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6mil plastic over slab?

Old 01-12-2020, 08:28 AM
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Default 6mil plastic over slab?

I am helping convert a friends 10x14 shed into a man cave. The shed is constructed on a concrete slab which can get come condensation on it. As this is a man cave he really does not want to spend too much money on it. He found some cheep $.80/sqft laminate at Home Depot. Would 6 mil plastic be good enough to keep any moisture from the slab away from the laminate flooring?
Old 01-12-2020, 08:34 AM
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You don't need to seal a concrete floor before installing laminate; however, to prevent moisture from the concrete floor from affecting your laminate, install a vapor barrier of polyethylene film. per google
Old 01-12-2020, 08:46 AM
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Originally Posted by Lorne Greene View Post
I am helping convert a friends 10x14 shed into a man cave. The shed is constructed on a concrete slab which can get come condensation on it. As this is a man cave he really does not want to spend too much money on it. He found some cheep $.80/sqft laminate at Home Depot. Would 6 mil plastic be good enough to keep any moisture from the slab away from the laminate flooring?
10 x 14 isn't too big so expense should not be real high. Yes you need a vapor barrier and most use 6 mil although I have seen some say you can use 4 mil (I wouldn't). Personally I would use floor "underlayment". It is a combination of a poly vapor barrier and a felt or foam pad. It is not that expensive and it gives a much better job taking out some of the imperfection in the concrete and adds some insulation and give to the floor making it more comfortable.

https://www.homedepot.com/p/TrafficM...N1HD/205665112
Old 01-12-2020, 09:35 AM
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Agree with using the vapor barrier over concrete, and would make one more recommendation - proceed with caution on ultra cheap laminate flooring. The really cheap stuff often has manufacturing defects on the joints which render some of it unusable. In addition, the “locking” joints are marginal, and any temperature swing will cause separation after it has been put down.

I’ve done more than a dozen houses with laminate and vinyl flooring, and the cheapest initial cost product usually ends up needing to be redone with a higher quality product due to joint separation.
Old 01-12-2020, 09:44 AM
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May want to belt and suspenders it and throw down a coat of Red Guard/ARDEX 8+9 or hydroban etc. May keep mildew from forming under the 6mil.
Old 01-12-2020, 10:25 AM
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I would agree with bjm it is definitely going to get moisture and mold/ mildew under the plastic. Ardex is worth the little cost
Old 01-12-2020, 10:35 AM
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I would use an underlayment before the plastic. You do need to use one, moisture will come up and delaminate your floor if you don't. You'd be surprised how much comes thru concrete. It's not going to add that much to the cost of the flooring. the new vinyl planking usually comes with a backer, so you don't need to worry about it, but Laminate stuff? Yes, use a moisture barrier and I'd upgrade from just plastic if it were me, or I wouldn't do it at all.
Old 01-12-2020, 10:44 AM
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I don't know if it would be the same, similar or different effect when used under laminate but my tile guy has put down this membrane stuff on part or all of several different jobs for me on slabs that like to "sweat." Stuff absolutely works and as given us zero issues ever.

https://www.lowes.com/pd/MAPEI-Mapel...mbrane/3056573
Old 01-12-2020, 10:56 AM
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^^^^ I'd try something like red guard

I just saw a bayhouse ruined because they put plastic under the laminate, completely different application it was the second floor.

the A/C will absolutely pull moister through the slab and mold everything in contact with it

Last edited by willie g; 01-12-2020 at 11:20 AM.
Old 01-12-2020, 11:17 AM
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I tile the basements of all the houses I build. Every one gets two coats of red guard prior to tile and I already have 6 mil under the slab. $150/3 gal bucket does around 300 ft2. Cheap insurance

Old 01-12-2020, 11:30 AM
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6 mil Visqueen should have gone under the slab.
Old 01-12-2020, 01:53 PM
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I always thought that the sweating was the moisture in the air condensing on the cold slab
Old 01-12-2020, 02:06 PM
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For the flooring I’d check a habit for humanity reuse store. About 10 years ago I paid like .75 and it was quality. Turned a shed into a kids play house. That’s now a man cave. I got lots of stuff there including windows
Old 01-12-2020, 02:38 PM
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Originally Posted by KBH View Post
6 mil Visqueen should have gone under the slab.
That has worked for us.
Old 01-12-2020, 06:25 PM
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Probably want to waterproof like bjm said and may consider switching to tile. Laminate won’t do well in a high moisture environment.
Old 01-13-2020, 04:23 AM
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The laminate manufacturer probably requires vapor barrier underlayment on the install (never seen laminate that doesn't). Just follow their instructions.
Old 01-13-2020, 07:11 AM
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Originally Posted by KBH View Post
6 mil Visqueen should have gone under the slab.
Uhm, little late for that, lol
Old 01-13-2020, 07:28 AM
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get the pvc laminate and call it done! you will never have to worry about the laminate floor degrading! wont really solve a moisture issue, but you will not have to replace the flooring due to a moisture issue!
Old 01-13-2020, 08:23 AM
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Originally Posted by bjm9818 View Post
I tile the basements of all the houses I build. Every one gets two coats of red guard prior to tile and I already have 6 mil under the slab. $150/3 gal bucket does around 300 ft2. Cheap insurance

This right here.

Anything moisture trapped between the plastic and the slab will become a science experiment in time.
Old 01-13-2020, 08:33 AM
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It depends on the source of your "moisture".

Concrete has a lot of thermal mass and is therefore prone to collecting condensate when a rapid temperature change occurs. If this is the cause of the moisture, then you will have problems unless you de-couple the flooring from the concrete. This can be done by putting down a false floor or "sleepers".

Moisture barriers help prevent concrete "wicking" moisture through from the ground but honestly this is somewhat minimal. The same result could be obtained by applying an epoxy sealer to the surface. It will never prevent sweating though. Insulating the slab and conditioning the space above the slab are the best and only means to prevent sweating in the long run. Is the space going to be continually conditioned?

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