Wood Floor, Concrete slab, and Moisture
![](https://secure.gravatar.com/avatar/688863ee6396a5176059297d1c8d152a?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fj.png&r=g)
Hi all, I had a question about proper installation of a wood floor on a concrete slab.
Assuming the slab has been moisture tested (and passes with flying colors), would it be alright to just put a plywood subfloor and finished wood floor on top? Or would I still be at risk of a mold issue in the future?
Reading about this online has pointed me in a thousand different directions. Some places say just to put tar paper under the subfloor, some places say I need a vapor barrier, some places say I need to float the wood floor, and some places say I need TWO layers of plywood subfloor in different directions. I’m really confused.
Then there’s the issue of nailing through the vapor barrier. Am I correct in assuming that by nailing through it into the concrete, I am creating a weak point in the barrier for moisture to come up?
Honestly, I thought installing wood floors would be easy, but this whole moisture factor is opening up a can of worms….
Thanks in advance for any replies.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
Hi Jay,
Providing the slab is flat (1/8" within 8' radius: self leveler can correct some deficiencies), I'd suggest installing a high quality floating floor, such as Kahrs. These are made to go over slabs: no additional subfloor required. The underlayment material that accompanies a float-in installation provides all of the waterproofing you'll need, as long as the slab stays as dry as it is now, which it should.
While floating floors are a bit more expensive on the front end than a traditional nail-down floor, in my experience the cost difference is more than made up by eliminating the need for all that additional plywood. They're also a lot quicker to install.
Good luck,
Chris Koehn
TimberGuides Design & Build
Vancouver Island
Jay,
There are a lot of variables. Is your slab below grade or above grade?
I would install a poly vapor barrier on top of the slab - then install a high quality subfloor like advantech - we typically install it diagonally in the room - then felt or a product like aquabar and then your floor. A few nails in your vapor barrier will be ok.
Even if the slab test ok now - there is no telling what may happen down the road so I would not leave the vapor barrier out. we just had to tear our an enitre house of hardwoods that was installed over a slab that had been there for 20 years with no problems - out of nowhere, the hardwoods buckled and there was mold on the subfloor. No vapor barrier was previously installed.
Martin, the area is below grade (basement)
Thank you all for the replies. I'll start moving on with the project and see how the tests go.
I don't think the testing is much use. First of all, how are you going to test? Tape plastic to the slab? Buy a test kit? Buy a Tramex meter? Buy a meter for ASTM 2170? Even if the slab seems fairly dry now, I would still stick with an engineered floating floor over a vapor barrier. A slab below grade is very unlikely to stay dry forever, even if it is dry now, so the more precautions you take the better. If it were my job I'd probably try to talk you into a porcelain tile floor.
Jay,
Installing a wood floor over a basement slab is always somewhat risky, because many basement slabs are subject to occasional moisture-entry events.
You didn't mention whether there is any rigid foam under the slab. Ideally, your slab should be insulated.
Here is more information on how other people have install a plywood subfloor over a basement slab:
Fine Homebuilding: The Stay-Dry, No-Mold Finished Basement
Fine Homebuilding Q&A: Finishing a basement floor
GBA Q&A: Basement floor insulation retrofit
GBA Q&A: Floating plywood floor
How about a floor at grade / ground level?
When we first moved into our house we did one room and our contractor used a ‘painted’ moisture barrier applied directly to the slab after a moisture test came back higher than they would have liked. They glued the wood right over that. House was built in 1975, and we're in the Los Angeles area.
Is a poly barrier better / more effective than a painted barrier?
Also, while the radon levels in our house aren’t extremely high, they’re still above the action level - and although I know that a pipe into the slab and vented through the roof is the best option, and we will most likely be adding this at some point, is there a recommendation for a moisture barrier or sealant that will keep Radon from coming up through the slab? RadonSeal seems like it’s completely unreliable, any other options?
Joe,
Q. "When we first moved into our house, we did one room and our contractor used a ‘painted’ moisture barrier applied directly to the slab after a moisture test came back higher than they would have liked. They glued the wood right over that. House was built in 1975, and we're in the Los Angeles area. Is a poly barrier better / more effective than a painted barrier?"
A. Yes, but that's water under the bridge right now. If your floor is performing well, I wouldn't worry about it.
Q. "While the radon levels in our house aren’t extremely high, they’re still above the action level - and although I know that a pipe into the slab and vented through the roof is the best option, and we will most likely be adding this at some point, is there a recommendation for a moisture barrier or sealant that will keep radon from coming up through the slab?"
A. No, not that I know of. If you want to reduce the radon level in your home, the best approach is to install a sub-slab depressurization system. The work should be performed by a certified radon mitigation specialist. For more information, see All About Radon.
If your radon level is only a little higher than you would be comfortable with, trying measures short of sub-slab depressurization is reasonable, if you recognize that they might or might not work. An electronic monitor can help you track progress, although a conventional test to confirm afterwards is a good idea.
I think that the reason anecdotal evidence about the effectiveness of concrete sealants is so mixed is that there are almost always plenty of other routes of entry for the radon. So your chances are having a measurable effect go up if you also seal even crack, seam, or penetration through the foundation and slab. Other factors include the degree to which there's an easy way for sub-slab gasses to get out to the atmosphere without going through your basement.
If I recall correctly, materials such as plastic sheets are somewhat permeable to radon, which is part of the reason it's better to suck it out from under the slab rather than trying to block it. But at that same time, stopping air flow helps even if the air sealing layer is not a complete radon barrier. The rate of radon transfer is much higher through a substantial air leak than permeating through a solid material. So if you are close to having a radon level you are comfortable with, air sealing by any means may be adequate, even if it's done with a radon-permeable material.
Martin, thanks for your response.
Sorry, I wasn't clear. We're going to be adding engineered hardwood to 2 additional rooms shortly and so I was wondering what the original poster was asking about the correct way to lay this floor over a slab. I guess a poly barrier is best? And then glueing the floor right over that is OK? Or is that only OK for the painted barriers? Do I need a layer of plywood over the poly barrier? The other floor is holding up very well, we're just trying to minimize chemical layers in these other rooms as much as possible since they're children's rooms.
Any recommendations on specific poly barriers that anyone likes.
Charlie, thanks for the info. This is exactly what I was thinking. We will be adding the depressurization system, I just figured why not look into sealing the floor with Radon in mind, if at all possible.
Joe,
If you're talking about real hardwood, you should follow the guidelines provided in this document: Installing a hardwood floor over a concrete slab.
If you are installing an engineered flooring product, you should follow the installation instructions provided by the flooring manufacturer.
The guidelines you linked to show a vapor retarder applied above the concrete. In the case of a project I am working on now the slab on grade at the first floor is above a vapor retarder, 2" rigid, and crushed gravel. Is the vapor retarder above and below the slab OK?
Josef,
Installing an additional vapor retarder above a slab that already has a vapor retarder (polyethylene) under the slab is OK. There won't be any problems if the slab has vapor barriers on both sides.