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Vapor barrier under exterior slab-on-grade??

Vapor barrier under exterior slab-on-grade??

Vapor barrier under exterior slab-on-grade??

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??


RE: Vapor barrier under exterior slab-on-grade??

For exterior applications, there are no good reasons to use a vapor barrier.  Further, for sloped applications, a vapor barrier, particularly plastic, reduces the friction interface between the concrete and the ground and will promote slippage of the slab.

RE: Vapor barrier under exterior slab-on-grade??

Thanks Ron,

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!!

RE: Vapor barrier under exterior slab-on-grade??

What are the possible consequences of not laying a vapor barrier prior to pouring a monolithic slab?  What can be done to prevent any problems after the fact?

RE: Vapor barrier under exterior slab-on-grade??

FredRay, if you are referring to an INSIDE slab, the consequences may be little or great, depending on several geological factors.  The use of a VB is prophylactic, so it is better to always use it.  One reason is the fact that concrete is subject to capillary suction (water will "wick" up, as witness a concrete block set into a shallow puddle).  Thus damp basements, leading to mould, rot, and health (IAQ) problems.  A good drainage layer, such as 6 inches of crushed stone, is also an essential capillary break..  In addition, concrete is prone to vapour diffusion, whereby the small gaseous H2O molecules can easily work their way through the microscopic interstices in the concrete matrix.  Some concretes, such as a high-volume fly ash (ASTM Class F) are more water and vapour-impermeable than others, but the additional protection of an extremely low permeance Vapour Diffusion Retarder (VDR) is essential.  Thirdly, in areas prone to soil gases, such as radon gas (often coming through small fissures in a granitic formation), a low permeance shield is needed (along with sub-slab depressurization techniques to keep the radon out of the building entirely).  The funny thing about radon gas is that one house may be radon-free, while its next-door neighbour could be riddled with it – all depending on the nature of the fissures in the rock underlay.

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??

By the way, no sub-slab VB will  prevent water ingress due to high seasonal water tables!  If a building must be placed below such WT,  excellent perimeter drainage at the level of the footing base is essential, to at least create a local cone of depression.  In addition, to enable surface water to readily penetrate down to the drainage tile, a well-drained backfill must be used, and eavestrough downspouts shunted beyond the foundation line, etc. - basic, good design and practice.

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??

Not necessarily for industrial slabs-on-grade.  VB do work fine for slabs with subsequent coverings such as carpet, tile, etc.

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.

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