Pole Barn Concrete

[foggy1111]

I have purchased a new 30x40 pole shed and found a guy to do the site fill and compaction and do the concrete work after the pole shed is built (I like that one guy is responsible for this work...no excuses for not doing it right). The way the building is put up....he will use the lower boards for the concrete forms....which is a common practice around here. He is putting in 1/2" rebar on a 3' grid....and a six-bag mix. But....I forgot to talk with him about putting down a vapor barrier under the concrete slab. I spent considerable time getting a good tight building with decent sidewall soffits and ridge venting to reduce problems with condensation.....but I forgot all about the under floor barrier.

What are some thoughts? Is a vapor barrier needed under the concrete or not? How much extra should I pay to have my concrete & fill guy install the vapor barrier?

 

[radman1]

On my shed (47x72'), I used a single large, thick sheet of plastic. It should not cost much for the installation and the plastic is relatively cheap. A rough guesstimate for installation is about 1-1.5 times the cost of the plastic. The downside to the vapor barrier/plastic is the cement curing time goes up significantly and needs to wait longer before the cement hardens to work the cement such as power troweling. I poured mine in AM and couldn't walk on it until about midnight to power trowel it. (It was the cool time of year when I poured my cement.) Make sure the cement is not too wet. With the vapor barrier, the excess moisture has no place to go but evaporation.

Whether you need it or not is another question. If you have lots of rain or questionable drainage, then yes. If you are worried about radon, plastic is probably the cheapest barrier available. The plastic is cheap. Installation is cheap. Ask your installer what it will cost.

 

[Cat_Driver]

Ya why not add a thick platic barrier, it's cheap enough and may keep ants and termites from popping out where ever they feel like it.

In fact I think you can also get a bug guy out and spray the foundation with some sort of life time insect killer that would help.

 

[biggerten]

Get it down, it will help keep the concrete from getting slippery. He should also provide you with a choice of finishing, don't go for mirror smooth. It's tempting, I know, but you want *some* texture.

Especially since you took pains to ventilate to prevent condensation, and the water vapor will just come up through the concrete like it's not even there.

As for the concrete taking longer to cure, that's a good thing.

The moisture barrier is cheap (check out Menard's) and will take him very little time to install.

Edit: By getting slippery, I mean after the fact, like some spring day when you're trying to lift something heavy and a foot goes out from underneath you. This is a safety thing.

 

[LD1]

Get it down, it will help keep the concrete from getting slippery. He should also provide you with a choice of finishing, don't go for mirror smooth. It's tempting, I know, but you want *some* texture.

Especially since you took pains to ventilate to prevent condensation, and the water vapor will just come up through the concrete like it's not even there.

As for the concrete taking longer to cure, that's a good thing.

The moisture barrier is cheap (check out Menard's) and will take him very little time to install.

I agree 100%. Without the barrier, humid times of the year, it will be like some one hosed it down...all the time.

Longer to cure is a very good thing. The longer it takes to cure, the stronger it is and that means less surface cracks. Also, get it as thick as you can. The less water they use in the mix, the stronger it'll be. Once you have it finished, it's a good idea to keep the top wet, to make it take longer to cure. Hose it down with a fine mist a every few hours for about 7 days.

The reason for all of this is becase concrete doesn't "dry" it "cures". There is a chemical reaction that takes place(thats why it gets hot) and it needs water for this to happen. If all the water evaporates before the cement can cure, it isn't as strong.

 

[/pine]

The plastic also tends to support the chairs that hold up the steel (in sandy soil)...another benefit is the concrete will retain the water longer for a harder cure....keeping it wet for the first 3 days is optimal

 

[6sunset6]

One other thing you might want to think about or 2 actually if you are in a winter climate . Insulation board on top of the vapor barrier and tubing in the floor for heat.
I did a floor with vapor barrier and no insulation right next to a floor with both.
The floor with out insulation would act like a condensate sink on certain days and be very slippery.
On the tubes. i. e. radient heat in the floor. I heat my house like that but I never thought about it for my barn/shop. Hot water panels on the roof with some photo voltaic to run a pump and the floor can be 40d in the winter for free. If you do the tubes you need the insulation.
If you do the tubes now you can add the rest anytime. I kick myself everytime I think about it, which is often Nov to March.

 

[Willl]

I know it rains alot here but I'm on good drainage and without any plastic barrier under my slab.

Going on 8 years with no humidity/condensation problems.

My PB vents up through the side panel ridges and out the roof ridge vent. Doesn't everybody's ?

 

[foggy1111]

I am putting up an unheated and uninsulated building. I am not too sure about how every other building vents.....but I wanted a vermin and bug proof building....so I have sealed up all those siding vents and have put venting in via the soffits and at the roof peak. I think people do it differently for different purposes. I want to keep out the critters and still vent....thus my venting solution.....sorta like a house vents.

Thanks for all the responses....I think I will talk to my masonry guy about adding the plastic vapor barrier.

 

[foggy1111]

One other thing you might want to think about or 2 actually if you are in a winter climate . Insulation board on top of the vapor barrier and tubing in the floor for heat.
I did a floor with vapor barrier and no insulation right next to a floor with both.
The floor with out insulation would act like a condensate sink on certain days and be very slippery.
On the tubes. i. e. radient heat in the floor. I heat my house like that but I never thought about it for my barn/shop. Hot water panels on the roof with some photo voltaic to run a pump and the floor can be 40d in the winter for free. If you do the tubes you need the insulation.
If you do the tubes now you can add the rest anytime. I kick myself everytime I think about it, which is often Nov to March.

I also have this type of heat in my home garage and in my basement. I think its the greatest. Many of the heated pole buildings here have that type of heat too....and most really like it. THis building is not heated however....I am not even putting electric in....just some ridge lites.....very basic storage.....but secure and vermin and bug resisitant - I hope.

 

[Egon]

Is a vapor barrier needed under the concrete or not? How much extra should I pay to have my concrete & fill guy install the vapor barrier?

Maybe try looking up the and using the resources of "The Portland Cement Association" and then maybe you will find some answerers.

They have lots of old creditable documented knowledge at hand so you don't have to make any guesses!

Just do a Google!

Might even clear the fog!

 

[Ford850]

Vapor barriers will stop moisture from coming up through the floor if you have a damp or wet base, but floors sweat for other reasons. Most sweating is caused by moist or humid air and a cold surface. The cold surface condenses the moisture, pulling it from the air, not from the concrete. A good example is how much more mositure forms on a cold beverage can on a humid day compared to a dry day. Either way the mositure is coming from the air, not through the can.

 

[Duffster]

Vapor barriers will stop moisture from coming up through the floor if you have a damp or wet base, but floors sweat for other reasons. Most sweating is caused by moist or humid air and a cold surface. The cold surface condenses the moisture, pulling it from the air, not from the concrete. A good example is how much more mositure forms on a cold beverage can on a humid day compared to a dry day. Either way the mositure is coming from the air, not through the can.

^^^^^^^^^^^What he said^^^^^^^^^^^

Vapor retarder is an over rated product, insulation would be money much better spent

Not sure what thickness of concrete your pouring, but IMO rebar 3' o.c. is a little wide. I would use 3/8" steel on a tighter o.c.

 

[LD1]

Vapor barrior may very well be overrated and may very well not make a difference in the floor sweating......but when I poured mine 30 x 30 x 4" a total of $1230 in concrete, the barrior (which you can put down yourself, just go buy a roll of plastic) was only $20. Not a big deal wether it works or not..................but it will also make the concrete take longer to cure...which is a very good thing.

So if you dont want to put it down as a vapor barrier, put it down as a retarder to make your concrete stronger. Either way it's pocket change compaired to the cost you are already willing to spend on the concrete.

 

[Ford850]

Vapor barrior may very well be overrated and may very well not make a difference in the floor sweating......but when I poured mine 30 x 30 x 4" a total of $1230 in concrete, the barrior (which you can put down yourself, just go buy a roll of plastic) was only $20. Not a big deal wether it works or not..................but it will also make the concrete take longer to cure...which is a very good thing.

So if you dont want to put it down as a vapor barrier, put it down as a retarder to make your concrete stronger. Either way it's pocket change compaired to the cost you are already willing to spend on the concrete.

That's very true. I agree it would be a good thing and cheap insurance if sub-surface moisture might be a problem, and also a great retarder. Just don't buy into the story about how it will stop sweating, which is a common issue with unheated buildings. Also, depending on who is pouring the floor it could cost quite a bit more in labor costs since they will be waiting all day to finish it. If you are doing it yourself, that's not an issue.

 

[biggerten]

Also, depending on who is pouring the floor it could cost quite a bit more in labor costs since they will be waiting all day to finish it. If you are doing it yourself, that's not an issue.

If the concrete is mixed properly, there's no issue there, I don't know where that got started. My concrete was poured at 7AM, screeding, floating and finishing was done by noon.

Get good guys, use the vapor barrier and don't look back. The insulation and in floor heating was a good part of this discussion, but won't be an issue for you.

Good luck.

 

[EddieWalker]

I love vapor barriers.

They never rip.

They have such a high R value that they stop the concrete from forming condensation.

They never trap water in between them and the concrete, along with sand and silt that holds moisture.

But of course, there are a few things that I don't understand.

If you put a piece of concrete in a puddle of water, how come the water never works its way up to the top of that piece of concrete?

When there is a crack in a slab that does not have a vapor barrier, and it's raining outside, why doesn't the water come in through that crack?

If you pour a cup of water on top of the concrete, how come the water never goes down through the concrete? just up from the ground to rest on the top of it?

If a slab with a house on it doesn't have a vapor barrier, why does water work it's way up through concrete in the garage/shop, but not inside the house and ruin the carpet?

OK, so I'm not being serious and I already know that nobody will answer my questions if they already believe that water comes up through the ground and through the concrete to rest on top of the concrete. I remodel homes and cut into concrete slabs fairly often. I've seen homes with vapor barriers and I've seen them tore and holes wore through them. I'm sure it's just the homes here in East Texas that has that happen to them, but the funny thing is that they don't have any moisture on top of the concrete. Another personal observation is that when replacing bathtubs, I never find any moisture on top of the concrete, yet there is a big hole under the tubs for the drain. Tubs almost always have pretty big gaps around their drain lines because it's so hard to get it just right when doing the rough in on the plumbing before the slab is poured. This is just another example that proves to me that water isn't coming into buildings from the soil under the building.

I think the vapor barrier has it's place and is actually very important when needed. Somehow it's become a wonder cure for things that don't exist, and it doesn't have anything to do with. The reason for it is to keep moisture IN the concrete when pouring a foundation. Concrete needs to keep it's moisture level for a certain amount of time to cure properly. If you pour concrete over porous or very dry souls, the moisture in the concrete will seep into the soil and out of the concrete. This will make for a weaker pour. The vapor barrier keeps the water in the concrete.

If you need it, then it's a great idea. If you think it's going to stop water from coming up out of the ground and forming puddles or wet areas on top of your concrete, spend the money for piece of mind. It's your money. But it's doesn't actually accomplish this.

Eddie

 

[biggerten]

from Choosing a Vapor Barrier - The Concrete Network

Choosing a Vapor Barrier

Bottom line: If you want to make sure your slab on ground gets dry and stays dry, use a vapor barrier.
Cure with waterproof sheets rather than with water or curing compounds.
All vapor barriers should conform to ASTM E-1745; but consider going with a barrier that has water vapor permeance less than 0.03 perms.
A 10-mil barrier is probably OK for residential construction if you're careful not to puncture it during construction.
If laser screeds or heavy placing equipment will be on the barrier, go with 15 mils.
Starting with lower water-cement ratio concrete will let the slab dry faster. Keep w/c at or below 0.5.
Vapor barriers also block gases, such as methane or radon. Radon is the second leading cause of lung cancer in the U.S., according to the American Lung Association.
Typical 8-mil polyethylene sheeting costs 5 to 7 cents per square foot. A 15-mil vapor barrier can be about 25 cents per square foot. Sure, that's an increase, but it's better than a damp, discolored slab.



I guess you got all the arguments.

 

[foggy1111]

Like so many things in life....this issue is about as clear as mud. But from Eddies Post....a vapor barrier doesn't seem like a big deal except when used to slow the curing process....which seems to me to be the biggest advantage. Then too.....just closing the overhead door on my new shed will greatly slow the curing process by keeping the sun off the concrete and by slowing evaporation....probably much like keeping it covered.

I had hoped a vapor barrier would help to prevent condensation forming on the floor and interior walls/roof in an unheated and uninsulated building...but from this discussion I no longer think that's the case and it just don't seem like a big deal one way or the other.

 

[yooperdave]

Adding fly ash will densify the concrete making moisture more difficult to migrate thru the slab.

Vapor barriers only ****** moisture at best. Often they are punctured or misinstalled not helping the slab as intended.

Yooper Dave