Vapor barrier under exterior slab-on-grade??
Vapor barrier under exterior slab-on-grade??
(OP)
I'm in the process of putting together plans for a small boat ramp. From what I can tell from a state DNR fax, they recommend placing TWO layers of 4-mil vapor barrier underneath the concrete ramp.
I understand the purpose for a vapor barrier inside a stucture but what is the purpose of this on an exterior slab?? Any idea what the DNR is thinking??
Thanks,
---
Andrew
I understand the purpose for a vapor barrier inside a stucture but what is the purpose of this on an exterior slab?? Any idea what the DNR is thinking??
Thanks,
---
Andrew





RE: Vapor barrier under exterior slab-on-grade??
RE: Vapor barrier under exterior slab-on-grade??
I think I see what they are trying to show. I think the picture of the base is only for the "push-in" slab that goes into the water. The two vapor barriers make the slab easier to push into the water.
Thanks Again!!
---
Andrew
RE: Vapor barrier under exterior slab-on-grade??
RE: Vapor barrier under exterior slab-on-grade??
VDRs are rated according to their permeance ratings, in either (Imperial/US: grain/ft^2 * hr – inches Hg) or (SI: ng/Pa*m^2*s), and are usually categorized as Type I or Type II (Type 1s are better, having a metric permeance of up to 15 nanograms…, as opposed to a Type 2, with a permeance of up to a maximum of 60 ng/Pa*m^&2*s. As you can imagine, an aluminum foil has a very good permeance rating (0.03 mm = 0 perm), as does 6 mil polyethylene (3.4 perm SI). By comparison, standard 1-2-3 concrete has a perm SI of 184, and ½" gypsum wall board has a perm rating of 2026 SI!
To convert Perm SI to Perm Imp., multiply Perm SI by 0.0174
The big trouble I have with most applications of poly under a slab is the actual practice, which guarantees that it will be perforated in hundreds of places before the slab is placed. To avoid this, I generally spread a small layer of sand over the crushed stone before laying on the poly (which I lap and seal also). Then, heavy boots will not ruin the thin membrane as readily.
As to your second question, I have no experience in this area - with sealers, top membranes, etc. Perhaps another forum member can respond to this?
Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.
RE: Vapor barrier under exterior slab-on-grade??
Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.
RE: Vapor barrier under exterior slab-on-grade??
Aton is right on the money. The only thing I would add is that without a vapor barrier, on an inside slab, you run the risk of the finish materials (tile, paint, carpet or sealers) being damaged by moisture coming through the concrete. The safest and best practice is to always use a vapor barrier for interior slab-on-grade.
RE: Vapor barrier under exterior slab-on-grade??
For slabs without covering, a vapor barrier directly below the slab increases the potential for curling and diminishes the slab-life.
In addition, if your project is a tilt-up concrete system, the slab will usually be placed first, before the roof structure, exposing the slab to exterior rain. With a vaport barrier, you run the risk of perching water directly below the slab. After the roof in installed and subsequent floor finishes placed, the trapped moisture can wreak havoc on the finishes.