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Community and Q&A

Vapor barrier under the basement slab

user-647948 | Posted in General Questions on

I am building my own home in Idaho (climate zone 5) and have a full basement and want to put a 10mil poly under the slab. My concrete contractor is very concerned and has “never heard of this” he is asking for technical information to send him. Also he puts a sealer on the top of the concrete after it is poured, would this be a issue?


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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what your concrete contractor has never heard of.

    Has he never heard of installing a polyethylene vapor barrier under a slab? If so, his name must be Rip Van Winkle. Perhaps he fell asleep in 1948, and just woke up.

    Or maybe you're saying something else -- that he usually uses 6 mil poly, and he has never heard of using 10 mil poly. Is that what you are saying?

  2. Expert Member

    There are sealers that can be applied immediately after pouring. One of which is called "Cure and Seal". They allow moisture to escape. If your contractor is relying on the absence of a vapour barrier to allow the moisture exit through the underside, and using a regular sealer, they don't know what they are doing.

  3. jk96 | | #3

    Is it a walkout basement or is the full basement under grade?

  4. user-647948 | | #4

    thanks guys it is a walkout basement,
    he has never heard of using any poly under the slab, (and he had done it for years!!) I was going to use 10mil poly, but he is concerned about how to install, and some literate proofing this is not crazy. I will check and see if he uses cure and seal.
    thanks for you thoughts

  5. GBA Editor
    Martin Holladay | | #5

    You can print out this article -- use large-size font -- and let him read it. Call him up in a week and see if he still wants to be your contractor. In the meantime, pick up the phone and get some other bids:
    Polyethylene Under Concrete Slabs

  6. jk96 | | #6

    The reason I asked about being a walkout is that we built our house 3 years ago and had the same discussion you had with your contractor. He stated to us that if it was a walk-out basement then poly was not needed and would also weaken the concrete as all of the water had to evaporate through the surface. I thought he was crazy. I asked a few other contractors who told me the same. I'm interested in any further discussion on this in regards to walk-outs. Our walkout is through the garage which steps down from the conditioned space.

  7. GBA Editor
    Martin Holladay | | #7

    Whether or not the basement is a walk-out basement is irrelevant to this discussion.

    The problem appears to be concrete contractors who haven't taken the time to subscribe to a single trade magazine or attend a single course on concrete. Some of these contractors were apparently trained by their grandfathers.

    These contractors are enabled by state regulators who have no educational requirements for residential concrete contractors.

    Any Jim or Bob can buy a power trowel and a pickup truck, unfortunately.

  8. jk96 | | #8

    I was just curious if there was difference. We did not end up putting a vapor barrier down, just 4" of clean gravel. I guess we have been lucky so far with no moisture issues to date.

  9. davidmeiland | | #9

    Whether or not there is a moisture issue depends on at least a few variables. If you're built on a dry spot and the soil drains well, your chances of success are better than if you're in a wet area and there is clay. If the house has a lot of air leakage then you might not notice a humidity load from the slab. If you then tighten the same house, persistent high humidity might emerge. One or two occupants who work long hours out of the house might have fewer problems than if three people are home all day.

    I was at a house today where a large art/craft area has a concrete floor with no vapor barrier under it, and there are humidity issues. Their best choices are to either install a floating floor over a vapor barrier, ventilate more, or both. Their first instinct was to get a dehumidifier.

  10. user-647948 | | #10

    thanks every one, I will send the info on and some stuff from building science of america. I can't wait to have the Manuel J discussion in a few weeks. !!

  11. latexia | | #11

    hey there... thought i'd throw my $0.02...

    by no means am i a concrete contractor... but i like to read and gather information...

    i too am in the search for a contractor for doing my basement slab. so far, all of them told me the same thing... no vapor barrier... why did i ask... because the concrete can't cure properly... all the water in the concrete won't be able to sip down... instead it will go up and the surface, once dried, will become dusty... also referred to as chalking.

    one guy told me that this might not be the case if i use 35MPa... and he is not guarantying the work.

    so what am i left with? vapor barrier and dry pack 1200 sq ft and then pour SLC over it? vapor barrier and then go 35Mpa? vapor barrier and then Go 25MPA and then extra sealer to prevent chalking? sell my house and go in the middle of the Arizona desert?

    problem with 'old' concrete people is that they work the same way they were doing it 40 years ago and they show their new employees how it was done way back then...

    oh... and i am not telling you about in slab radiant floor concrete people... they think that heat rises... so wrong... heat radiates... warm air rises... not the same thing!

    if heat was only going up... why is the Sun heating us while it's high up in the sky?


  12. GBA Editor
    Martin Holladay | | #12

    La Texia,
    We live in a big country. In some areas of the country, contractors have been reading responsible trade magazines for 30 or 40 years. In other areas of the country, contractors don't read much, and they're still doing work the way they were taught back in 1979, when they joined the crew as a teenager. Back then, they learned what to do from the older guys on the job site.

    Concrete should be mixed with as little water as possible -- just enough water to make it possible to physically place the concrete. Excess water weakens the concrete. If you have soupy concrete, a fast-and-dirty solution is to place the concrete over crushed stone, with no poly, and hope that the excess water drains into the stones below. If you are a responsible contractor, though, you will tell the driver of the Ready-Mix truck that the concrete is too soupy to place, and you'll insist on stiff concrete.

    For more information on this topic, see Polyethylene Under Concrete Slabs.

  13. Chaubenee | | #13

    Concrete will take time to dry and give up its moisture. Putting plastic and foam underneath wont harm its ability to dry and cure. I used ten inches of Crushed number two gravel which ended up being 162 yards. I also used 2 1/4" eps and 10mil poly. Around the perimeter and on top of the footings it sits and with one inch of xps isolating it from the basement walls. It sounds a little hollow and there is a slight echo because the slab doesnt touch the footings or foundation. But it will be super warm and super dry. Find another mason.

  14. STEPHEN SHEEHY | | #14

    I'm no expert, but my understanding is that, except for some surface moisture, concrete doesn't "dry" in the sense that water in it evaporates. It cures, through a chemical reaction.

    I'm pretty sure they didn't put poly under the bottom of the Hoover Dam so the concrete could dry.

  15. GBA Editor
    Martin Holladay | | #15

    You're right, concrete cures -- but the typical residential concrete mix has more water than necessary for curing. The extra water is added to make the concrete easy to place.

    The excess water -- that is, the water beyond what is needed for curing -- has to evaporate or drain. While it's true that a layer of poly under the concrete prevents any excess water from draining, and thereby extends the time between when the concrete is placed and when it is firm enough to be power-troweled, it's the concrete contractor's job to wait -- not to hurry up the job by leaving out the poly.

  16. Chaubenee | | #16

    Stephen, no one lives in the Hoover Dam. There was no need for poly. And yes, the water in concrete will be given up for months to the air. It cures, but the H2O does not magically transform. I poured my slab in December and I am still running a high powered box fan out the basement window of my new build now. Of course with the rainy winter, before we were dried in, the extra water was helpful to the concrete but to the wood not as much. I just finished framing up and in my case we are roughing in plumbing and ducts. But we expect that moisture will continue to be released by the concrete for some time. That is a drying process.

  17. GBA Editor
    Martin Holladay | | #17

    Concrete continues to cure (and gain strength) for at least a month, and some cases longer. The hydration process (a chemical reaction) requires water, but not much.

    Impatient concrete contractors should remember that reducing the amount of water in the mix not only makes the concrete stronger (although somewhat harder to place), it reduces the waiting time between placing the concrete and troweling.

  18. Chaubenee | | #18

    Also, having done this work, I know that keeping it wet longer will also directly await the strength of the concrete. After the pour of the slab it rained for days. Once we framed it rained more and by the end of thirty days the concrete was still wet. Finally after completely drying the house in, with use of a very high powered box fan, I have been blowing air out the basement window (I held off installing the basement window there) in order to reduce the moisture level from the house. It is pretty bone dry, seemingly now, but I know there is still moisture lurking in that concrete. I won't place insulation into the wall for some time nor will I Sheetrock until perhaps late May. East does it.

  19. Expert Member
    MALCOLM TAYLOR | | #19

    No builder waits six months from when they pour their slabs until they close the walls in because of potential moisture in the slab. The advice here needs to be tempered with a bit more real life experience.

  20. user-647948 | | #20

    just an update,
    I went ahead and told my concrete contractor that I really wanted to do poly under the slab, So I went and got a 6mil poly and put it down my self after work and taped the seams with inches of overlap. The next day the concrete was poured and I came out and checked on it and it was looking good. I got a call then next day from the concrete guy saying my slab has cracks, and he tried to work them out but they still came back. The slab otherwise looks good and has a few cracks at the edges. after doing some research I think I have plastic shrinkage cracks, they seems superficial and don't think they will affect other than the look.
    anyone else had this issue with placing poly under slab?

  21. STEPHEN SHEEHY | | #21

    Are there any control joints in the slab?

  22. user-647948 | | #22

    yes control joints every 10 ft

  23. davidmeiland | | #23

    Did he use wire mesh or rebar in the slab? What about fiber?

  24. Chaubenee | | #24

    Is this a basement floor? If so, no one puts rebar in a basement floor. There is nothing that rebar can do to help a basement floor. And Malcom, what the hell are you talking about? I didn't wait that long to dry the house in to let the concrete cure, I went a few months framing my house because it took time to build it. I don't own a massive construction company at my beck and call and I am sorry I could not get my house framed in less than thirty days. The footings were done in November, the foundation walls were done in December and the slabs in the garage and basement floor were poured a few days before Christmas. The framing commenced a few days after New Years and we had it decked by mid January. By early February we were framing both floors and by late February were were dried in. A few weeks ago the wondows and doors were installed. It was a rainy winter and my slabs got wet, wet, wet during that time. Why don't you comprehend better? And in 2 story 4800sf house there is a lot of moisture that will be given up by not only the concrete (210 yards of it by the way) but by the wet wood. What "real world experience" do you suggest I temper? I am sorry I am not a Mercedes driving erudite Dbag, but in order for me to build a house during winter, I need time because I work for a living in my self-made world with 15 employees depending on me 12 hours a day, seven days per week. I am not a home builder by trade with a Haaavaaard degree... Who the hell are you?

  25. Chaubenee | | #25

    And where do I tell anyone to wait six months to close their house in, Malcom? Stop placing words or statements into my text boxes. The Sheetrock won't be done until May not because I am worried about moisture, but because it takes considerable time to rough in all my electrical and mechanicals and plumbing. Again, not an owner of a construction company and rely on myself, my friends and family and select contractors who are up to the job. I did jot advise OP to wait that long anywhere in my posts. I simply explained that drying the house takes time and more wet weather you have during concrete time and frame up makes for more moisture in a building. There is some real world facts for you. This is why I don't associate with builders; they are highly arrogant and most of them started with daddy's money.

  26. GBA Editor
    Martin Holladay | | #26

    A few cracks probably won't affect the performance of your slab.

    Slow slab curing usually reduces cracking; the traditional way to make sure a slab cures slowly is to install a layer of damp burlap on top. There are other methods that can be used, especially in hot, windy weather: once the slab is firm, it can be covered by hay or temporary poly, or it can be misted with a sprinkler.

  27. Chaubenee | | #27

    I used hay. The rain did the job the sprinkler would do. Just keep it wet longer. The wetter it is for a long period, the stronger it will be. That is real world experience from pouring, screening, floating and even stamping concrete for thirty five years. Skip the rebar or mesh if it is your basement. This isn't the garage floor. You won't be driving on it, it won't be subject to frost heaving, the will be little load on it except a small furnace/boiler or maybe a pool table and if you have the crushed gravel over virgin soil and foam, it is not going to be settling on you.

  28. Expert Member
    MALCOLM TAYLOR | | #28

    Easy Joe. For The answers on GBA to be useful they need to be accurate but also tempered by knowledge of how houses actually get built. Readers also need to be able to recognize what practices make no appreciable difference to the quality of the construction, what needs worrying about and what doesn't.
    Take something simple like wall sheathing; To maximize the resistance to shear it would be better if all plywood was nailed at 3"o.c. on both the edges and in the field. It should also have the edges blocked. But if you saw me routinely telling posters it was necessary for their builds, wouldn't you call me out as suggesting something time consuming and expensive that didn't in most cases add anything to the house?
    Your advice was that you need to keep the slab moist beyond 28 days and should delay closing in the walls six months later because there might still be excessive moisture. It made no sense. Your two later replies back-track on that, but they weren't what I was commenting on.

  29. latexia | | #29

    Sheesh Joe......... how about a stress pill?

    There is enough violence in the world... and specially Europe and Middle East... we need not bring it here...

  30. Chaubenee | | #30

    I don't like being denigrated by arrogant people who misconstrue my words. I don't take such crap from anyone, not online and not in person. I ever said any such thing or gave any such advice so don't attribute statements to me that I did not make lest the original poster infer that I recommend such nonsense. I wasn't handed a silver spoon and all my life I have been up against the silver spoon boys. So Malcolm, I really have no idea where you are coming from but my real world experience is with blood sweat and tears, not paper or my wife's or Dad's money. Brush up on reading comprehension before you try to insult me.

  31. Chaubenee | | #31

    And to you with the fake alias, I never made threat or violence.. What the hell are you talking about? Is the arrogance, lecturing, lack of comprehension and dealing insult systemic in this industry? I am a perfectly nice guy until you start sticking arrogant, pointy gingers in my face. Read what I posted a few times if you don't get it. I NEVER TOLD THE MAN TO WAIT SIX MONTHS FOR HIS SLAB TO DRY TO BUILD HIS HOME. Is this a vision or comprehension issue? It takes a little guy many months to build a home and it is not because of wet conditions or waiting for the concrete to cure. I guess I am the only poor slob on "pay as you go" and who doesn't get a house built in 90 days. So be it.

  32. GBA Editor
    Martin Holladay | | #32

    OK, time for the forum moderator to step in. I'm not taking sides here. Several people have had a chance to express themselves. Now that that's happened, the topic is over, as far as I'm concerned.

    Comments on this Q&A forum should concern building science, building methods, product specifications, energy efficiency, or green building. No one is interested in reading comments that discuss the personality of other GBA readers or commenters.

    Any attempt to deviate from these guidelines on this thread will be deleted.

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