Vapor barrier?

[89efhatch]

I'm having a slab poured next week in my new pole barn and my concrete guy says I don't need a vapor barrier. I was under the impression that it's pretty standard.

It's a 30x38 pole barn. Floor will be 6" thick. And it's sitting on 6-18" of crushed stone. About 90 tons in total.

And the space will be used mostly for my machine tools/welding equipment/vehicle work.

So do I need one or not?
Any thoughts or opinions are appreciated.

Mike

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[bigdav160]

I think this is one of those "depends on what part of the country" questions.

 

[The Cobbler]

even if it is deemed "not necessary" , I would do it to keep vapour from wicking thru the slab

 

[Sumboodie]

2" foam would be ideal.

 

[malibu101]

My thoughts but I ain't not no concrete expert.

If someone can tell you how it could possibly in any way be a bad thing (other than cost but I don't see how unrolling some plastic could be that much more)- research it, ask more questions, and weigh the answers.

I think they might be trying to cut some cost corners on the twist of saving you material and labor. Pouring over plastic, I think, makes for a longer wait until finishing but ends with better curing.

 

[ericm]

It'll be hard to put the vapor barrier under the slab later if it turns out you need it.

Your codes may require it.

 

[Dagny]

Wish people would say where they live.

 

[ConCretin]

While limiting moisture intrusion into structures is always desirable, you only need a vapor barrier if a) you live in a non-arid climate and b) you intend to coat the floor with an adhered floor covering such as epoxy. Water vapor transmission through the concrete could interfere with the bond of the floor covering and cause it to fail.

If that's the case, I'd opt for a true 15 mil vapor barrier such as Stegowrap. Poly is better than nothing but it's technically a vapor retarder not a true barrier and will likely be full of holes by the time the concrete is in.

In addition to the cost, there is a downside to vapor barriers. Because mix water is blocked from leaving the concrete into the base below, it all comes to the surface inn the form of bleed water. If you don't carefully control slump and/or have a finisher who knows how to properly deal with bleed water, a range of surface defects can result.

Hope that helps. Good luck with your project.

 

[89efhatch]

Wish people would say where they live.

Upstate NY. Binghamton area

 

[89efhatch]

While limiting moisture intrusion into structures is always desirable, you only need a vapor barrier if a) you live in a non-arid climate and b) you intend to coat the floor with an adhered floor covering such as epoxy. Water vapor transmission through the concrete could interfere with the bond of the floor covering and cause it to fail.

If that's the case, I'd opt for a true 15 mil vapor barrier such as Stegowrap. Poly is better than nothing but it's technically a vapor retarder not a true barrier and will likely be full of holes by the time the concrete is in.

In addition to the cost, there is a downside to vapor barriers. Because mix water is blocked from leaving the concrete into the base below, it all comes to the surface inn the form of bleed water. If you don't carefully control slump and/or have a finisher who knows how to properly deal with bleed water, a range of surface defects can result.

Hope that helps. Good luck with your project.

Thanks for the input. The guy doing the work did mention the water all coming to the top making it harder to deal with. I want to avoid future moisture problems though

 

[rustyjames]

Given the location, I would do the vapor barrier.

 

[Pntyrmvr]

Of course the guy doing the work mentions the water coming up!

It's a longer day for the concrete finishers as the water proportioned by the batch plant to chemically react with the cement powder is actually where it should be for strength: in the mix not soaked into the ground below.

A guy like that makes me wonder about his commitment to a long lasting product.

If there are drawings or specs for this pour put the under slab membrane right in there along with air entrainment, thickness, and strength of the concrete.

 

[ConCretin]

Thanks for the input. The guy doing the work did mention the water all coming to the top making it harder to deal with. I want to avoid future moisture problems though

While your floor covering might notice the presence or lack of a vapor barrier, it's unlikely you ever will. The amount of moisture that will come through a slab is usually irrelevant to the conditions inside the garage. The most common moisture issue in a garage is condensation, which is the result of warm moist air from outside coming into contact with cool interior surfaces. A vapor barrier won't prevent this.

Half the guys on here think they have a vapor barrier when in truth, what they have is a degraded sheet of 6 mil poly that is so full of holes it barely slows the moisture down.

Not trying to talk you out of it but you are quite a ways down the road with your finisher at this point. If you insist on a vapor barrier and a slump low enough to avoid surface issues, you'll likely have a pretty disgruntled finisher on your hands If you go this route, ask the batch plant for a mid range water reducer and tell them you are trying to keep the mix water to a minimum while providing a workable slump. Otherwise they'll just use it as an excuse to cut back on the cement.

Btw, don't depend on the batch plant to control mix water. The truck driver is going to add as much water as the finisher asks for unless you are there to monitor it. Also, don't even consider air entrained concrete. The combination of air entrainment, a vapor barrier and interior steel troweled concrete is a recipe for utter disaster.

Give my Guide to Floor Slabs in the link below a read for more on the subject of slabs.

 

[89efhatch]

While your floor covering might notice the presence or lack of a vapor barrier, it's unlikely you ever will. The amount of moisture that will come through a slab is usually irrelevant to the conditions inside the garage. The most common moisture issue in a garage is condensation, which is the result of warm moist air from outside coming into contact with cool interior surfaces. A vapor barrier won't prevent this.

Half the guys on here think they have a vapor barrier when in truth, what they have is a degraded sheet of 6 mil poly that is so full of holes it barely slows the moisture down.

Not trying to talk you out of it but you are quite a ways down the road with your finisher at this point. If you insist on a vapor barrier and a slump low enough to avoid surface issues, you'll likely have a pretty disgruntled finisher on your hands If you go this route, ask the batch plant for a mid range water reducer and tell them you are trying to keep the mix water to a minimum while providing a workable slump. Otherwise they'll just use it as an excuse to cut back on the cement.

Btw, don't depend on the batch plant to control mix water. The truck driver is going to add as much water as the finisher asks for unless you are there to monitor it. Also, don't even consider air entrained concrete. The combination of air entrainment, a vapor barrier and interior steel troweled concrete is a recipe for utter disaster.

Give my Guide to Floor Slabs in the link below a read for more on the subject of slabs.

Are you saying if I don't intend on using epoxy that I might not need the the vapor barrier? I just plan on sealing it. I don't see myself going with epoxy
Thanks for all the help

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[89efhatch]

Also the vapor barrier I was thinking about going with is 15mil from a local supplier of unknown brand

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[ConCretin]

Are you saying if I don't intend on using epoxy that I might not need the the vapor barrier? ]

Yes. You won't be able to tell whether you have one or not in everyday use. A lot of people put them down "just in case" or because "it's now or never" but unless you are going to apply a floor covering, a vapor barrier is not mandatory.

On the other hand, they are relatively inexpensive to install and as long as you understand the potential risk associated with bleed water and improper finishing, you'll have it if you ever need it.

While more costly, the 15 mil products are vastly superior to 6 or 10 mil poly, which are very easy to damage and degrade over time. To be effective all seams and penetrations need to be taped.

 

[Sumboodie]

Why has no one mentioned 2" foam?

 

[Retroman]

While limiting moisture intrusion into structures is always desirable, you only need a vapor barrier if a) you live in a non-arid climate and b) you intend to coat the floor with an adhered floor covering such as epoxy. Water vapor transmission through the concrete could interfere with the bond of the floor covering and cause it to fail.

If that's the case, I'd opt for a true 15 mil vapor barrier such as Stegowrap. Poly is better than nothing but it's technically a vapor retarder not a true barrier and will likely be full of holes by the time the concrete is in.

In addition to the cost, there is a downside to vapor barriers. Because mix water is blocked from leaving the concrete into the base below, it all comes to the surface inn the form of bleed water. If you don't carefully control slump and/or have a finisher who knows how to properly deal with bleed water, a range of surface defects can result.

Hope that helps. Good luck with your project.

Lots of Class A vapor barrier used here in the Mojave Desert.

 

[RangerSVT]

Just read LLWillysFan's guide to floor slabs, great write up, thankyou! But also found a link to this as a comment under his write-up. I haven't read through this one yet, but sharing it here as it discusses vapor barrier...

https://www.garagejournal.com/forum/showthread.php?t=322454&highlight=grade

 

[Raisedonadeere]

Try storing a couple bags of cement on a non vapor barrier floor for a couple of years, maybe a metal tool box that has fallen into disuse sitting in a corner somewhere for a few years.

over the years I have seen the difference between with and without vapor barrier and it is not just the affect on humidity in the building although that is much greater that most suppose in a well sealed building with no climate control. In an open air building with everything pretty much moving around you might not notice much but when things start getting laid there and they kind of pile up in a corner or against the wall somewhere you will find things work out quite well when there is a good vapor barrier under the concrete. Makes a really big difference in basements, not as much in a open air garage but when things start stacking up you are going to see the insect friendly, crud friendly environment taking its toll. Moisture wicks through concrete. And it exerts pressure against whatever is trying to retard it such as floor coverings.

And when properly understood it adds very little to the overall cost of the pour.
After understanding the science of vapor pressure, concrete curing etc, as far as I am concerned it is a no brainer. I hire the concrete guys that talks vapor barrier and seems to understand it. Or else as in one job where the guy seemed to be ignorant of these things I ask him if he is happy to just do the job like I ask him. If he falters on that in any way I just opt out of doing the job for a while.

 

[Jazz1]

Also the vapor barrier I was thinking about going with is 15mil from a local supplier of unknown brand

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Stegawrap is designed for this purpose and is resistant to puncture and tearing. I think I paid $200 for 100’x 24’. Used in crawl space of home as well excellent in portable garages for preventing moisture from entering space/

 

[wingzero2sh]

To my knowledge slab pulls moisture from ambient humidity

 

[Armorpoxy]

We would recommend insisting on it.

 

[Amnashaikh]

A vapor barrier is a standard and recommended practice. It protects moisture from rising to the top. Polyethylene material is normally used as a vapor barrier.

 

[b-boy]

Try storing a couple bags of cement on a non vapor barrier floor for a couple of years, maybe a metal tool box that has fallen into disuse sitting in a corner somewhere for a few years.

I agree with this. I have a garage that has no vapor barrier under the slab. Anything metal rusts in there.

I have a pole barn with a vapor barrier. Since I moved all my tools in there, no issues with rust.

Even the feel of the air in each building is different. The old garage has a musty smell and just feels damp.

I'm in Buffalo NY.

 

[Raisedonadeere]

To my knowledge slab pulls moisture from ambient humidity

Actually the vapor moves from higher humidity to lower humidity. In the case of a slab of concrete, in many areas the moisture in the earth under the slab creates a higher humidity area than what you want in your building and so vapor travels from underneath, through the concrete into your structure.

If the concrete temperature is below the dewpoint of the air in the structure of course moisture from the structure space will condense onto the concrete so in that sense I suppose you could say the concrete "pulls" moisture. Moisture from underneath the slab wicking into the building would just add to that problem.

 

[Armorpoxy]

Since we deal with hundreds, of not thousands of garages a month, moisture is a common problem and in our opinion other than some nominal cost, there is no downside to putting vapor barrier down.

It can't be added later on so you only get one chance! As Nike says "Just Do It" ! (our opinion of course)

 

[ConCretin]

All else being equal, I'd always opt for a vapor barrier but it's not accurate to say there is NO downside.

If I have a contractor that doesn't know how to deal with the added bleed water and is going to float it into the surface off my slab, I've got to find another guy or pick my poison. Do I want to deal with some moisture potentially rusting the cans of paint I leave on the slab OR do I want a chalky, blistered, de-laminated surface. How many times have we heard that complaint on the GJ?

Most moisture complaints we read about on here are the result of condensation from warm moist air from outside the garage coming in contact with cool interior surfaces. I doubt most of us would even know if we have a vapor barrier or not.

In addition, although it isn't relative to the average garage slab vapor barriers contribute to slab curling. By keeping the bottom of the slab moist while the top drys, large slabs will curl. In fact, a large supermarket chain up this way stopped using vapor barriers for that very reason.

For such a simple material, concrete is surprisingly complex. Do I install vapor barriers under my slab? yes I do. In fact I wouldn't dream of throwing down some Home Depot poly and calling it a vapor barrier cause it ain't. There are circumstances however where they cause more harm than good. Never hurts to have all the facts.

 

[danfromsyr]

Binghamton is in the Southern Tier.. 'downstate'

Upstate NY. Binghamton area