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Re: Vapor Barriers

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I worked for a number of years for an office (SE) here on the central
California coast.  The standard detailing for slabs on grade called for a
6" rock cushion as a capillary break with no plastic vapor barrier.  I
don't know if this would be applicable for other areas/climates, but seemed
to work well here.  Either gravel or coarse sand meeting noted
specifications serves as the "rock" cushion.

Paul Guthrie, PE
San Luis Obispo, CA

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From: Michael Davis <michael.davis(--nospam--at)ferc.fed.us>
To: seaint(--nospam--at)seaint.org
Subject: RE: Vapor Barriers
Date: Thursday, October 15, 1998 6:37 AM

"Caldwell, Stan" <scaldwell(--nospam--at)halff.com> wrote:
 
>"the vapor barrier is just intended just stop upward vapor transmission.
>That is why I think it may be OK to punch holes in the vapor barrier"

>Huh!  I don't get it, y'all!  Why bother to put down a visqueen barrier
>if you intend to poke holes in it?  If you had a boat, would you
>purposely poke holes in the hull?  Since the purpose of the visqueen is
>to keep moisture out of the floor, whether liquid or vapor, any
>penetrations are detrimental.  Besides, the contractor will usually find
>creative ways (rebar, crushed stone) to puncture the visqueen without
>any help from the engineer!


Stan,

The main purpose of using a vapor barrier is to reduce the humidity level
in the building to reduce or eliminate condensation problems.  The holes in
the boat analogy is not a good one - rather, the barrier is more like
housewrap, it doesn't stop the wind from washing the heat out of the
walls in the winter, but it does slow it down considerably.  If you have a
few small areas where the housewrap is not sealed properly, it's not
going to make much of a difference to your fuel bill.  Likewise, a few
small holes in the vapor barrier is not going to significantly increase
vapor transmission. 

Vapor transmission requires a large surface area to move a lot of
moisture.  A small amount of vapor transmission can be handled by the
natural (or mechanical) ventilation of the building.  Have you ever been in
a crawlspace of an older home where there is no vapor barrier, but just
an exposed soil surface?  I've read that several gallons of water
evaporate from the soil daily into these homes, resulting in condensation
on the joists and subsequent rot.  Installing the barrier with a layer of
gravel to hold it down and protect it makes a huge difference,
substantially lowering the humidity level in the crawlspace and in the
house.

Vapor barriers aren't designed to stop water penetration.  If this is a
concern (and it usually is!), then drain pipes along the footing leading to
a
sump pump or lower ground outside should be installed.   


Michael S. Davis, P.E.