Page 1 - 2018-01-CFR Volume 112 - So You Don't Think You Need An Under Slab Vapor Retarder In The Desert - January 2018
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Volume 112—January 2018
This article was authored by Peter Craig, one of the industry’s foremost in-
dependent concrete floor consultants with Concrete Constructives and an
associate of LGM, and the Technical Department of Stego® Industries,
makers of the widely-specified Stego® Wrap Vapor Barrier. Peter has au-
thored over 25 industry publications, is a frequent lecturer and has more
than 44 years of experience with concrete slabs, specializing in diagnosing
and correcting moisture-related flooring & coating problems. This article is
used, with additions by me (LGM), with their permission. Stego Wrap is
also a product LGM recommends for use as an under-slab vapor retarder.

   Facts: It is all too easy for those who design and construct buildings in very dry regions of the country

   to not give moisture from the ground the consideration it deserves. Cities such as Phoenix, Arizona, Las
   Vegas, Nevada and Albuquerque, New Mexico for example each receive on average less than 10 inch-
   es of rain per year. They also have water tables well below the surface of the earth. How can there pos-
   sibly be a need to protect a concrete slab-on-ground from the entrance of moisture from below?
   For the answer, we first turn to science and a report issued by the Building Research Advisory Board
   (BRAB) in Publication 596 of The National Academy of Sciences (NAC). In section 6.0 of the publication
   the committee states their belief that regardless of the depth of the water table, or the amount or fre-
   quency of precipitation, it is likely that 100% relative humidity exists beneath concrete slabs-on-ground
   even under favorable soil and drainage conditions.
   In support of the committee's belief is the evidence that, since its publication, numerous floor failure in-
   vestigations conducted by respected firms have measured the relative humidity (RH) in the ground be-
   neath concrete slabs to be at or close to 100%.
   Now, no one would question 100% RH in the ground beneath a slab in Florida where the water table
   may be only 6 feet below the slab. But in Albuquerque, New Mexico— where the water table can be
   over 1,000 feet below the surface of the ground— how can this be?

1  Commercial Flooring Report                                                 January 2018
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