Moisture barrier or not?

[duraleigh]

Newbie here. Thanks for the forum.

I am ready to pour the concrete slab for my new garage and workshop. The garage will be unheated and the workshop will be heated and cooled.

My assumption is the smart thing to do is a moisture barrier throughout.

Assuming that's correct, is 4 mil poly adequate?

TIA,

Dave S.

 

[nonhog]

Couple of thoughts here. I went with 2 and in spots 3 layers of 6mil and what I had left over of 4 mil . I went with the extra 4 mil in the high traffic areas
The areas the workers would be using wheelbarrows to shuttle the concrete.
and doorways where they would be walking back and forth .
(and because I had it)
Necessary ? who knows . Vapor barrier YES if there is any question of drainage. I knew it was needed at my shop due to the moisture "wicking" up in the house garage slab.
My house (and shop) sits below road level.
Be sure it goes up on the edges or moisture can get under it.(between it and the slab)

Some will say use a vapor barrier/ insulation combo or all in one.
depends on your area (real cold?) may be worthwhile.
Just my thoughts , time to study see what's right for you.
use the search feature here and maybe google it.
Good luck

 

[duraleigh]

Nonhog,

Thanks for the quick reply. Sorry I didn't say my location....Raleigh, NC.

I'll search here on the forum but I'm 95% sure already That I'll put at least 4 mil down. Traffic is not a problem in that the truck will be able to discharge from 3 of the four sides....no framing up yet.

 

[boiler7904]

Definitely do a vapor barrier throughout the slab in case you decide to heat or cool it down the road.

4 mil won't last through the pour in one piece. Between the crew walking on the plastic, your reinforcing steel poking at it, and the tools the crew will be using, it is going to be punctured (a lot). As soon as you start to puncture a vapor barrier, you might as well have left it out of the whole floor since water vapor moves in all directions - not just straight up and down.

You'd be a lot better off using a couple layers of 6 mil or a single layer of 10 mil. Use tape that is compatible with the vapor barrier to seal all joints, the edges of the slab, and any pipe or conduit penetrations through the floor. The cost to do it right up front is minimal compared to the regret you'll feel later when you start having moisture problems especially if you want to apply an epoxy floor coating.

If it isn't too late, think about adding a 2' wide strip of 2" rigid foam insulation along the inside of the foundation wall and under the perimeter of the slab. It will reduce or eliminate heat transfer out of the shop. Probably not a big deal most of the year for you but it will help keep the building warmer in the winter without using too much heat.

 

[zeebad1]

I met with a gentleman yesterday that does most of the concrete work in shops around here, and his work is second to none.

He told me that a vapor barrier isn't necessary, and the moisture on floors comes from temperature difference between the concrete, and the air.

His comment was if concrete is really that porous, it's not good concrete. He mentioned that there are a lot of old concrete livestock water tanks around the country, still holding water, and swimming pools too.

 

['the epoxy floor guy']

In my opinion there is ONLY one reason to install a vapor barrier.

When pouring concrete the best concrete uses the least amount of water in the mix in proportion to the amount of cement. Once placed keeping that slab as wet as possible for as long as possible will give the floor the best strength.

especially when really hot and dry placing a vapor barrier under the slab will keep the moisture that is in the newly placed concrete in the concrete for as long as possible. The same effect can be accomplished by applying ample amounts of water to the sand/fill/ground just before placing the wet cement, THEN after the surface has set 1-2 hours, keeping the floor even flooded with water will create the strongest concrete.

just like zeebad1 said the moisture on concrete when "sweating" is condensation. Condensation is a warm moist air coming in contact with a cold (below the dew point) surface.

 

[rinny_tin_tin]

I met with a gentleman yesterday that does most of the concrete work in shops around here, and his work is second to none.

He told me that a vapor barrier isn't necessary, and the moisture on floors comes from temperature difference between the concrete, and the air.

His comment was if concrete is really that porous, it's not good concrete. He mentioned that there are a lot of old concrete livestock water tanks around the country, still holding water, and swimming pools too.

Concrete is porous - and if its holding water, then it has been plastered

 

[nonhog]

He told me that a vapor barrier isn't necessary, and the moisture on floors comes from temperature difference between the concrete, and the air.

His comment was if concrete is really that porous, it's not good concrete. He mentioned that there are a lot of old concrete livestock water tanks around the country, still holding water, and swimming pools too.

In my opinion there is ONLY one reason to install a vapor barrier.

just like zeebad1 said the moisture on concrete when "sweating" is condensation. Condensation is a warm moist air coming in contact with a cold (below the dew point) surface.

I am no expert and have seen the condensation happen before like after opening a garage door and large amounts of warmer moist air comes inside.
But that does not explain why the house garage seems to "wick" condensation
(no open doors) and my shop (with vapor barrier) does not.
How come MOST epoxy guys suggest the plastic test before applying epoxy ?
Bad concrete ? So when it rains hard outside and I leave a plastic tote on the floor its actually wet underneath , condensation from warm air ? NO
bad concrete (?) maybe but I doubt it.

 

[rinny_tin_tin]

Newbie here. Thanks for the forum.

I am ready to pour the concrete slab for my new garage and workshop. The garage will be unheated and the workshop will be heated and cooled.

My assumption is the smart thing to do is a moisture barrier throughout.

Assuming that's correct, is 4 mil poly adequate?

TIA,

Dave S.

Here is the definitive paper on the subject.

http://www.stegoindustries.com/docs/Vapor Barriers Under Concrete Slabs.pdf

The short answer: directly below the concrete and steel: 4 inches of No. 57 gravel and 4 inches of sand on top of that. NO vapor barrier.

 

[mad57]

ok for the lousy 50 60 bucks for the plastic why wouldnt you its cheap insurance, once that crete is in its in!!!!not too mention its code in my town to have it. good luck what ever your decision,mike.

 

[nonhog]

More food for thought............
http://www.architects.org/emplibrary/UnderSlabVaporBarrierPPP.pdf


Also my question re: gravel and sand . If thats all you did how would that stop moisture in a case like mine . My shop is 4-5 below road level my neighbor across that road is 10-15 feet above road level .
Maybe it would I don't know, too late now.
I have talked with someone who went with gravel and sand w/o vapor barrier
and is not happy has to run a dehumidifier alot .
Your decision . I say study till your satisfied, put off the pour till your good with what you choose.

Heck my slab is a little over a year old (not well proven yet) but so far I am glad I went with VB . Good luck/have fun !

 

[rinny_tin_tin]

More food for thought............
http://www.architects.org/emplibrary/UnderSlabVaporBarrierPPP.pdf


Also my question re: gravel and sand . If thats all you did how would that stop moisture in a case like mine . My shop is 4-5 below road level my neighbor across that road is 10-15 feet above road level .
Maybe it would I don't know, too late now.
I have talked with someone who went with gravel and sand w/o vapor barrier
and is not happy has to run a dehumidifier alot .
Your decision . I say study till your satisfied, put off the pour till your good with what you choose.

Heck my slab is a little over a year old (not well proven yet) but so far I am glad I went with VB . Good luck/have fun !

Your problem seems to be a different class than what is discussed in the paper - namely, it seems to be more of a waterproofing problem. If your slab is submerged with groundwater - a vapor barrier isn't going to prevent water from seeping through the porous concrete and into your basement. The slab should be well elevated above the water table for any expectation of interior dryness, AND buffered from any soil moisture with a layer of course gravel and sand. As stated in the paper, concrete has been shown to wick water up (and defy gravity) as much as 30+ feet. The gravel and sand layers create sufficient surface tension to defeat wicking tendencies. In areas such as New Orleans where the buildings essentially *float* you do what you can and forever combat the termites.

 

[radial coupe]

well im a concrete finisher and i can tell u first hand it needs a moister barrier. have done it both ways and the floors that dont have the barrier will always sweat. the only reason a finisher will tell u it dosent is drying time he will get done quicker if it does not have the plastic.

 

[Mojo]

I'm definitely no expert and I went back and forth on whether to lay down the barrier. My shop is 1500 sq foot. In the end, I didn't do it. I talked with the concrete guy who had been in concrete for 30 years. His opinion was that the barrier doesn't let the concrete dry properly and leads to more cracking. Not sure if that's true or not. Its been almost a year since we poured the foundation and I put epoxy over it. I haven't had any cracks to date and no issues with moisture. A friend of mine has a similar sized shop, did the barrier and has a number of decent cracks in his floor. Just a data point for you. As I said, I have no expertise in the area.

 

[mpraddict]

If you have a concrete guy telling you a vapor barrier (actually vapor retarder) isn't needed, he's only speaking from installation experience. Typically, these guys are long gone before the problems arise. Typically without one, you'd have issues with floor finishes lifting from the vapor "pushing" through the slab. That alone indicates vapor DOES come up through concrete. Generally, the floor guy gets a call back when that happens, not the concrete guy.
If you don't plan on finishing your garage, that's not as much of an issue, but why would you want that extra moisture in your space inviting rust? It's a relatively low cost for the benefit.