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RE: Vapor Barrier

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What is the purpose of the "vapor barrier"?

I have typically understood this barrier to actually be a capillary break
between the slab and the supporting base rather that a vapor barrier as
found in above grade walls.  This is a key element especially if the area
will be interior to the building envelope.  If this is the case, the
performance of the capillary break will be largely dependant upon surface
area rather than the integrity of the element.

Based on this, I generally specify that holes be punched in the barrier
before placing sand or gravel on top of the barrier and that the sand/rock
be dampened just prior to placing concrete.  

Hal Riddle

> -----Original Message-----
> From:	Jim Kestner [SMTP:jkestner(--nospam--at)]
> Sent:	Tuesday, October 13, 1998 1:06 PM
> To:	seaint(--nospam--at)
> Subject:	Vapor Barrier
> I would like to get opinions on what I consider conflicting requirements
> regarding vapor barrier under slab-on-grade. ACI 302.1-R, 2.4.1 requires
> that if a vapor barrier be used that it be covered with 3" of sand
> before pouring the slab. This minimizes plastic shrinkage cracking and
> curling. It also controls bleed water coming to the surface. This is
> logical and well documented. However, this procedure is risky when
> pouring a slab not under cover. What are the chances of rain occuring
> after the sand is placed over the vapor barrier but before the slab is
> poured? Now, you may have water trapped which could be very expensive to
> deal with or if ignored, worse than having no vapor barrier at all.
> It seems like ACI requirements address the finishing and performance
> aspects of the slab, but have ignored the practical aspects of what the
> contactor must deal with. How do you other Engineers and Contractors
> deal with this?
> Jim Kestner
> Green Bay, Wi