Page 2 - 2018-01-CFR Volume 112 - So You Don't Think You Need An Under Slab Vapor Retarder In The Desert - January 2018

Again, we turn to science to understand the upward migration of mois- The experts at LGM
ture in soils. There are two basic means by which moisture in the
ground can rise from the water table to a higher elevation: capillary ac- specialize in consultation,
tion and diffusion. correction and resolution for
As it relates to our discussion, ca- flooring complaints, claims,
pillary action is the rise of water in installation and performance
liquid state to an elevation higher
in the ground than the water table. issues.
For this to occur, the interstitial
space (gap) between soil particles No issue is too big, too small
must be extremely small such that or too far away for us to
the adhesive attraction of water
molecules to the soil particles handle.
forming the narrow passageways
exceeds their cohesive strength. The combination of adhesive attrac- All ads are interactive. Just
tion, surface tension and the cohesive strength of water can cause liq- click on the ad to enter their
uid water to rise many feet above the measured water table if the soil
conditions are favorable to capillary rise. website.
The potential rise of water in soils by capillary action is well known and
understood by those in the geotechnical field. When such a condition
exists, or is of concern, the typical solution is to remove (to a specified
depth) the finer soil material and replace it with a coarse fill material
(such as crushed stone) where the gap between particles is wide and
not conducive to the capillary rise of liquid water. In the drier regions of
the country, where the water table is well below the surface of the
earth, capillary action is seldom the cause of moisture-related flooring
or coating problems.
The second means by which moisture rises from the water table to a
higher elevation is diffusion. Diffusion is a well-established scientific
principle where molecules move from an area of higher concentration
to and area of lower concentration until a state of equilibrium is
reached. As it relates to our discussion,
in the ground, regardless of the depth
of the water table, water will undergo a
phase change from a liquid to a vapor
and the vapor will rise upward by diffu-
sion through the soil structure in an at-
tempt to establish a state of equilibrium
with the drier environment above (i.e.
within the building envelope).

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2 Commercial Flooring Report January 2018