Greenhouse Slab Vapor Barrier?

[SteelPE]

I am currently designing a foundation for a greenhouse (110,000 square feet). Code is IBC2015.

We typically call out 10mil vapor barriers under all of our slabs.... would you still call out a vapor barrier under a greenhouse slab? Though process is, there will be adding so much water for that plants that it won't really matter if a little extra vapor comes from under the slab.

 

[r13]

I agree with your assessment, vapor barrier is not required for this case.

 

[MIStructE_IRE]

I would agree.

 

[dik]

Any chance for frost heave? in in the current use or maybe later use? even at edges?

Dik

 

[STrctPono]

Just some food for thought....

1. Isn't 10 mil. a vapor retarder. Isn't 15 mil a vapor barrier?
2. Is owner going to be doing anything with the floor? Tile or epoxy overlay?
3. Any chance the building occupancy could change in the future?
4. Do you ever install vapor barriers for other purposes such as in conjunction with an active soil depressurization system for radon gas mitigation?
5. Does a vapor barrier protect the bottom layer of steel in the slab from accelerated corrosion if you are less than 3" clear cover?

 

[SteelPE]

1. Isn't 10 mil. a vapor retarder. Isn't 15 mil a vapor barrier?

I don't really know, we have historically indicated a 10 mil sheet under the slab. To be honest, I don't quite know why we need to specify the barrier at all. Seems like something the architect should be calling out. There is no benefit for the structural engineer to be calling this out... but there certainly seems like there is a lot of downside placing this information on the structural drawings.

2. Is owner going to be doing anything with the floor? Tile or epoxy overlay?

It's a greenhouse for mass production of plants, I doubt there will be any type of floor finish other than concrete.

3. Any chance the building occupancy could change in the future?

It's a plexiglass greenhouse. I suppose the occupancy could change..... they could also decide to erect a 150 story building on top of our foundation. Should I try to accommodate that? I am tasked with designing a foundation for a greenhouse not something else that the greenhouse may change to in the future.

4. Do you ever install vapor barriers for other purposes such as in conjunction with an active soil depressurization system for radon gas mitigation?

No

5. Does a vapor barrier protect the bottom layer of steel in the slab from accelerated corrosion if you are less than 3" clear cover?

This is just a slab on grade, not structural. We are showing WWM in the slab but really it's only there for crack control (no, no one (owner and GC) is going to want to install bars at 18" o.c.). Column grid is 7'x24' and we are providing continuous slab haunches to pick up the interior columns spaced 7'-0" o.c.

 

[EZBuilding]

I don't really know, we have historically indicated a 10 mil sheet under the slab. To be honest, I don't quite know why we need to specify the barrier at all. Seems like something the architect should be calling out. There is no benefit for the structural engineer to be calling this out... but there certainly seems like there is a lot of downside placing this information on the structural drawings.

I would highly recommend that you include language in your proposals/contracts which indicate that you are not responsible for any architectural elements such as life safety or waterproofing. Your structural notes should at best reference to see architect for this information. I expect the majority of Structural Engineers - myself included - don't fully understand the need or use for these elements but have historically continue including this information in their structural notes.

 

[GeorgeTheCivilEngineer]

A barrier will stop water being lost to the sub-base during curing. I'm not sure how big a problem that is really, but even in agricultural sheds, there's a polythene sheet placed below the slab.

 

[dik]

Why 10 mil? we usually use 6 mil.

Dik

 

[GC_Hopi]

I would keep the barrier for similar reasoning to George. It helps the curing of the slab.

You probably have done this but the slab design may consider traffic loading depending on the client. They will likely use forklifts or pallet jacks, if this is an industrial operation, to be moving soil, fertilizer, etc around or there may be post loading from shelving.

 

[r13]

In order to avoid conflict with the specification, I always call out the "vapor barrier/retarder" on our drawing without given thickness. For slab curing alone, I wouldn't provide sheeting, as the subgrade should be properly wetted, and maintain moist throughout the concreting.

 

[dik]

so you can use 4mil?

Dik

 

[r13]

See spec.

 

[dik]

That's where we differ... I rarely use a spec... I may have 3 or 4 pages of comprehensive project notes... too often with old projects, drawings can be found, but the specs are missing. On a major project in Winnipeg, and one of my first with SNC, the project manager (different company) when he issued the Tender drawing on which the Contract was based, forgot to include the specifications in the package... if it weren't for project notes, there could have been an incredible Extra to Contract.

Some clients require a spec and the spec is coordinated to my project notes (over 400,000 B text document)

Dik

 

[r13]

We always include specifications in the bid/design package. In most occasions, I would only list the steel grade, concrete strength and reference drawings on the first sheet of foundation drawings.

 

[XR250]

@GC_hopi,

It is my understanding that unless proper curing practices are enforced on the top side of the slab, a vapor retarder will promote slab curling. So maybe they are better off without it unless they plan to manage that.

 

[Craig_H]

It has been my understanding that a vapour barrier on the underside of a slab-on-grade would provide a lower friction surface, thereby preventing uncontrolled shrinkage cracks from forming. By allowing the slab to shift slightly against the ground, cracks would be more likely to form at control joints as intended. I've always understood this to be the reason why a vapour barrier was specified below slabs on grade on the structural drawings. Perhaps someone can debunk or confirm this?

 

[dik]

XR250... correct

Dik

 

[dik]

Debunked confirmed?... it's actually to minimise moisture from the soil wicking through the concrete and affecting finishes and environment.

Dik

 

[SteelPE]

Wow, my head is now spinning over this simple question. In this instance, I may just bring the issue up with the owner. I am on the fence about it. We have no finish materials that I am aware of as it's just suppose to be a concrete slab. Honestly, I don't even know if the owner is going to even know what I am talking about.... as I have had to hole his hand through this whole process.