Vapor Retarder Under Slab?

[anojones]

I know this is a controversial issue- but I need to make a decision in the next few days. We are currently building a 30X72 pole barn (pictures to follow at some point), it will have a full bath and finished studio space for my wife. The studio area will be 18X24 with a 18X6 bath/utility room, heated/cooled by a PTAC unit and will have some type of residential flooring placed onto the slab (not sure what yet). There will also be a 18X30 insulated shop (with the concrete floor) again heated/cooled by a PTAC unit- the rest of the space will be garage/storage. It's a pole building and we are pouring a 4" slab on gravel (3" gravel at one end, 18" at the far end due to the grade). There will be no footers. We essentially live in the desert- about 14" rain a year and very little humidity, and there shouldn't be any water collection issues around the perimiter. This will not be a living space, but I anticipate my wife will use it fairly often for hobbies and it will probably remain heated/cooled year round (although I'm puttiing in the ability to winerize the plumbing).
The contractor doesn't think an under slab vapor retarder is necesary, and he's probably correct, but he doesn't seem too excited about it but doesn't mind if I lay one before the pour. I'm worried about curling of the slab etc with the barrier, and don't really want to deal with the work or expense if not needed. I could just do the studio portion of the slab and leave the rest.
I've never done this before- do I just lay the barrier up to the forms, or should the barrier extend a few inches beyond the pour? Anything I should know. Any thoughts appreciated.

 

[Superduper]

14 inches per year is not Seattle, I know, but desert? Anyhow, just my 2 cents but I would definitely install a vapor barrier throughout. Concrete is like a giant sponge and you can't really isolate water where you want it because it will migrate. Not only will a vapor barrier reduce moisture into the barn area but will also reduce or eliminate efflouresence, the white powdery substance that is so yucky looking. The effective cost of the plastic can't be that much, could it?

As for a 4" slab, well, you are having a contractor do it so I presume you guys are doing it according to code but it seems to me that a 4" slab without a footer is awfully thin and just asking for trouble. You're building a fairly large structure and cracks that develop later on is no fun. Around here, slabs are up to 12" thick and the perimeter is excavated even further for extra support capacity.

 

[Highbeam]

12" slab eh? That's just silly. This is a pole barn so the slab is not supporting the building and can be between 4 and 6" thick to be typical. Even our large industrial warehouses here in the puget sound area are 6" or less, some with rebar and some without. Of course the footer and foundations are not the slab and are often much thicker. Did you mean 12" footers?

 

[KaiB]

Ano...In your climate, it would certainly serve you well to lay poly down as it should serve to help retain moisture in your curing concrete; all the better for it. I agree that 4" with some steel is plenty for a floating slab.

Do you plan to sheetrock the studio area? If so, how have you designed framing and insulation?

I ask, because I am currently designing a large project (pole structure) with about 1500sgft in office, guest quarters. I still cannot get by without "floating" a stud frame btw. the poles to isolate the rock from the pole structure. If I do this, I then have the problem of windows integral to both the pole structure and the finished and cased rock.

I may be wrong, but I believe that if the rock is hung on a frame attached to the poles I'll have flex issues.

Ideas...?

 

[anojones]

I've read through a number of the pole barn threads here (too many to count really) and it doesn't seem that sheetrock ends up being a problem due to flex, I'm sure there are exceptions. We are going to have "commerical" girts: the girts (2X6) are attatched in between the poles on blocking on the flat. This has two advantages, it stiffens up the barn in our high wind area and allows us to finish the perimiter walls without having to do any more framing (the girts are flush with the 6X6 posts on the inside of the building). We will sheetrock the studio portion and probably use OSB in the shop. We will have to reinforce the slab for a load bearing wall for the studio, as I don't want to heat a 12' ceiling in that area and want to have an 8' ceiling.
Sounds like I should add a vapor barrier- it's going to be tough to squeeze in- plumber for underslab on Tuesday and pour on Wednesday- I'll have to do it Tuesday night. I suppose it's just laying out the material over the gravel and taping the seams- never done it or seen it done.

 

[LogChain]

Anojones,

I'm not an expert, but last week I formed and poured a 25'x50'x4.5" concrete slab for a pole barn here in East Tennessee (70-90" annual rainfall). I had professionals finish the concrete. I used black 6-mil plastic (20'x100' roll @ $80 + tax) from HD. It took a couple hours to roll out and cut the plastic. I overlapped the seams about 2 feet with no tape. Even duct tape adheres poorly to the plastic. I brought the plastic up to but not over the forms. The forms were 2x lumber and provided a ledge for stone work. Plastic over the forms would have interfered with concrete filling the forms. For straight forms, plastic over the forms is probably OK and better for retarding water loss during curing and suppressing lateral water infiltration. The basement slab of my house had plastic extending about 6" up the surrounding block walls. The basement floor shows no signed of seepage or moisture after about 17 years of East Tennessee. For what it's worth.

- LogChain

 

[KaiB]

Yep...at its most basic app., the poly is laid over the substrate. You need not tape seams if you have good overlap, but I have seen guys use duct tape. Your 'crete will thank you, and you won't spend much at all doing it.

It will get roughed up as the trades walk on it; not to worry.

I'm with you on the rigid design (using horizontal 2x6 girts btw. poles in the enclosed area); I'll still need girts on the exterior (to provide 1.5" out to match with the skirt), but these can be reduced to 2x4. I'm actually examining doing the 2x6 girts, wraping with Tyvek, nailing on the 2x4 exterior girts, and then the siding.

All this on the exterior part of the structure which will be finished as office, of course. Should be fun in the Oklahoma winds.

 

[RichNJKubota]

Check out
Shelter Companies
Click on "Building Products" on the left side and then click on "Building Products Brochure" toward the bottom right side.
It's a PDF file. Scroll down to page 11 (I Think) and you'll find a vapor barrier that is superior to the poly that you find at HD.
Conventional poly degrades rapidly under a slab. This stuff at Shelter Supply lasts many, many years.

 

[have_blue]

Conventional poly degrades rapidly under a slab. This stuff at Shelter Supply lasts many, many years.

Sounds like a sales pitch to me.

Environmentalists say that 1 mil poly bags last 1000 years in a land fill. Even if their estimate is 90% high, I think 100 years would suffice for a pole barn. <LOL!>

 

[Farwell]

I put 2 inch foam board as well as a vapor barrier under my pole barn concrete. The extra cost is worth the insurance even if it does nothing, in spite of the controversy. The cold concrete with warm humid air will add moisture to your floor in many cases. My garage floor will have 1/8 of water standing on it when the humidity is high, it has no moisture barrier or insulation. My pole barn is well insulated and heated during the cold season. It costs me around $35 a month to heat it with a natural gas fired Hot Dawg furnace. I have no moisture problem in the pole barn.