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House is on a slab. Installer and one flooring company says the laminate I am considering has underlayment attached and I don't need vapor barrier. Another installer/flooring company advises it is very much necessary to prevent future problems with buckling laminate and or mold/mildew. Who's correct.

isherwood
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Home owner12
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2 Answers2

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Lots of slabs are covered in carpet without any problem. It depends on your specific slab. To know you need to test the slab. Take a trash bag tape it down to the floor. In 24 hours pull it up. If the floor looks damp or there is moisture on the bag a barrier will be needed if it is dry no barrier is needed. I have epoxy painted to seal damp floors made a huge difference in the humidity. Even though you have underlayment on your flooring it is segmented, moisture under there is not desirable. If dry with the bag test use the bag for trash and start your floor. If wet decide on the best path forward and use your bag for some trash (don’t waste the bag just because it has some tape on the outside. Added since some don’t understand: Many slabs have a vapor barrier under them, so if your plastic test comes up dry no additional barrier may be needed. I mentioned carpet because it holds moisture but grows mold and mildew faster than laminate and if a slab can handle carpet it can handle laminate, the plastic test is how to know if it is wet / dry at any age.

Ed Beal
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    The OP is asking about laminate, not carpet. – Lee Sam Aug 28 '19 at 23:57
  • If you read I mentioned carpet as it has no built in vapor Barrier and would be more prone to problems than laminate but even with a built in barrier it is segmented . – Ed Beal Aug 29 '19 at 16:21
  • Carpet is more forgiving than laminate. The OP doesn’t care about carpet. Maybe you have info about painting the Panama Canal...it’s made out of concrete too. – Lee Sam Aug 29 '19 at 17:54
  • Actually carpet is a huge mold source if moist much worse than laminate – Ed Beal Aug 29 '19 at 18:16
  • Again, the OP is asking about laminate, not carpet. – Lee Sam Aug 29 '19 at 18:21
  • Get it through your head, I mentioned both, carpet will grow mildew and molds if there is moisture quicker than laminate as that was part of the question or are you not able to understand a parallel explanation? – Ed Beal Aug 29 '19 at 18:33
  • Oh, now with your edit to your original answer. – Lee Sam Aug 29 '19 at 19:08
  • Shame on people who have to change their answer just to win an argument. – Lee Sam Aug 29 '19 at 19:34
  • Carpet in a basement will allow vapour through. And if the carpet is of non organic material then mold will not grow. Laminate flooring has an organic portion(wood) and as such must be installed with a vapour barrier in the basement flooring assembly. To further reduce the chance of mold, good air movement will help in keeping the area dry. – user68386 Aug 30 '19 at 20:24
  • This is simply not true, carpet is the leading source of allergens in a home dirt , dust, dander, hair all promotes mold and mildew growth in the carpet and all that is needed is moisture. if the slab is dry no barrier is required for laminate or carpeting. I have had 4 homes 3 with basements 1 slab that all had carpet. Only 1 needed to be sealed as it failed the plastic test. The others had no problems and one was a long shag (ok it was the 70’s and that was cool back then). – Ed Beal Aug 30 '19 at 21:43
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ALWAYS use a vapor barrier on concrete slabs.

You cannot control moisture coming up from the ground. Water tables change based on weather, season, etc. It might test fine now, but with a change of season, you could have too much moisture. The only way to insure keeping the slab dry and thus your laminate flooring dry, (and flat,) is to install a vapor barrier.

The “plastic bag” test is only for testing NEW concrete slabs to determine if the slab has cured (dried out) sufficiently to install wood flooring.

If the water table rises or if there is a pool of water from an exterior downspout and it seeps in under your slab, it will come up through the slab at control joints, expansion joints, cold pour joints, etc. and affect your flooring. Just a minute amount of moisture could make each board grow 1/64” (or even more). When that happens the boards will expand and then after a few boards it will cause your floor to expand by 1/4” or so...that’s what causes buckling.

I’ve seen floors buckle up 2” due to moisture from a rising water table. Even if it goes back down later, it will always leave a ridge.

Why take a chance? (Read the manufacturer’s installation instructions and you’ll see they recommend it too. They even recommend it on wood frame floors in kitchens and bathrooms and laundry rooms.)

Lee Sam
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  • You should read the instructions on concrete treatments. All the etching dyes and epoxy paint for flooring instruct to test with a plastic sheet say to verify the moisture is low enough for the product to work. This is useful on any slab not just new slabs. – Ed Beal Aug 29 '19 at 16:25
  • @EdBeal Again, the OP is asking about laminate not about an etching dye, etc. You’re right it’s useful on old and new slabs for etching dyes, etc. – Lee Sam Aug 29 '19 at 16:53
  • This basic test tells if you have a moisture problem to begin with. If there is a problem with moisture that shows up water on the plastic more than a vapor barrier may be needed. – Ed Beal Aug 29 '19 at 18:15
  • Even in the case of insulated slabs with poly sheeting and closed-cell foam underneath? – isherwood Aug 29 '19 at 19:24