Plastic between slab and ground

[FltEngCPO]

I love this sight, so many great resources to make a great garage!

I'm in the forms stage of a monolithic slab and I have seen pictures of slabs on here with a plastic vapor barrior laid down before the pour. I asked the guy doing my concrete work if that was the plan and he said no, nobody does that in Oklahoma. So my question is.... Is that a BS answer to shut me up or is moisture not an issue in this part of the country? Should I insist on the vapor barrier?

 

[rustluver]

I would definitely insist on the vapor barrier. It's cheap insurance againest condensation problems when your building is enclosed.

 

[scooterseats]

Always put a vapor barrier down before a pour, even in Oklahoma.

 

[ddawg16]

2" of sand...1 layer of 6mil plastic....2 more inches of sand....

 

[Bruce4310TX]

Yep u gotta have that Vapor barrier if you dont it will sweat buckets, insist on it you will regret it if you dont any metal thats not primered or painted will rust right up.....

 

[hdshinn]

Most concrete finishers that I've known don't particularly like a plastic vapor barrier as it prevents excess water from being absorbed by the ground thus it takes longer for the bleed water to evaporate to the point the slab can be finished. My personal preference is for a 2" layer of polyiso rigid foam insulation under the slab, and that installed over 6" of pea gravel. Depending on how quickly your new building is enclosed, please understand that there's a good deal of moisture in that slab that can take quite a while to evaporate. I've got a 24 x 40' slab with average 4' concrete stem walls and I fought condensation on my tools for a couple years during winter.

I'm in the Pacific Northwest (WA state) so your mileage may vary.

 

[slghmmr88]

Yes it is done here in OK. It is under where I work and when the repairs were being done from a ruptured 6" fire sprinkler line, I was working offsite but came by to open up for the concrete crew because the person in charge was not able to get there in time, took a look to see the rebar all in place and tied, trucks showing up and pumper getting ready, called my supervisor who was the one in charge and asked "did you decide to not replace the vapor barrier?" . The response was "probably too late now" I said " You're in charge not me and you can do what you want." Seemed to have second thoughts and told be to remind the concrete subcontractor about the vapor barrier, and boy you should have seen everybody scramble and phone calls being made. Guys started untying rebar and soon the plastic showed up and luckily there was a big crew there and got the barrier slid into place stretching it out from the middle underneath the rest of the rebar and shifted the hot mud to another job and reshuffled the rest of the trucks, btw it was 1500 sq ft x 6" inside a building.

 

[demographic]

If they don't put a vapour barrier in they don't get paid over here.

 

[Justanoldguy]

Think of concrete as like a sponge.
It is porous eg full of holes.
It will **** any moisture out of the ground if no barrier is in place.
Put a few drops of water on concrete and see what happens.
It spreads as quick as fire.

 

[Justanoldguy]

I know the barrier is a pain in the **** for contractors trying to finish concrete surface.
But the plus side is it gives more time to get a better job done and it doesn't dry out too quickly as it would with no barrier. That is when concrete develops cracks. Been placing concrete for 45 years so have some knowlege on what happens.

 

[darkk]

??? I had mine done with 6mil plastic. It helps the concrete from drying too fast from the bottom. It isalso good to help prevent ground moisture from seeping up onto the floor after it is cured. For the $100 bucks it's going to cost, it'scheap insurance. It sure as hell ain't going to hurt anything...I kept my 30'x30'x6" slab soaked after with a bar sprinkler for a solid 7 days to prevent surface cracks and help the hardness of the cured concrete. Drying out to fast is very bad. Do some *google* searches on how to cure concrete properly. An informed consumer is a smart consumer...

 

[Daniel Dudley]

Concrete can be as much as 20% air by volume. Concrete cures as opposed to drying, so you have to keep it wet, as water is the catalyst. But after it cures, the water dries out of it, and the concrete is full of microscopic voids. This makes the concrete porous, and a poor vapor retardant. If you have a concrete slab over damp ground, and there is drier air above the slab, moisture will pass through the slab by osmosis, in an attempt to balance out the moisture differential. Fine for a diveway, but if you try to put a dehumidifier in a garage, it will only draw MORE moisture out of the concrete. You will never get it dry.

The moral to this story is use a vapor barrier. IF you HAVEN'T used a vapor barrier, products like Radon Seal can be used to create a vapor barrier on the surface of the concrete itself. Radon seal is mixed with water, and soaks into the porous concrete, causing the concrete to re cure in the microscopic voids. It is NO substitute for a vapor barrier on new concrete, but it can be a Godsend for damp basements and garages where there is no other recourse. It can also be used on non vapor barrier floors prior to painting the floor, as it will prevent the vapor pressure from lifting the paint.

TO sum up, vapor pressure is a big problem. Vapor barriers are no joke. Curing concrete to full strength by keeping it wet is also a big issue. Keep it wet and covered or use a cure seal. There are a lot of guys out there who know how to pour concrete who know nothing about how it cures, or why they should use a vapor barrier. Ignorance can lead to problems later on down the road, so don't get fooled.

 

[ConCretin]

All good advice. Didn't see it mentioned but make sure they tape the seams and if you can, keep an eye on the crew to make sure they don't damage it. If you are using wire mesh, install it on supports. The crew will trash your vapor barrier trying to pull up the mesh

One final thought, If you are going to coat the floor, I'd upgrade to a 35 mil product like Stegowrap, which is superior to plain 6 mil poly. It's a little pricey (about .30/sf) but you only get one chance.

 

[FltEngCPO]

Thanks gang, I will be informing my concrete guy in the morning.

 

[brownbagg]

everything said here is correct concern vapor barrier but one no body mention, vapor barrier are reguired by the building code.

 

[rsa]

The online-May issue of Journal of Lightweight Construction (subscription required) had an article, "Better Garage Floors Ten tips for pouring stronger, crack-free concrete slabs". They recommended stepping up to a 10-mil or thicker vapor barrier that's more resistant to punctures and deterioration. They specifically mentioned VaporBlock, Stego Wrap and Permiator. I'm sure there are many others.

The authors also specified placing the vapor barrier above all subbase.
The vapor barrier should be placed on top of all the subbase, immediately below the concrete.

 

[rsa]

everything said here is correct concern vapor barrier but one no body mention, vapor barrier are reguired by the building code.

Even on an unheated, detached garage?

 

[akdiesel]

Great points. The main purpose for the vapor barrier up here and in most mountainous regions is Radon gasses. The barrier is installed after the footings and wall have been installed so the edges are are not sealed nor are the seems for larger areas. I suppose it is simply to cut down on the amount of gas that enters.
Moisture can / will still enter crawl space or basement since the edges and the joining seems are not tapped or installed prior to the foundation installation.
If you are worried about water seeping your best bet is to install a french drain.

 

[mad57]

I have 2 garages one i built 30x40 back in 92 never had a vapor barrier installed when its hot and humid you can see wet areas in shapes in the crete, i built a small office in the one and did use dry loc basement sealer paint on the floor some 10 yrs later and what a diffrence it made i was able to carpet and pad this area, on the new garage i did use the plastic barrier and it is much better insde.

 

[Justanoldguy]

The vapor barrier should be placed on top of all the subbase, immediately below the concrete.

The only way I have ever done it.

 

[CCShopgirl]

We started building our house back in 1983.. poured foundation that year and the architect called for a Vapor Barrier. It took time for the water to drain and settle out, but I can tell you to this day we have never had an issues with the basement floor. It is today as it was the day it was poured so glad we did it~

CC

 

[MScott]

I will be having a floor poured soon and will be installing pex for infloor heating as well as foam for insulation. What is the the correct order to install these items? Is it gravel, foam, plastic, mesh or rebar, and then pex, or should the order be different?

 

[Falcon67]

Never used it on a garage slab, and didn't on the new one. Never had any moisture problem either. It's too f'n dry here anyway. They didn't use any on the $200K house next door either. So - put it down there if you want, or don't worry about it IMHO.

 

[toyotadriver]

We didn't use it in my two shops and the current house garage. We haven't had any problems with moisture caused by lack of vapor barrier.

However, I would still spec one.

 

[robertearl]

I had them remove the vapor barrier around the rebar where I had connected my Ufer ground. I do not believe that the Ufer ground will work if the concrete does not come in contact with the earth. The rest of the slab had vapor barrier.

Reb

 

[rockchucker]

100% Vapor Barrier all of the time under just about any slab if anything is going to be built on it. I have only poured 2 small slabs without it. These were both for the Sheriff for the placement of 2 block style Outhouses right on top of the sand. They were both vandalized very fast and the poly holding tanks were burned out within a few weeks. Thanks Moses Lake.

Just my 2¢

 

[BADSIX]

my brother inlaw has a dary ranch and he has a huge wast tank thats 10 to 12 tall full of water and cow ****. now it doesn't get wet on the out side and it has no sealer on the inside. your floor weting problems are from a rapid change in tempature and humidity. kinda like taking that cold can of beer out of the frig on a warm humid day. the can gets wet on the outside and i'm sure the beer didn't leak through the can.

 

[John Timmins]

Have the ground treated for termites before the slab is poured too !

 

[Beaumont67]

Never used vapor barrier 6ml plastic...on my 1991 garage build.

Instead:
Poured the 24x30ft. slab of concrete on top of 1" foam board...Zero issues with condensation &/or mold smell.

 

[xrdad]

Would the barrier be spec'd for freezing climates too?

It's not usually used in construction (from what I'm told). I'm wondering if the barrier could cause water to stay in the concrete and cause damage in a cold snap?

 

[ConCretin]

Vapor barriers are pretty typical in cold weather climates as well. Once concrete has set sufficiently it won't be affected by residual mix water or moisture migrating in from air or soil.

 

[FltEngCPO]

Vapor barrier went in (much to my contractors chagrin) between the sand and concrete. Almost had to put water on it immediaty from the heat (above 105 every day since pour). Had few very small (almost cant see them) spiderweb surface cracks but it held the first night. The next day they put relief cuts (1.25" deep) in it and then sealed everything. I have been running a fan sprinkler on it from noon to 9pm every day since. Not looking forward to that water bill. I will post some pics soon.

 

[bigdav160]

I don't think the use of plastic hurt anything. Decades ago when I was in the home building industry I saw hundreds of slabs poured directly on the ground. (Ok, it was scraped of the top soil and sometimes sand brought in)

I think the vapor barrier under the slab is a practice brought in from areas where the ground water is close to the surface. But found to aid in proper curing in areas where the ground is hard and dry.

 

[boiler7904]

Vapor barrier should be under every slab that is in an enclosed space or is likely to be enclosed in the future like an attached carport. 6 mil poly isn't the best choice because by the time the mesh or rebar is installed, the poly is usually torn to shreds. Stegowrap or one of the other purposed designed concrete vapor barriers should be used instead. Whether using standard poly or a specific vapor barrier, all seams and penetrations (drains, plumbing supply, electric conduit / ground rods, etc.) through the material should be sealed with a compatible tape (not duct tape since it will eventually dry out and not form a seal).

Side note on the installation of mesh or rebar: Insist on it being installed on chairs (wire or plastic) or concrete bricks sized to keep the mesh in the middle third of the slab thickness. Don't let your concrete guy tell you that they use a rake or hook to pull the mesh into the center of the slab. It doesn't work. As soon as the pull the mesh up, another guy on the crew walks through the same area pushing it back to the base material where it does you no good. I had a job a couple of years ago where some plumbing was missed and the slab had to be cut. Noticed that the mesh was in the bottom 1/4" of the slab or completely out of the concrete in one place and made the contractor rip it all out and start over.

 

[v8garage]

everything said here is correct concern vapor barrier but one no body mention, vapor barrier are reguired by the building code.

Building codes vary greatly. There are no building codes whatsoever in some rural areas especially for shops, barns etc.
V/8

 

[brownbagg]

oh there are building code, might not be enforced due to lack of money in the budget but there are building codes everywhere

 

[nonhog]

Fought with my builder and ended up paying a bit more because it takes so much extra care and time. Time? maybe a little because of the way it cures not enough to piss off the workers wives. They would not be late for dinner. He also said it makes it so slippery. again I use to do concrete.

I went with 2 layers of 6 mil.

I'm sure my house garage was poured w/o one and needs to be taken out and redone. What fun.

 

[toyotadriver]

oh there are building code, might not be enforced due to lack of money in the budget but there are building codes everywhere

There is an international building code but my county has not adopted any codes at all. You can build whatever you want, however you want, and wherever you want.

The cities have adopted various building codes but the county has not.

 

[Neuswede]

I will be having a floor poured soon and will be installing pex for infloor heating as well as foam for insulation. What is the the correct order to install these items? Is it gravel, foam, plastic, mesh or rebar, and then pex, or should the order be different?

You have it right. The PEX is attached to the wire mesh with tie wraps, and you will use a block/stand to keep the mesh suspended during pour. Some attempt to just hook it and let it settle back into the wet concrete. There is alot of info circulating that the PEX at the bottom of the slab is OK, but I still believe that it should be contained in the lower 2 inches of the pour (for a 4 inch pour).

Remember to install 1" insulation around the perimeter to help combat the foundation from wicking heat from the slab. Also remember to tape the joints on the vapor barrier and the insulation.

 

[Leadfoot3232]

I've had three poured near Tulsa over the years and never had plastic under any of them without any problems..I had my new pole barn shop floor poured last weekend in that heat..I watered it for 3 days, so i can relate on the upcoming water bill..