn't offer the thermal break that Slab Shield does, but I bet it is a good bit less expensive, so that may make it a nice option for certain budget sensitive jobs. This will be a good one to have in our spec arsenal for the future. Thanks again.

  7. Riversong | | #7

    The "thermal break" of the Slabshield is a joke, since it is only ½" thick, and the 4' wide rolls requires an excessive amount of seam taping (always the weak link in a vapor barrier).

    I'm not familiar with Stego, but Tu-Tuff is commonly available, and comes in 9' and 12' wide rolls for ease of installation. It is made of cross-laminated virgin high-density resins and it withstands considerable abuse.

    For a true sub-slab "thermal break", XPS is the industry standard.

  8. user-869687 | | #8

    Robert, when using heavy plastic sheeting as a vapor barrier, why not use EPS insulation? XPS has that controversial blowing gas. EPS has less R-value per inch but you can add inches.

  9. J99aAMQzYo | | #9

    "The "thermal break" of the Slabshield is a joke,"
    It's not when option "B" is thin plastic sheeting measured in MM. I'm not saying that it's the end-all-to-be-all, but if the cost is essentially in the same ballpark, then you bet I'll take SlabShield over poly (or Tu Tuff, or any other thin membrane). Sure, 2-4" of XPS is optimal, but that's a lot more $$ and additional section build-up, either of which may not be options. And as for tape being the weak link, the exact same thing holds true for XPS. So ... this meets a good medium ground: cheaper and thinner than XPS; taped just the same; and thicker and warmer than sheeting. In this economy, compromise is the name of the game. We can preach about perfect design until the cows come home, but then there's the reality of our client's budgets.

  10. Riversong | | #10

    Andy CLC (whatever that means)

    Slabshield, which claims R-2.9 (as part of an assembly), is manufactured by a company that specializes in radiant "insulation" with the typical claims of "up to R-11".

    XPS does not require taping at joints to be an effective insulation. But any vapor/radon barrier must be continuous and that's why I pointed to the potential weakness of taped seams, which are far more problematic if spaced 4' on center rather than 12'.

  11. user-716970 | | #11

    Dr Joe has some differing thoughts on underfloor poly...
    I asked him about radon in the comments section...

  12. Riversong | | #12


    I disagree with Dr. Joe. With a good subslab radon mitigation system that effectively depressurizes the soil, an intact concrete slab might be good enough. But, if there is any positive soil gas pressure, then no concrete slab will be tight enough to prevent radon intrusion, which can pass through a crack too small for the eye to see.

    For this reason, a non-perforated subslab membrane is an essential backup to a radon mitigation system, and also mitigates against liquid water diffusion or capillary movement through the slab in the event of pooling or high water table.

    The tighter we build, the more important becomes radon mitigation strategies. With 14,000 to 21,000 radon-induced deaths per year in the US, a belts and suspenders approach makes sense. this is just one of the reasons to avoid using 6 mil poly under a slab, since it's full of holes even before taking it out of the box.

  13. user-831496 | | #13

    Thanks for the suggestions.I am going to review them this weekend. There is not any insulation under the existing slab and as testing done on my basement did not show any evidence of radon. I am mainly concerned about any voc's migrating through the cement. Aside from those barriers spoken about above can concrete sealers be used under the new cement? My contractor suggested using a product called Hydra Block. I didn't think these products were supposed to be used for this purpose. I also worry about the product off gassing in it since I am chemically sensitive. Am I correct in assuming that the physical barriers mentioned above would be better?

  14. Riversong | | #14


    Have you tested for the VOC's or other organic chemicals you're worried about? Or is all this just precautionary?

    If you really have a problem or think you might, I would get rid of that contractor, who seems rather ignorant, and hire an occupational health specialist who can advise you on the best course of action.

  15. user-831496 | | #15

    It's really a precaution. We are rebuilding my home using the existing foundation. We figured that since we have the opportunity why not take precautions. I am concerned since this superfund site is just blocks away. The EPA has claimed that the contamination is contained but there is still monitoring going on. The contractor is another issue. We started out with a green builder who quit the project because of difference of opinion on design leaving us scrambling. We just can't afford the prices some of the other green builders are charging so we are trying to navigate this project on our own.

  16. Riversong | | #16


    If you're doing a major rebuild, then this is the time to remove the existing basement floor and install a proper soil-gas mitigation system, which is the only way to assure that nothing below the foundation can diffuse into the house. With a passive vent stack through the roof, a depressurization fan can always be installed later if any problems appear.

    Sorry to hear about your problems with "green" builders. It appears that far too many think that "green" refers to the color of money.

  17. J99aAMQzYo | | #17

    Robert, not to get off topic, but since you asked ... CLC stands for Certified Lead Carpenter, a designation I worked very hard over a number of years to attain. One of the skills taught in that program is how to use team-building skills to support and coordinate all of the workers and trades on the job site to ensure the best possible outcome of the project for the client. And how NOT to shoot others down or brow beat them in order to make yourself feel more important. It’s a great curriculum that many in our profession could benefit from.

  18. Riversong | | #18

    It's too bad that the CLC training didn't also include the skill to differentiate between criticism of faulty ideas and practices or inappropriate presentations and the 'shooting down or browbeating of others for the sake of self-importance'.

    Then, perhaps those who earned such titles wouldn't be so self-conscious as to either mistake legitimate critiques for personal attacks or suffer the need to append an obscure set of letters after their names in order to appear more credible than others.

  19. J99aAMQzYo | | #19

    Assuming that one views "whatever that means" as a "legitimate critique" then you're absolutely correct and I retract my prior statement with full apologies.

  20. Riversong | | #20


    "Whatever that means" was a simple statement expressing both the obscurity of the millions of varieties of such post-nominal letters and the obvious self-importance with which they are deliberately used.

    My critique of ideas, principles and practices is what followed. But the proof of my contention that individuals identify with titles for their own sense of self-worth has been proven by your over-reaction to a simple statement about the obscurity of three letters appended to your name and your non-response to the substantive critique I presented.

    This should be a forum about ideas, principles and practices - not about personalities and egos. In fact, and contrary to a too-common post-modern pretension, there are no people on this website - merely words.

  21. J99aAMQzYo | | #21

    Arlene, sorry for momentarily diverting your original thread. I hope that you've gotten a few good possible paths to consider for your project. Please report back and let us know where it ends up.

  22. Craig, GC | | #22

    Hey, great question. I agree with the consensus of this tread that thin set IS vapor permeable. As a different line of thinking, Have you looked into "RedGard?" I love this product. Typically it is used under tile and it is being used in some applications as a shower pan liner. It seems perfect for a situation between slabs because it acts as a crack isolation membrane and vapor barrier, so it should seal any cracks in the original slab and prevent any new cracks from leaking vapor through. I don't know if it is best practice to caulk existing cracks first. You could even put another coat of redgard on your top slab for redundancy. I worry it would be expensive, but not compared to health tragedies, and probably not compared to any product of equal effectiveness. I found their site at You will need to look into it further for your application, but it is my favorite choice I can think of. Also note. 6 mil "vapor barriers" DO NOT block out all vapor. An amazing amount of water vapor still comes through, even when taped. That is why sealed crawlspaces do not use them. Looking into products for sealed crawl spaces, like the special tapes and nylon vapor barriers they recommend will give you some other great options.

  23. Robert Hronek | | #23

    I would be curious as to the extent of the rebuild and what access there is to the basement slab? Are you trying to reuse a foundation but the rest of the sturcture is gone or is it a gut and update? The ease of getting equipment in as well as removing debris on bringing new materials in would be factors.

    I agree with Robert in that the best solution would be a new slab over rock, foam, and a vapor barrier. Of course budgets get in the way and can prevent that.

    Since there is not really a way to get a vapor barrier under the existing slab you have to look above it. I will ask is possible for someone to place a vapor barrier above the slab and another layer of contrete on top?

  24. user-831496 | | #24

    The problem is that we have already started building. We are re-using the existing foundation and poured an extension in the back of the house. We had four inches of concrete with radiant heat and porcelain tile on top of that but the builder had to rip all that out down to the old floor since the tiles were damaged during demolition and we needed the height. I just remembered that prior to putting the tiles on that floor we would have white salty looking stains on the floor. It made me worry that contaminants could migrate through the concrete. The contractor was going to use the self leveling cement on the floor then we are installing new radiant heat and porcelain tile again. Putting a vapor barrier above the slab with a layer of concrete is the way we will have to go at this point. Now we have to decide what to use. I have been looking into all of the suggestions and am waiting for the concrete sub contractor to get back to me. I am still unsure as to whats best. Thanks to all for your input.

  25. Riversong | | #25


    The "white salty looking stains" were probably efflorescence - dissolved mineral salts carried by water through the concrete and left as crystals after evaporation. This indicates that there is no vapor barrier under the slab and almost certainly no insulation.

    If you're pouring a new radiant slab on top of the existing one, then it needs to be thermally isolated from the concrete below - both because that slab is probably in direct contact with the ground and because the old concrete will absorb too much of the heat and reduce the surface temperature of the new floor. I would suggest a minimum of 1" XPS if the new slab is only 2" thick.

    Then I would recommend a high-density (not lightweight) concrete for thermal mass with short polyester fibers to prevent shrinkage cracks. If the old slab is not cracked, then no additional reinforcement is necessary in the new layer, but if there is evidence of settling then 6" welded wire mesh should also be used to reinforce the new slab against shrinkage or settling. The mesh also makes it easy to attach the PEX tubing at regular intervals. The closer the tubing is to the new floor surface, the quicker the response and the less heat is lost to the slab and ground below.

  26. David Meiland | | #26

    I would shoot for 1" of concrete cover over the tops of the tubes. If the slab is 2" thick then the WWF would have to be on top of the tubing to do much good.

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