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RE: Vapor Barriers

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I might be missing something here, but I have a couple observations
1) Generally, the soil is saturated before pouring a slab. I understand that
the water can percolate out - but:
2) The concrete is poured wet - some water must settle below the concrete
whether the sand between barrier and slab is wet or dry.
3) How much water is will remain once the slab dries? Doesn't the heat
generated in the curing process help draw the water up to the surface? Won't
this help to wick the excess moisture from below into the slab?
4) Assuming the sand is compacted well prior to placing the slab, Isn't the
moisture retained rather negligible?

Dennis Wish PE
-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Wednesday, October 14, 1998 1:17 PM
To: seaint(--nospam--at)seaint.org
Subject: Vapor Barriers


Thanks for all your responses.

No one except Brian Smith really addressed the constructibility issue
regarding if it rains and the top 3" of sand gets saturated what does
the contractor and engineer do about it? It won't drain since the vapor
barrier is usually taped at the joints and turned up and taped to the
perimeter foundation walls. About the only way it will "dry out" is by
evaporation and will 3" below the surface still be wet. Anyone else have
a better answer?

Someone ask what is wrong with having the vapor barrier directly under
the slab? The top surface of the slab will dry out eventually while the
bottom will stay wet. This will cause the slab to curl (just like a wet
piece of paper sitting on a table...the top dries and the bottom stays
wet). The 3" of sand acts as a blotter so that the slab cures and dries
more evenly. Plastic shrinkage cracking and bleeding is also much more
common with a vapor barrier directly under the slab.

We typically only use a vapor barrier under slabs that have finish
material on the slab that could be damaged by excessive moisture. We
might use 4 mil polyethene in residential, 6 mil in commercial and
something heavier for special conditions. If the water table rises to
the slab elevation or above, it is not intended to prevent water from
coming thru the joints. If that happens, you need a structural mat to
resist hydrostatic uplift and some waterproof membrane.

Although some of you referred to the vapor barrier as a capillary break,
I believe it is the course self draining granular subbase below the
vapor barrier that is intended to be the capillary break (ask your
friendly Geotechnical Engineer) and 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 (although I have only done this once or
twice many, many years ago....in California) for the reasons that
William Riddle stated.

I believe there are 3 items that cause problems with finish floor
materials: upward water flow (capillary action), upward movement of
vapors, and condensation (sweating). Each of these problems can be
solved by: course granular subbase, vapor barrier and a good HVAC
system.

Thanks again for making the discussion worthwhile and interesting.

Jim Kestner, P.E.
Green Bay, Wi
Go Packers!