Underlayment for basement subfloor?


Old 04-07-16, 06:45 PM
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Underlayment for basement subfloor?

Hi all,

I'm about to install laminate flooring in my basement. The concrete floor is very uneven, so I have decided to install 16" on center "joists" (with 2x4s) that will be shimmed as necessary to make a level plywood subfloor. Plywood will go on top of the "joists". Self leveling concrete is not an option due to high cost as I would probably need 7 or 8 bags to get the entire floor level, probably more.

My question is about using an underlayment in this setup. My floor is dry, I did the plastic sheet taped to the floor test and didn't see any condensation, even after a very wet 2 weeks. Benefit of living on very sandy soil. Do I need a plastic underlayment of any kind in this situation? If yes, then would I put the underlayment directly on the concrete, over the joists but under the plywood, or on top of the plywood? It seems like letting air circulate between the concrete and the plywood would help evaporate the little moisture I might have given how dry my basement is. If I should use an underlayment, would plastic sheeting be the way to go, or another option?

Appreciate any thoughts on the subject.
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Old 04-07-16, 07:15 PM
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I think the general consensus here is that laminate is a poor choice for a basement as it doesn't handle moisture well.

However, if you are satisfied the basement is and will always be dry....make sure you choose a laminate that is approved for below grade installation...many (most?) aren't, and then follow the manufacturers instructions regarding a vapor barrier and other below grade installation requirements. Most will require a vapor barrier. And that applies whether or not you have a plywood subfloor underneath it or not.

You might look at the dri-core system as an alternate way to do the subfloor.

Remember that wood that is against concrete has to be pressure treated.
Old 04-08-16, 08:53 AM
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I would not recommend dri-core to anyone. I used it in a basement store room several years ago. The floor got wet (not flooded) from seepage and the dri-core fell apart. The plastic separated from the OSB and the OSB turned black with mold.

It all went to the landfill. An expensive mistake.

Here's some tips for below grade laminate install although it sounds like you have already done the research.


Old 04-08-16, 09:55 AM
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Thanks for your response. Would the vapor barrier need to be installed above the plywood subfloor in this case? I am familiar with laminate installation instructions, e.g. a vapor barrier goes on top of the subfloor and beneath the laminate, but I'm not sure if I need any vapor barrier
below the subfloor or sleepers since this is different than an above grade installation.

I will be using pressure treated lumber for the sleepers that will be making contact with the floor. Would it be recommended to let the 2x4s dry out before attaching them to the floor or can they go on wet? I was planning on using liquid nails glue to fasten the sleepers to the floor. If attaching them wet I would worry about the glue failing and lead to possible shifting. But I suppose once I have the plywood fastened to the sleepers, that would prevent any major shifting.
Old 04-08-16, 05:45 PM
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You are not just protecting the laminate against moisture problems, but also have to consider trapped moisture under your plywood as well. We have has numerous threads from people who have had sleepers over slabs and discovered total rot only after a complete failure down the road. Your safest route (if you can call it that) is to go with self leveling compound and a thin skim. Make sure you prime the slab first so that you get a good bond with the SLC. Also, consider many of the floating vinyl floor options as moisture will not effect should you have a 100 year flood situation. Laminate is such a time bomb under grade. It is less expensive upfront, but if you do have a water situation, it will be tremendously more expensive long term.
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