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Community and Q&A

Carpet in Basement

thegiz | Posted in General Questions on

How’s everyone doing, I should just change my name to basement floor at this point. So I have the right set up I believe. 6mm poly, 1/2 inch eps on top, 1/2 inch plywood, followed by carpet. I’m in westchester county, NY, zone 4 I believe. Walls are not insulated. So my questions are:

1. can I use peel and stick carpeting on top of wood? Will this be too hard for toddlers?

2. is there a floor finishing I can put directly on eps and then use area rugs with padding (this is for kids playroom)

3. I want to build and insulate walls last. Any way I can loose lay 1/2 inch plywood so I can lift up later or do I insulate and build walls right over wood? Is there a flooring I can use such as hardwood that wouldn’t require me attaching to concrete?

Thanks as always

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  1. Expert Member
    BILL WICHERS | | #1

    1- Carpet squares should work fine, which is probably what you're thinking of. Just make sure you have a clean surface to stick them too. It might help to prime the wood first, but you can always do a stick test with a single carpet square to check.

    2- You need some type of subfloor to protect the EPS sheets. EPS itself is probably about the weakest of the rigid foams in terms of resistance to "chunking". You said you're planning on putting a layer of 1/2" plywood over the EPS which should be fine.

    3- You could potentially put rigid foam directly against the basement walls (which I assume are some type of masonry foundation), then put plywood (or drywall) directly over the rigid foam panels. You don't necassarily need to build out studwalls here. More detail on your walls would help to get more detailed advice though.


  2. thegiz | | #2

    Hi Bill thank you for the advice.

    The walls are old stone foundation walls but there is a layer of concrete making the walls flat. I have no problem with using 1/2 inch plywood on floor but:

    1. Is there anything with enough strength for this floor that is slightly thinner than 1/2 inch plywood? Or maybe some other combination thinner wood, stronger foam.

    2. If I do floors first what kind of gap so I need to leave between floor and wall. I would prefer skipping the stud wall build. Is there anything wrong with foam against the wall and sitting on the wood floor when walls are built last?

    3. Can I float the floor in any way without increasing thickness? Trying to skip screwing it down in case I need to remove it.

    1. Expert Member
      BILL WICHERS | | #4

      1- You could try 25 PSI rated foam (both EPS and XPS are available with this rating), and maybe 1/4" hardboard instead of 1/2" plywood. You'll have more risk of punch through from point loads (heavy things with small feet, like pianos or pool tables, for example) this way though due to the thinner "hard" layer.

      2- Typical expansion gap around the perimeter of a floating floor is 1/2", but that can vary a bit depending on the size of the floor. What I would do is to put the floor down first, leaving that 1/2" perimeter gap, then put rigid foam and drywall or plywood directly over the basement wall and down flush to the floating floor. You could use tapcons to secure the drywall/plywood through the rigid foam directly to the masonry wall without needing any studs in between. The downside to this is it's difficult to hide things like wiring, and drywall is a little more prone to squishing than plywood if you use tapcons like this.

      3- You could use a double layer of 1/8" hardboard with seams staggered, but you need to be sure the seams are pretty well seperated when staggering (maybe a foot), since the 1/8" hardboard seams are big weak spots. Staggered layers of 1/4" hardboard would be much better, but also 1/4" thicker. I'd really try for the thicker material if at all possible. I don't see any issue with floating the floor though, just keep in mind that you might have a bit of a "hollow" feel to the floor in any low spots since the floating floor will try to span over any depressions in the underlying "real" floor and those areas will squish down when stepped on.


  3. Jon_R | | #3

    > in case I need to remove it

    Given the multiple ways basements can get wet, this is a good idea. So is a wall-to-wall rug vs carpeting. IMO, with just people walking and the right density foam, you can go thinner on the plywood.

    1. thegiz | | #44

      By wall to wall rug do you mean it is just not attached to anything. I was thinking just laying it down all the way to edge or using a double sided tape. Easier then having to rip off a tack strip in case of removal. Not sure if it would wiggle around constantly. I also thought of peel and stick. If an area needs to be replaced just remove and put new squares

  4. Expert Member
    NICK KEENAN | | #5

    I would see if you can negotiate throw rugs rather than carpet. Much easier to throw away when they inevitably get wet.

    If you do a foam wall, it doesn't sit on the floor, it hangs off of the wall. So no reason why it can't sit on the plywood subfloor. That said, I like to have a bottom plate at the bottom of the wall, it gives something for the bottom of the drywall and the baseboard to attach to. You want that wall assembly to be airtight, and having a bottom plate makes that easier. And if you use moisture-sensitive insulation (like polyiso) that can't directly touch concrete it provides a buffer. It also means fewer concrete fasteners into the wall. If you're going to do a bottom plate, I'd put that in before the subfloor and just run the subfloor up against it.

    In terms of floating the subfloor, the only way I know to do that is two layers of plywood screwed together with overlapped seams. Luaun plywood is 5mm so two layers would be 3/8".

  5. user-6623302 | | #6

    What is driving the desire to reduce the plywood thickness. If it is cost, remember that skimping on materials may lead to an unsatisfying result and actually costing more. Also remember, it is a basement. Paint the floor and put down indoor/outdoor carpet with a pad. The kid do not care.

  6. thegiz | | #7


    It's all about ceiling height along with not worrying about mold. My wife really wants carpeting, I could try to convince her with painted concrete and outdoor rugs. Maybe there is an economical way to put something hard enough over the 1/2 inch eps and then use cheap outdoor area rugs.

  7. Jon_R | | #8

    +1 on giving serious consideration to a thick pad (with a wall-to-wall rug) and no EPS or plywood.

    6 mil poly over the floor may create mold underneath it and odor (see Fig 7 here). A fully adhered low perm coating plus the ability to dry upwards reduces this risk.

    1. Expert Member
      NICK KEENAN | | #10

      The problem with basements is there is no thermal drive for drying. Moisture tends to flow from warmer to cooler. Typically in a basement the walls and floors are cooler than the interior year-round. The thermal drive is toward the outside. But a well-constructed basement has a moisture barrier, so the moisture just gets driven into the moisture barrier.

      This is why carpets in basements always get moldy.

    2. Expert Member
      NICK KEENAN | | #11

      I don't see an issue with mold between the poly and the floor if the poly is taped at the seams and edges. It may be an issue down the road for the guy who has to take it up, but for the people living in the house there's no way for the mold to get out.

  8. thegiz | | #9

    Any recommendations on the exact products/materials to use in this situation. How often or when would I have to clean this flooring, take it out or dry it.

    1. Expert Member
      BILL WICHERS | | #12

      I would epoxy seal the floor instead of using poly. No mold risk that way, and it'll be easier to clean forever. Be careful doing this though, either use one of the new water based epoxies, or put the epoxy down on a friday and then leave town for the weekend. Many epoxies will stink you out of the house while they cure. I actually write into my project contracts that epoxy floors (we do a LOT of those commerically in mechanical rooms) are to be applied Friday, and ONLY Friday, for the safety of the other trades. I have had many of my subs thank me for that too.


  9. thegiz | | #13

    Thanks Bill I will look into an epoxy, water based would be easier to use. Any you know of in the big orange and blue stores?

    So Jon what thick pad and rug (wall to wall) are you referring to. I’m assuming the pad would be open celled to allow air flow and drying. The rug would be some kind of indoor/outdoor rug. Wall to wall? How would you tack it down with a pad?

    1. Expert Member
      BILL WICHERS | | #14

      I like the PPG products. The only downside is some of them are only available in 5 or 55 gallon sizes. That's not a problem for a 10,000 square foot mechanical space, but it's bit much for the typical residential project :-)

      They do have some 1 gallon size products (two 1 gallons, since there are two components to the epoxies) which would be good sizes. They aren't cheap, I think the one I used was around $200 for the two gallons, but I got a deal on it from my local PPG store. If you go tell them what you want to do, they can recommend one of their products. Many of the epoxies they'll have to order in, and they might have to have someone call you back to talk about those since the people in the paint stores aren't always familiar with the more exotic coatings that their company sells.

      I haven't used the Rustoleum products the box stores usually carry. I wouldn't be surprised if you can get good results with them, but I think they are very expensive for the quantity of material you get in the kits when compared to the commerical products I'm more used to using.

      A note about anti-skid/slip additives too:
      There are three kinds of anti-slip additives I've seen for epoxies. The first is a sort of plastic chips that get sprinkled in. These look OK, but the floor will still be pretty slick, especially if it gets wet. I don't usually recommend these for my commerical projects. These are the most common anti-slip additive you see.

      The second kind is a bit like coarse sand. This is my favorite -- it's grippy, but not too aggressive. I recommend this one for garages especially and prefer it for my commerical projects.

      The third one is pure evil. It is metal swarf, basically sharp little bits of metal. It is AWESOMELY GRIPPY, and works spectacularly well as an anti-slip additive. The evil part is that if you fall on it, it will cut you, as in your hand will bleed if you break your fall with your hand. It will slice your pants if you sit on it and slide (ask me how I know...). I think this one is dangerous. I specifically disallow this type of anti slip additive on all of my commerical projects.

      In your particular project, you MIGHT be OK without an anti-slip additive, or use the plastic chip type which is the least aggressive. The reason for this is that as the anti-slip additives get more aggressively grippy, they also make the surface more difficult to clean. That's the trade off.


  10. thegiz | | #15

    Thanks Bill for all the advice. Once I epoxy the floor I have narrowed down my options to:

    1. Leaving concrete sealed with epoxy. Using some kind of carpet pad that can be laid directly on concrete, not sure what, with outdoor carpets on top. Probably cheapest option.

    2. Trying dmx one step carpet. They claim you can add carpet right on top of it. I read in articles on here that these products are just too thin and do not offer any guaranteed R value. This product claims it allows moisture to evaporate from bottom with an air gap. I guess this is kind of a similar concept to leaving air permeable carpet pad directly on concrete. I could use peel and stick outdoor carpet directly on top and if I start to see a mildew problem remove them pretty easily.

    3. Putting down the 1/2 eps and staggering 2 layers of 1/4 plywood. I would loose an inch of height, probably the most expensive option. If I notice a mold problem or some kind of flooding takes place, a lot harder to remove.

    1. Expert Member
      NICK KEENAN | | #16

      DMX just makes absolutely no sense to me. It's advertised as "an air gap on the bottom side of the membrane to evaporate moisture causing mold, and a completely waterproof surface to repel accidental spills." And they recommend taping the seams. So how is anything going to evaporate with a "completely waterproof surface" with taped seams? And even if it did allow evaporation, you don't want that anyway, you want to keep water vapor out of the interior.

      I suspect that what it really does is make your floor somewhat tolerant of small amounts of water seeping in. The water has a place to go under the dimples, where eventually it can be absorbed by the concrete. The taped waterproof surface keeps it from evaporating and introducing vapor into your living space, and keeps your carpet above the water level.

      That might work, in a way. But it turns me off that their marketing blatantly misrepresents what's going on.

  11. thegiz | | #17

    DC either they have no idea what their product does or they thought it sounded better to explain it that way. I wonder if I put a thick enough carpet pad with a moisture barrier on top it would boost my r value and protect me from condensation. Not sure if the dmx would flex too much with the added padding or it would make the whole idea fail. Something like this:

    Prime Comfort 1/2 inc. Thick Premium Carpet Pad with HyPURguard and SpillSafe Double-sided Moisture Barrier

  12. jberger | | #18

    If you are looking for something from the box stores, the orange store sells a pad with air channels under the lifeproof brand.
    It's also available in 2 sizes since you are concerned about overall height.

  13. thegiz | | #19

    Thanks Jayson, wonder if that would work better alone than the dmx one step carpet or if you could use both by placing pad on top of dmx plastic. I don’t know if there would be any benefit to having an air channel between concrete and an additional air channel under the carpet.

  14. Expert Member
    Michael Maines | | #20

    Hammer, just to be clear, this is the project with the 1/2" thick slab and at least one mystery void below the slab, correct?

  15. thegiz | | #21

    Yes detective, you found me. You sound condescending are you here to offer advice or try to bully me.

    1. Expert Member
      Michael Maines | | #22

      I am not trying to bully you or to be condescending. I think the others giving you what would normally be good, safe advice should understand that you have a non-standard situation. Adding non-structural layers such as carpet padding or 1/2" EPS with 1/4" hardboard over a slab that I wouldn't even call marginal, more like a wash coat over soil, does not seem smart to me. But clearly you are committed to doing it.

      It's been standard practice on GBA from the beginning to ask that posters keep follow-up questions to their original thread instead of starting a new thread every time. It helps keep the main page from clutter, but more importantly it helps those of us providing advice to understand the situation. And since Q+As are at least as much for future readers as for the original poster, they can see the advice we provide in one place instead of scattered through multiple posts. It's basic forum etiquette.

    2. Expert Member
      MALCOLM TAYLOR | | #23


      It's hard to tell if you are really looking for advice, or just confirmation of what you want to do. Following the suggestions made in this thread without doing the remedial work which is necessary to fix the problems you have detailed in many other ones you have posted makes no sense. Fly at it, it's your house, but if you don't want any criticism of your approach, you need post on your own blog, not a building science website.

  16. thegiz | | #24

    I’m not sure how you could evaluate my slab condition and thickness over an Internet forum. If I said that I think my slab was thin at any point it was because I thought it might be. I have since had professionals look at my situation and corrected me. I appreciate everyone’s advice here. I’m trying my best to find a safe and economical way to finish my floor. If I asked multiple questions it was because I was doing my research. A recommendation to rip out an entire slab would be extensive and impractical for most people. If I was set on doing it my own way why would I ask so many different questions. Anyway you are right that I’m not a building scientist, I’m not going to start my own blog I just won’t ask for advice. Even though this is called green building advisor

    1. Expert Member
      BILL WICHERS | | #25

      A lot of the concern is people here don't want to see you invest a lot of time and money just to have to do everything over if you have an underlying problem. Typical concrete slabs are 4" thick. Thinner than that is a "rat slab", and not really expected to hold much up. Even thinner than maybe 1.5-2 inches is starting to get into "not really a slab" territory.

      Basically if you already have a thin slab, then it's weak. Keeping the flooring materials thin will not help spread out load, so you end up with even more risk of point loads (pianos and pool tables :-) punching through and damaging your floor.

      I would look at it like this to be practical:

      When you walk around on your existing slab does it seem solid to you? No squishy/crunchy spots? Can you jump on it without it cracking or deflecting? Do you think you could safely put whatever the heaviest thing you expect to put down there is without the floor cracking? If the answer to all of that is "yes", then you're probably safe to follow the floor finshing advice everyone has given you in this thread -- but understand that you'll have a weaker than normal floor, so you need to limit what you put on it.

      If the answer to any of that was "no", then you really need to fix the underlying problem slab first before finishing it, or at least use some heavier subfloor materials to spread the load (sort of a "band aid" for the weak slab). If you only have a few thin spots and a void, you might be able to just manually remove the trouble spots with a hammer and/or chisel attachment for a rotary hammer, then patch/fill with concrete mix from the box store. It won't matter much if it doesn't look perfect if you're planning to cover it up with insulation and carpet anyway.

      I think mostly everyone here just wants you to end up with a good result and not have to redo things down the road. No one likes to waste time and money after all :-)


  17. Expert Member
    Michael Maines | | #26

    For anyone following along, to get a full picture of the situation, these are the other questions Hammer has asked about his basement floor:

    I might have misinterpreted a comment in one of them to understand that his slab is 1/2". That's an example of why it's better to keep questions on the same topic to the same thread, as I have asked Hammer to do in a few different threads now.

    Hammer, none of us get paid to be here, we do it because we like helping people and sharing solid building science information, which can be hard to find for free online.

    1. Expert Member
      NICK KEENAN | | #33

      So it's not my imagination that we kept answering the same questions over and over.

  18. Expert Member
    Dana Dorsett | | #27

    Neither EPS nor XPS are fire rated, and can't be used without a timed thermal barrier against fire on top of it in this floor application. At 1/2" even the plywood will not meet spec, but 3/4" would.

    There is no way even Type X EPS/XPS can be used under some non-fire rated flooring without the fire barrier, and without a subfloor to spread the weight it would quickly become dented by everything from furniture to dropped coffee mugs, etc.

    At any thickness EPS/XPS on the floor, it needs 3/4" subfloor, or a double layer of 1/2" plywood to meet fire code. With either of those the bottom plate of studwall can and should be fastened to the subfloor.

    Tiling the floor would not need insulation or subfloor, but any rug will have moisture issues. With even the modest R value of the rug above the slab temp would average near the subsoil temperatures, which will often be below the dew point of summertime ventilation air in Westchester county. Even if the bottom of the rug is still above the dew point, the relative humidity of the entrained air can easily exceed 75%, creating mold conditions on the bottom of the rug.

    1. Jon_R | | #28

      A good point, but:

      "R316.5.13 Floors
      The thermal barrier specified in Section R316.4 is not required to be installed on the walking surface of a structural floor system that contains foam plastic insulation where the foam plastic is covered by not more than a nominal 1/2-inch-thick (12.7 mm) wood structural panel or equivalent.

      Carpet + pad also provides additional thermal resistance, but no idea what exactly "equivalent" is in this section.

      P.S. "not more than" appears to be a typo.

      1. Expert Member
        BILL WICHERS | | #31

        This is an interesting question, since most of the fire ratings have to do with vertical flame spread -- fire rising on flammable stuff, basically. That's why there is a "vertical flame test" for stuff like wire, to make sure the fire self-extinguishes in some set length of vertical cable to prevent a fire spreading.

        A horizontal surface might need an ignition barrier, but are you sure about a thermal barrier? 1/4" hardboard counts as an ignition barrier, and two 1/8" layers or one 1/4" layer should meet that requirement. Structurally there are other issues, and I'd be concerned with the seams, but it seems to satisfy the code requirement as written.


        1. Expert Member
          Michael Maines | | #38

          Bill, yes, this comes up often when building and remodeling basements. You can't leave foam plastic exposed, if the space is accessible, with a few specific exceptions. Thermax and I think another brand of foil-faced polyiso have passed flame-spread and smoke-developed tests so they can remain exposed, but shouldn't be used on floors because they can absorb water. Jon shared the NYS code section above; here is the IRC version: See section R316.5.13.

      2. Expert Member
        Dana Dorsett | | #55

        Thanks for that Jon! (Some typo, eh? :-) )

        So a mere half inch plywood IS sufficient (if barely) for basement slabs.

        And clearly using Type-X or denser foam to make it more "walkable" without the plywood or OSB wouldn't meet code, even if were somewhat more rugged against damage from foot traffic & furniture.

        1. thegiz | | #56

          If code states you need 1/2 inch wood over foam then isn’t this
          High compressive eps with a name tag, says you can put flooring right over it but if you couldn’t do this by code

  19. thegiz | | #29

    Bill, I never felt like the floor was weak or feared it would crack, then again I can't see anything underneath. I would def be taking a risk with a less than solid sub floor.

    Dana, based on what you are telling me I need at least a 1 1/4 inch floor for code. 1/2 inch eps with 3/4 plywood. I know those osb subfloor panels that dricore sells are only an inch thick. Unless it is really a 1/4inch foam with 3/4 plywood, or as Jon is saying you can have 1/2 inch plywood over a foam insulation. Not sure where that code comes from.

    Jon you liked the idea of a thick carpet pad with rug rather than the eps+plywood. The big orange store makes life proof carpet pad that is supposedly safe to put directly on concrete. I think it's somewhat new, Jayson linked it above. Not sure if it actually works and doesn't absorb moisture or mold. Maybe dmx one step carpet is better engineered but I don't know.

    Overall its either I build at the least an inch to 1 1/4 subfloor for carpet, (floor would be strong but loose ceiling height) I use one of these systems dmx one step carpet or life proof carpet pad (not sure if they actually work) or place outdoor/indoor carpet directly on concrete. Otherwise I have to say bye bye to any carpet

    1. Jon_R | | #30

      I can tell you that I have vapor permeable pad and carpet over concrete and have absolutely no mold odor as long as I keep humidity below 60%. And this is in colder Z5 with a partially exposed basement (which makes the concrete colder in the Spring).

      I have had multiple occasions when I wished the carpet could be easily pulled up and was glad there wasn't any wood there.

      1. Expert Member
        NICK KEENAN | | #34

        While air sealing and insulation are good, indoor humidity control is the sure bet.

        The only issue I have is that most portable dehumidifiers have trouble running 24/7.

        1. Jon_R | | #35

          My portable dehumidifier never comes close to running 24/7. And it would run the same amount if I had a foam insulated basement floor. On the other hand, a better insulated floor would save a little energy and be slightly more comfortable.

          1. Expert Member
            NICK KEENAN | | #36

            I'm not saying it has to be actually running 24/7, but it has to be prepared to run at all times. My experience with portable's is that they tend to shut down a lot. The condensate drain tends to be fussy, if it's gravity feeding to a drain it's easily clogged, if it has a pump the pump isn't reliable. There are stupid design flaws, like the condensate tray that shakes loose from the vibration of the compressor and then the whole thing shuts off. Most won't restart if they lose power. Or there are ones that have a pump, and will restart after losing power, but the pump has to be turned on manually each time it restarts. The designers don't seem to understand how they're used.

          2. Jon_R | | #37

            Mine has worked well. Had a couple die over many years, the result was nothing more than "smells slightly musty, check the dehumidifier".

          3. thegiz | | #40

            Jon, I have a portable dehumidifier, do you manually empty the water tray or do you have it draining somewhere? Did you seal your floor underneath the pad, Bill outlined how to epoxy the floor, I think epoxy strengthens and seals the floor against moisture. Am I getting the idea right or the recommendation for epoxy is if the basement floor remains uncovered

          4. Expert Member
            BILL WICHERS | | #41

            My portable dehumidifier drain line would often clog, and it was nearly impossible to clean the connector -- plus the hole in the side wasn't really big enough for the hose fitting to fit through. Eventually I'd had enough of that, so I drilled a hole in the side of the tank, screwed in a bulkhead fitting, and connected the drain hose to that fitting. That was nearly a year ago, and I've had no further problems. I used to have to clean it nearly weekly.

            BTW, a bit of bleach in the drain hose periodically can help keep it clear of gunk.

            Hammer, epoxy doesn't "strengthen" the floor, but it does seal it. The epoxy can also lift if you have oils or water coming up from within the concrete, so you need a clean, dry surface for application. Once the epoxy has cleared, it makes a pretty decent vapor barrier.


          5. Jon_R | | #43

            I have a hose that drains the dehumidifier. No vapor barrier on the floor, but would be better if I had a fully adhered one.

    2. Expert Member
      BILL WICHERS | | #32

      Products don't have to meet the written code if they are listed assemblies for a specific purpose. This is why the code says all the stuff about thermal and ignition barriers for rigid foam, but there are some products (Dow Thermax, for example, Johns Manville has a similar product) that are specifically lab tested AND LISTED (UL, etc.) to have certain fire resistance WITHOUT the code-mandated protection. That is why sometimes the code says things like "bla bla bla EXCEPT where materials listed for the purpose are used". This is similar to the "except under engineering supervision" line in the code -- we engineering folks are supposed to know enough to be able to design things to be safe in the PARTICULAR application in question.

      The code is a GENERAL set of rules that is supposed to GUARANTEE a minimum level of safety in ALL cases. The code cannot anticipate everything, so they left a way for engineers (through careful design) and manufacturers (through careful testing) to provide builders with options for unique situations.


      1. Expert Member
        Michael Maines | | #39

        Bill, I'd word that slightly differently--IRC sections R316.3 and R316.4 specify which tests the product has to meet:

        1. Expert Member
          BILL WICHERS | | #42

          I was simplifying a bit. My point was more that there are products out there that can "meet code" without having to be installed using the general code requirements. The important part is that the testing and certification that those products work has to be done by testing labs, not people at home :-)

          I've seen people try to make their own versions of listed products before, and I think we've even had a few discussions on GBA with people asking about that, usually along the lines of "why should I pay them to do something I can do myself". That doesn't work, because you don't have a controlled manufacturing enviornment at home to ensure product consistency, and you aren't a recognized testing lab that can do controlled tests to make sure your "product" works as expected.

          I suppose the simplest way to word the general rule is: "build it per code, or use materials listed to be used in your application, or hire an engineer to design a solution".


  20. thegiz | | #45

    So I reached out to the manufacturer of lifeproof carpet pad. For the sake of Maines I did not create a new post. But here is what they said:

    Good morning,

    This is in follow-up to your recent questions about Eco Cork Foam.

    Question: I’m very interested in using your product for a basement floor. There doesn’t seem to be anything similar on the market. My basement is in a 100+ year old home. The floor never floods but I have occasional dampness from moisture wicking up. Can I still use this pad on my concrete floor? Can I apply a waterproofing paint or other product on the bare floor before putting down carpet pad? Thank you in advance for your help.

    Answer: Thank you for your interest in LifeProof Carpet Cushion. It sounds like you have some moisture vapor to contend with and no pooling or sitting water which is good news. The great news is that you absolutely can put a coating down but not really necessary. There are some things to consider when applying coatings over concrete. Moisture levels need to be tested to ensure a coating will even adhere to it. If the moisture level will allow for it to dry, very often, it has to be etched with muriatic acid to create enough of a surface profile to bite to. Having a waterproof coating between the concrete and coating, can often create an ideal environment to trap moisture and the result is concrete alkalinity that often has devastating effects on concrete causing physical breakdown and premature degradation.

    Personally, I would not recommend a waterproof coating in conjunction with LP pad as it is an unnecessary step. LifeProof Carpet Cushion was created specifically for these types of conditions. LifeProof Carpet Cushion is comprised of polyethylene foam which is waterproof. The built in air channels will allow the moisture vapor to dissipate and it is infused with Ultra-Fresh to inhibit the growth of mold, mildew and bacteria. You truly can worry less with LifeProof Carpet Cushion.

    So does that make sense from a building science perspective? I was planning on using theroseal or drylok on my unfinished wall. Would the same science apply to my walls? Would I better off with a breathable paint, like concrete paint or something permeable like 100%acrylic

    1. Expert Member
      NICK KEENAN | | #46

      They were good until "the built in air channels will allow the moisture vapor to dissipate." You want a vapor barrier, period. You want to control the vapor, not dissipate it.

      If all you're worried about is vapor wicking through the concrete, you just need a vapor-proof layer. It doesn't have to be particularly strong and it doesn't have to stand up to pressure. Polyethylene will do.

      If you're worried about liquid water coming through the walls (most likely rainwater from above) or floor (most likely groundwater from below) no coating or barrier is going to be sufficient. You need to redirect that liquid someplace lower than the level of the slab.

      I feel like you keep asking the same questions over and over and we keep giving the same answers.

    2. Expert Member
      NICK KEENAN | | #49

      The reason I get my dander up about claims like "the built in air channels will allow the moisture vapor to dissipate" is that many people have an intuitive sense about how moisture flows in a basement, and that intuition is at odds with the science. The field of basement remediation is chock full of charlatans who prey on the unwary, peddling ineffective and unnecessary solutions.

      So my charlatan alarm goes off when I hear something at odds with the science.

  21. thegiz | | #47

    I disagree that it is the same question because it is too different ideas. It’s similar to the idea of putting a poly vapor barrier over your stud wall and insulation. Moisture becomes trapped in the wall instead of drying towards the interior. EPS is water tolerant but it still breathes. In a new slab floor you put poly and then your insulation before slab correct? In an old concrete floor there is no poly or Insulation. So in a retrofit with a carpet pad resembling eps, probably not the same but breathable. Should you use a vapor barrier liquid or poly on the bare concrete or let the concrete breathe. Same idea with the walls if it is unfinished or uninsulated do you want a vapor barrier or waterproofer or should the wall breathe as well. I did what I could on exterior to redirect bulk water. Waterproof paint will never completely stop water infiltration and I understand that but will waterproofer do more harm than good. I’m not dealing with liquid water if that makes a difference. I don’t even have wet walls, just small areas of dampness on floor

    If you are trapping water vapor in concrete I guess it’s different than trapping it in a wall because there is no wood or organic material to eat. Will the moisture as vapor stay under the poly or cause a mold problem, I feel like there is 2 train of thoughts here on what is right.

    1. Expert Member
      NICK KEENAN | | #48

      Never let the concrete breathe. Ever. Nothing good can come from that. You want the moisture that is in the concrete and the dirt behind and below it to stay there. Concrete is completely unharmed by moisture. Any moisture that is in the concrete will do a lot more harm in the basement than it will stuck in the concrete.

      So you want a vapor barrier lining the entire concrete of your basement. But this doesn't have to be elaborate, a sheet of plastic under your finish will do. If you have no liquid water intrusion, and a vapor barrier, any moisture that gets into your finished wall and floors will be coming from the interior -- primarily from humid air condensing. Let me repeat that, because it's important: If you have no liquid water intrusion, and a vapor barrier, any moisture that gets into your finished wall and floors will be coming from the interior.

      There are a couple of strategies to keep that moisture out. All of your finishes should be air sealed to prevent the flow of air. An insulation layer keeps humid air off of cool surfaces. But the best way is to keep the humidity out of the basement in the first place by conditioning the air, either with air conditioning or a dehumidifier. You should also make all of your finishes out of materials that don't absorb moisture.

      To the extent that moisture that gets into your finished wall and floor will dry, it will only dry to the interior -- because you've put vapor barriers to the entire exterior. In practice you won't see much or even any drying, and it's better to assume that there will be no drying potential and plan accordingly. The reason there will be no drying potential is that there are two things that drive drying: moisture flows from wetter to drier, and from warmer to cooler. In a basement, year-round, the interior is warmer than the basement walls. Year-round, the moisture drive from heat is toward the exterior -- which you've blocked with vapor barriers. The only way the finished walls can dry is if the wetter-to-drier drive overcomes the warmer-to-cooler drive. Which means that the basement air has to be significantly drier than the interior of the wall or floor.

      Now, do you recall what was the only source of moisture in the finished parts of a well-sealed basement? Humidity from the interior. And what's the only way a basement wall can dry? If the interior air is significantly drier than the wall. So the only scenario in which drying is possible is the one in which it isn't necessary. I'm going to say that again too, because it's important: the only scenario in which drying is possible is the one in which it isn't necessary.

      Build your basement with the assumption that drying won't be possible, so it can't be necessary.

  22. thegiz | | #50

    DC good explanation, so in simplistic terms in my head basically first keep all water and vapor out from exterior. Concrete is not harmed by moisture so trapping it in concrete will do no harm. From there you are dealing with moisture from the internal air which is controlled with AC, dehumidifier, insulation, materials that don’t absorb moisture... in terms of applying this. 6mm poly on my slab will be fine. It will be under a floor covering. The walls if left unfinished couldn’t be draped by poly, it would look like every piece of furniture in grandmother’s living room growing up. Without insulating walls to prevent condensation (architect told me I can’t finish walls before an addition due to hassle with building dep) I would need a liquid vapor barrier. Now this is a stupid question but is drylok and it’s better half theroseal a vapor barrier and waterproofer or is there a liquid product that is more suited for this.

    1. Expert Member
      NICK KEENAN | | #51

      The web pages for both Drylok and Drylok Extreme say "Breathable film will not trap moisture in the masonry." That's not what you want. If you're going to paint you want a vapor barrier paint. I'd look at vapor barrier primers.

      There's a couple of reasons I like a sheet vapor barrier. If you concrete has a lot of moisture in it you're going to have trouble getting any paint to stick to it. If your rainwater management is good but not perfect -- say, a cup of water might get in during the biggest rainstorm of the year -- the sheet is going to perform better. If you have minor leakage, what you want to happen is the water stays behind the vapor barrier, runs down the wall to the floor where it goes under the vapor barrier, then soaks into the concrete and dissipates into the ground. (Incidentally, this is what I think the grooved channels in the carpet underlayment are really meant for.) You don't want that water getting on the other side of your vapor barrier. A plastic sheet is going to do a better job, because it can just move out of the way and let the water by. Paint has a hard time with water from behind, and if it leaks the water is in your interior.

  23. thegiz | | #52

    Came across this article when looking up vapor barrier primers. It argues that vapor barrier primers could out perform poly although this is probably dependent on situation and way over my head. They even referenced Joe L., I know he was recently interviewed on this site. Sheet would probably work best as you mentioned but wouldn’t have the look of a painted wall. Could use poly floor with a vapor barrier primer on wall or keep the whole system liquid just for continuity. For an average diyer i need to figure out the correct product and application. My local paint store might have the answer but sometimes I question their actual knowledge. Anyone use anything like this in actual practice or know what is sold at the big orange or blue guy? This would also have to be something that sticks to concrete.

  24. thegiz | | #53

    I was looking for liquid floor vapor barriers and came across this product:

    Claims you can apply to wet or dry concrete with up to 100% humidity. Seems pretty simple to apply. I wanted to epoxy the floor as Bill described but my understanding is that with epoxy floor it has to be 100% dry. This product claims it doesn’t matter. However I can’t find a retailer anywhere. Is there something else out there similar to this or do you know of a specialty store that carriers it?

    1. Expert Member
      Michael Maines | | #54

      The performance numbers look good: I would be concerned about it remaining in place with hydrostatic pressure (a damp slab trying to dry to the interior) but the installation instructions probably include specific techniques for old slabs. I'd call their technical support line to explain your situation and see what they say.

      In case you haven't seen it, here is a relevant article:

  25. thegiz | | #57

    Reading this over and there’s a lot of great advice. Btw the void in floor was just a random drywell. I’m thinking painted floor (latex epoxy) with large area rugs with pad is cheapest option and can easily be removed. Jon B mentioned this setup and he is able to control humidity and it is fine. What exact material pad and rug do you use? Do you have it attached at all, is it wall to wall? My other thought is just vinyl all over floors but without finished walls not sure if that makes sense and I would think more expensive.

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