Need advice on vapor barrier or not under slab?

[naturalgas]

My garage build was slowly moving along ( started first week of July ) which was ok with me. The GC I hired wanted certain contractors so we had to wait. He is a retired builder. So now my 36x40x12 garage is all framed (see my other posts) and he is pushing to get it done before snow and cold. I didn't care about waiting till spring for slab pour and siding. Well he was pushing me so slab is supposed to be poured next week and so far no wire mesh, fiber mesh instead and no vapor barrier. Now I'm ready to call off slab pour as that is very important to me to have a perfect burnished slab. Am I right about wanting vapor barrier? This is in Mass. My thinking is if I ever want to put down epoxy or tiles I need a barrier. I need some advice on this. I am ready to put the brakes on till spring. Thanks in advance. Jim


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[lakeroadster]

Sounds like you want the vapor barrier and the GC doesn't.

I know of no reason not to use a barrier... therefore demand it.

If he says no, find another GC.

And if you want to wait until spring for the pour, demand that to.

You're in control here, don't let the GC push you into anything you don't want to do.

Good luck, hope this helps.

FWIW: If you are planning to use fiber mesh in the concrete mix you don't need wire mesh.

UPDATE 03-06-2016

I know of no reason not to use a barrier... therefore demand it.

^^^^ WRONG ^^^^
Interesting article here about Vapor Barriers: http://www.stegoindustries.com/docs/Vapor Barriers Under Concrete Slabs.pdf

"Strength.

Nicholson also cored the concrete placed over polyethylene sheeting, a 3-inch sand layer, and a 3-inch sand-cement layer. Concrete placed over a sand bed was more than 30% stronger than concrete cast on the polyethylene"​

 

[Kaizen]

he doesn't want to use it as they will be floating it all night. with the plastic.....not sure i'd call it a vapor barrier....more like a ground water barrier....none of the water from the mix drains and at these temps it takes a long time for it to set up. you can insist if you want. use thickest you can find. but chances are it will get torn up unless you have sand and no pebbles or the concrete guys will just poke holes in it.

 

[kabinenroller]

I would definetly install he vapor barrier. In addition I would install ridged foam insulation under the slab and around the perimeter of the foundation. Even without in floor heat the extra insulation will all but stop condinsation from forming in the building. The insulation will keep the mass of the concrete "warm" this makes a differance when the outside temps are hot and the air is humid.
A portion of my building does not have hydronic heat, I insulated the floor the same as the hydronic side. I have absolutely no issues with moisture on the "cold" portion of the building. (There is a well insulated dividing wall between the two spaces, with a thermo break between the two floors)

 

[pattenp]

he doesn't want to use it as they will be floating it all night. with the plastic.....not sure i'd call it a vapor barrier....more like a ground water barrier....none of the water from the mix drains and at these temps it takes a long time for it to set up. you can insist if you want. use thickest you can find. but chances are it will get torn up unless you have sand and no pebbles or the concrete guys will just poke holes in it.

Exactly. It takes a long time for concrete to set when poured on top of plastic. You'll see the concrete workers sometimes poke holes in the vapor barrier to let settling water drain through. If you plan on epoxy covering you need the vapor barrier. A few inches of pea gravel on top of vapor barrier will help with water settling out of concrete.

 

[MagKarl]

There is no right answer, you'll only get opinions on vapor barriers. If you expect to get moisture, then a barrier might help, but drainage is key. If you have good drainage and slab grade set high, you may never have a problem. Did you run drains around your footings and backfill with drain rock, or just backfill with dirt? How much gravel is under the slab grade?

I have good drainage and lots of compacted crushed rock under my slabs, no plastic, and no moisture problems. I want a place for water that comes off of my vehicles to go, down through the saw cuts and cracks, through the gravel, not trapped under the slab at a barrier. That doesn't make it the right answer for you, just my experience and opinion.

 

[bczygan]

he doesn't want to use it as they will be floating it all night. With the plastic.....not sure i'd call it a vapor barrier....more like a ground water barrier....none of the water from the mix drains and at these temps it takes a long time for it to set up. You can insist if you want. Use thickest you can find. But chances are it will get torn up unless you have sand and no pebbles or the concrete guys will just poke holes in it.

this!

 

[Rookie2]

Absolutely vapor barrier ! 6-10 mil. Fibermesh will leave a ittle hair sticking up through the finish, mine did.

 

[matt_i]

My thoughts are that the vapor barrier helps keep water inside the concrete as it cures, instead of letting it leach out into the soil. Along the same lines slabs should be ideally wet-cured for several weeks or else sealed with a purpose-designed curing sealer. The crystal lattice has to "hydrate" to gain strength and if the water is gone, the number of reactions that can be completed is limited. You can flood the top, easy with a hose. Flooding the bottom can't really happen.

My thought is that the washed stone under the slab provides a break from the surface tension of ground water, the vapor barrier will assist, though.

6mil plastic would be minimum, at 10 mil, the guys won't be able to tear it up unless they try.

Rebar > wire mesh >>>> fibermesh in my opinion. The floor won't be smooth with fibermesh as above.

 

[TommyK]

There absolutely is a right answer and it is install vapor barrier and make sure they don't butcher it.

 

[naturalgas]

Thanks guys, talked to GC and concrete guy today. He will put poly down before the pour. I feel better


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[SunsetsAndFriends]

I would go with the a 10+ mil vapor barrier over thoroughly compacted stone. I would install 2" of Foamular 250 foamboard insulation with seams taped. I would recommend rebar instead of fibermesh. The fibermesh, as others have said, will leave "hairs" sticking up over the entire surface of the slab.

Lastly, I would wait until spring to pour. This gives you time to be sure you're choosing exactly what you want.

My 2 cents.

 

[joes169]

Glad the GC and concrete sub are on the same page as you now.......

We ALWAYS use vapor barrier under every residential floor, and I've yet to get a call about having a floor sweat, and we cn get some extreme temperature and humidty swings here in Wisconsin. Minimum is a single layer of 6 mil, 6" overlap at the seams, taped penetrations.

Personally, I wouldn't install foam under the floor, it won't sweat any less with foam than with plastic, and unless you plan to work out there in slippers in 70 degree heat, I doubt you'll ever value the cost of the insualtion.

Fiber's are no replacment for steel re-enforcement, even the fiber manufacturer's stopped making that claim years ago. As for the fibers left at the surface, they should be "burned" off fairly well from a good finish via a finish machine. THe balance can easily be burned off with a weed torch after the concrete cures.

As for pouring over poly in cold temps, it's really a non-issue in today's day & age. There's a multitude of elementary admixtures we can put in the concrete to aid in dealing with bleed water. We poured 86 yards of basement & garage floors inthe last 10 days, no "burning the midnight oil" for us. Actually, most days, we had the machine pulled off before dark, and we could have been more aggressive with accelerants.

Goood luck on your project.

 

[naturalgas]

Thank you sir.


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[nonhog]

I demanded it and so glad I did. I have issues with water coming up in my attached garage but not the shop or house, slab on grade.
I had to pay more do to extended finish time. Was worth it.

 

[machsnell]

I recommend at least 6 mil and 2" foam and rebar and microfiber. Standard fiber is great for Crack resistance but makes for a sloppy finish that is tough to epoxy.

Microfiber is amazing stuff. Can't see 8t in the finish and if you do micro and rebar the centers don't have to be as close and it is cheap insurance for cracks. Solid subgrade and well compacted stone is the first and necessary.

I always recommend to customers and I did my slab 1600 sf 5" of 4000 this way and I took a chance and only cut in one control joint and not a Crack 2 years later. Took over a year for the saw cut to finally Crack.

Fiber is good! Go micro tho. It's like 5 bucks a yard. 50 bucks a truck. Cheap

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[machsnell]

Go low slump also. 3 n0 more than 4 and don't let them add a bunch of water to it at the job

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[LegacyIndustrial]

Go low slump also. 3 n0 more than 4 and don't let them add a bunch of water to it at the job

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Ahh "Machsnell"
My fathers favorite thing to scream once the Christmas cheer kicks in. Hope all is well.


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[machsnell]

Haha. My dad said that A LOT. All is well and I am getting close to calling you. Things have been moving slowly but I am weeks away from getting spray foam and that's when you're up. Hope all is well

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[naturalgas]

Merry Christmas to all. Well looks like winter weather is arriving soon. Will be 60 here today and by Monday nite into Tuesday they are forecasting snow, sleet and freezing rain. My slab is scheduled for Tuesday morn. That's it I'm cancelling and waiting till spring. Concrete will be $40 a yard less in good weather also and will give me time to decide on lift location. And beefed up pads.


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[naturalgas]

Merry Christmas to all. Well looks like winter weather is arriving soon. Will be 60 here today and by Monday nite into Tuesday they are forecasting snow, sleet and freezing rain. My slab is scheduled for Tuesday morn. That's it I'm cancelling and waiting till spring. Concrete will be $40 a yard less in good weather also and will give me time to decide on lift location. And beefed up pads. No need on my part to rush things now. I have been dreaming about this build for 25 years. A few more months does not bother me at all to have it all go right.


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[marksland]

Good call on the floor. With poly underneath the concrete it would take forever to setup and finish.

I would definitely go with a 6 mil poly and wire mesh underneath the slab. If you are considering epoxy later, stay clear of fiber mesh. There are several discussions in this flooring forum regarding this topic.

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[brownbagg]

i did poly with fibers and its great, add character to the floor, the more screw up the slab is, the better they come out

 

[brownbagg]

concrete without vapor barrier will crack like crazy, the bottom of the slab will set quicker due to the excess water draining off while the top is still wet, the diferent will cause cracking, using vapor barrier it retain the moisture, so very important, you place at a lower slump. reducing the bleed water with create a stronger concrete and a stronger surface. excess bleed water will weaking the top surface with is cause of dusting

finisher love no vapor barrier because they can pour wetter and it will still set quick, but it will always crack. people say all concrete cracks, no just the poor workmanship jobs

 

[Colonel Clank]

In 1997 I placed a 24'x20' garage floor in central nj. In 1999 I dropped an iron ford v8 from about 5'. It took a chunk out of the floor about the size of a bic cigarette lighter but as of today has never cracked at all. It's also bone dry any time of the year.

18 years later I would do the exact same thing again. I used 5" of 4K or 4.5k with fiber and wire mesh on top of the thickest vapor barrier I could find at Home Depot...I don't remember exactly the mils, probably 6-8. The slab is about 6" above the surrounding yard grade on top of 3-4" of 3/4" gravel. I had a pro do the finish work and my buddies and I did the heavy work. In a few places I could see the glass in the finished surface but not after prepping for paint.

Btw, we placed the slab and apron in almost 100 degree weather and it was setting FAST! The Apron in front of the garage was placed at the same time but did not have vapor barrier. No cracks in that yet either. I never use salt or any other chemical ice treatment.


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[wagspe208]

This is always a discussion that has an obvious answer, but very different opinions.
I installed radiant heat tubing in my floor. I put down 2" foam, vapor barrier, wire mesh, tubing, pour.
The guys added 1 or 2% calcium to speed up the dry time. YES, cure time will be slowER in cool temps with a barrier due to water not leaching out. HOWEVER... if you don't want you floor to sweat, you WANT a barrier.
I think concrete will crack whether barrier or not. But what do I know..
Wags

Oh, I have friends who poured a pole barn with and without a vapor barrier. I also have a buddy who poured frost walls. The ones without the barrier complain.

 

[naturalgas]

I'm spending a small fortune on this build and the slab is right up the top on my priority list to get perfect. The slab in my two car attached has sucked since day one. And I was the GC on my house build. I don't want to deal with a contractor that is looking for quick buck. Bang it out and gone. The guy I'm dealing with is finding this out. I don't care. It is my dime.


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[INTMD8]

I bought my house with an existing pole building. When I drilled for the 2 post lift there did not seem to be a vapor barrier underneath and I was told there is no rebar, just fiber mesh.

There are a lot of saw cuts. Every 10x12 section or so. Other than the saw cuts there are no cracks anywhere.

Took epoxy just fine as well.

I think the vapor barrier is definitely a good idea but it's absence didn't seem to negatively effect anything in my case.

 

[Heel2toe]

Hey Jim feel free to PM me but Im curious who you are using for a concrete contractor?

I had a slab poured a couple months back in the same area and the specific details regarding what the contractor recommended sound all too familiar. I don't want to slander the company but I did not have a good experience and have some good feedback for you assuming you plan to work with the same folks that I did.

 

[naturalgas]

PM sent


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[jeff_gates]

It all depends on what you are doing.
We are doing in floor heating, so used 15 mil vapor barrier and 2" of Foamular 250 foam board. In November the air was freezing lots of ice and the floor was fine to stand on.

--Jeff

 

[bd8134]

My floor was poured with no vapor barrier or rebar. Just mesh and saw cuts. No cracks other than in the cuts. No moisture or damp problems. Epoxy and 2 post lift with no problems.
Each to their own on how they have their floor poured.

 

[machsnell]

My floor was poured with no vapor barrier or rebar. Just mesh and saw cuts. No cracks other than in the cuts. No moisture or damp problems. Epoxy and 2 post lift with no problems.
Each to their own on how they have their floor poured.

There is more to it than a blanket coverage on this is how it HAS to be done.

Type of soil and whether it is expansive, moisture content and cbr's of the soil as well as the compaction of the backfill. Most contractors don't do this well.

Vapor b and bar are not necessary always but CHEAP and easy insurance.

Only one way to find out if you need it but it will b e too late.

To each their own

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