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Under Concrete Vapor Barriers

The term vapor barrier or vapor retarder commonly refers to the plastic sheeting used under a concrete slab.  These products are used to block moisture migration and capillary action under concrete that is poured on ground.  Without a vapor barrier the moisture-dampness enters the concrete assembly over time to increase an Owners exposure to flooring issues, mold and costly repairs to re-establish a dry surface. 

The latest vapor barrier assembly and options of where to place the vapor barrier can be seen in the American Concrete Institutes (ACI) 302.R1-8 flow chart.  For year’s geotechnical recommendation provide just a few options that may include pouring concrete without a vapor barrier, pouring concrete on 2” of sand or directly pouring concrete on the plastic sheet.  Unfortunately the idea vapor barrier placement is an ongoing discussion for each project and the construction team’s preference.

The performance and quality of the plastic sheeting product has changed in recent years.   Older visqueen products commonly referred to in building specs as 6 or 10 mil offer a low puncture resistance rating and long term biodegradability.  Many renovation projects uncover higher than expected moisture vapor emission rates (ASTM F1869) and excessive in-concrete relative humidity levels (ASTM F2170) that have used low quality vapor barrier products.  Once established, the moisture readings will require the use of a liquid applied treatment process like the Synthetic30 technology to re-establish ideal moisture levels.     

High quality products can be established in new building specs.  Synthetics International recommends products with high puncture resistance ratings (ASTM D1709 method B), ultra low water vapor transmission rates (ASTM E96 procedure B) and non-biodegradable plastic resins.  Creating building specs based these physical properties will limit holes during installation, abuse of pouring the concrete on the plastic sheeting and resist damage during the preparation process. 

Even the best plastic sheet products can be deemed useless when improperly installed.   It has been documented that even small holes in the sheeting product can increase concrete moisture readings and hydrostatic pressure.  The reality is that most building specs, sheeting manufacturer recommendations and industry installation guidelines (ASTM E1643) are often not available to the field installers performing the process.  This allows bad habits and bad installations to continue on millions of projects each year. 

A good vapor barrier installation should be seen as a long term success.  The sheeting product will diffuse moisture-dampness from migrating through the concrete slab to interior spaces.  It will not resolve the moisture drive from newly placed concrete.  In fact, concrete takes longer to dry when a vapor barrier is used.  Only consistent weather and time will assist concrete in its drying process. 

The success of a vapor barrier installation can be seen years later when performing concrete moisture testing during a buildings renovation process.  A vapor barriers benefits are not immediate, since newly poured concrete has an amount of water that must dissipate (release) over time.  The greater the amount of water used during the pouring and placement of the concrete must be released over time.  The duration of time needed is unknown, but the idea drying conditions is a constant temperature-low humidity, no rain and wind (air movement) of the slab surface.  Since the drying conditions are unknown during the construction schedule, it has become difficult to install a flooring system.  The advent of fast track construction practices and early building completions can be achieved with an added expense of moisture treatment of the concrete before a flooring product is installed.   

Best practices are to weather proof the building and to acclimate the interior spaces.  A balanced HVAC system, allows the concrete slab to reach ideal conditions for the moisture testing process.  Most moisture testing is performed in a non-acclimated building and the potential for flooring issues remains. 

To explore this subject and other connections to concrete floor moisture, please contact our technical department at