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  Specifying Concrete Underslab Vapor Retarders
By Ron Beers, CSI


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For construction projects incorporating concrete slabs on-grade, one of the most critical building components affecting the long-term integrity of interior flooring materials is oftentimes left to chance, and in many instances not specified at all. While many architects, specifiers and flooring contractors understand the importance of concrete underslab vapor retarders, not all building professionals understand the different performance characteristics of underslab products. All too often, the wrong material is specified, which can lead to devastating consequences if the underslab vapor retarder is unable to keep moisture vapor migration in check.

Concrete underslab vapor retarders protect the integrity of both the concrete slab and any flooring materials applied over the slab by blocking the passage of moisture vapor emanating from the underlying soil. Despite outward appearances, concrete is not an impenetrable building material. Rather, concrete is permeated by microscopic voids that allow the passage of water and water vapor.

Concrete underslab vapor retarders are designed to intercept and block moisture vapor before it can reach the slab. As the term “underslab” implies, they are always installed below the slab, either below or on top of the capillary break. This positioning is critical, as no concrete top coat can protect slabs from moisture migrating from beneath the concrete.

To be effective, concrete underslab vapor retarders must withstand heavy foot and equipment traffic without being punctured or torn.Underslab vapor retarders are generally specified for slabs on-grade whenever site conditions indicate that moisture vapor migration may be a problem. Typically, these conditions include moderate to heavy annual rainfall, low-lying building sites, or those sites where soil testing reveals sufficient hydrostatic pressure. If underslab vapor retarders are not installed given these conditions, future degradation of both the slab and finish flooring materials becomes a very likely probability. Adhesively applied flooring materials such as tile or VCT are particularly vulnerable, as are water-sensitive materials such as wood parquet flooring.

In order to be effective, an underslab vapor retarder must possess a number of key physical performance characteristics, including low moisture vapor transmission (MVT), high tensile strength, high puncture resistance, and resistance to chemical or environmental attack. These characteristics not only ensure that the underslab vapor retarder provides an effective barrier to moisture vapor migration over time, but that it will also withstand the job site abuse routinely encountered during the setting of formwork, reinforcements and concrete. Underslab materials that are easily punctured or torn cannot provide an effective barrier to moisture vapor migration.

The use of pea gravel as a capillary break beneath the underslab vapor retarder minimizes the potential for damage.Until just a few years ago, there did not exist any industry standard requirement for underslab vapor retarders. Consequently, when a vapor retarder was specified, many contractors often took it upon themselves to select the material they felt was adequate. Often this was a 4 mil or 6 mil polyethylene film, selected primarily because of its low cost. While, superficially, polyethylene seems like a suitable material, in reality it is a poor choice for an underslab vapor retarder. Both the tear and puncture resistance of polyethylene are far too low to withstand the foot and equipment traffic experienced on most job sites. Further, because polyethylene film is not manufactured to meet specific MVT values, it is unknown whether polyethylene film is capable of providing an effective barrier to moisture vapor migration.

In 1996, the American Society for Testing & Materials provided the industry’s first benchmark for the performance of underslab vapor retarders with the adoption of ASTM E-1745, “Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs”. For the first time, an industry standard identified the desirable performance characteristics required for underslab vapor retarders. The specification went a step further by designating three progressive classes of water vapor retarders, from “C” to “A”, with class “A” establishing the most rigorous performance requirements.

To meet the requirements of ASTM E-1745 Class “C”, a concrete underslab vapor retarder must first possess three key physical properties: a MVT of .3 perms or less, tear resistance of 13.6 lbf/in., and puncture resistance of 475 grams. The material is then put through a battery of conditioning exposures that simulate environmental conditions that can negatively affect material performance, including wetting and drying, heat and low temperature conditioning, and soil organism exposure. The MVT of the post-conditioned material is then retested and must again pass the .3 perm standard in order to meet the Class “C” requirements. 

For Class “B” and Class “A” underslab vapor retarders, the MVT standard for both pre- and post-conditioned materials remains .3 perms or less, although the tear and puncture resistance are progressively higher. A Class “B” vapor retarder requires a tear resistance of 30 lbf/in. and a puncture resistance of 1,700 grams; Class “A” requirements are 45 lbf/in. tear resistance and 2,200 grams puncture resistance. Because the MVT requirement for all three classes remains the same, there is no inherent difference in the ability of a Class “C” or Class “A” underslab vapor retarder to provide an effective barrier to moisture vapor migration. Rather, the key distinction between the three classes of materials is their differing ability to withstand the abuse of installation without being punctured or torn.

Proper installation of an underslab vapor retarder requires that all seems are taped and any penetrations or damage to the membrane repaired prior to placement of the concrete.    It should be noted that, while in many instances consensus standards are drafted to include materials that are already manufactured and in use, at the time ASTM E-1745 was adopted there did not exist any concrete underslab vapor retarder that met the standard’s requirements. Fortifiber Building Products Systems was the first manufacturer to meet Class “C” requirements with the introduction of their Moistop Plus concrete underslab vapor retarder in 1997, and for nearly a year following its introduction, the product was the only one available that met the standard.It should be noted that, while in many instances consensus standards are drafted to include materials that are already manufactured and in use, at the time ASTM E-1745 was adopted there did not exist any concrete underslab vapor retarder that met the standard’s requirements. Fortifiber Building Products Systems was the first manufacturer to meet Class “C” requirements with the introduction of their Moistop Plus concrete underslab vapor retarder in 1997, and for nearly a year following its introduction, the product was the only one available that met the standard. 

Today, a number of manufacturers offer Class “C” and “B” underslab vapor retarders, while a handful also offer Class “A” vapor retarders, although these are rarely specified. There are also concrete underslab vapor retarders manufactured and in widespread use that meet the MVT and tensile strength requirement of ASTM E-1745, but fall short of the full requirements.

When specifying an underslab vapor retarder, consideration must be given to a number of factors, such as the use, location and type of capillary fill, exposure to foot and equipment traffic, and cost. In general terms, the greater the potential for damage to the underslab vapor retarder, a more durable underslab vapor retarder is desired. Understandably, the cost of underslab vapor retarders increases as one progresses from those that meet only portions of the ASTM E-1745 standard to those that meet the requirements for classes “C”, “B” and “A” (see table).

An underslab vapor retarder that meets the MVT and tear resistance of ASTM E-1754, but does not meet all of the Class “C” requirements, is still an appropriate material for those job sites conditions least likely to cause damage to the underslab vapor retarder. Examples include concrete slabs that do not employ a capillary break, or those that utilize pea gravel underneath the vapor retarder.

For installations where a capillary break is installed on top of the underslab vapor retarder, resulting in considerable foot and equipment traffic over the material, a more substantial product is often warranted, such as a Class “C” or “B” material. The type of capillary fill material being used is also an important consideration. Rip-rock or crushed stone are more likely to cause damage to the vapor retarder than pea gravel or river rock.About the author: Ron Beers is Concrete Products Manager for Fortifiber Building Products Systems. He can be contacted by phone at (800) 773-4777 or by email at rbeers@fortifiber.com

Regardless of the material selected, proper installation of the underslab vapor retarder is critical for optimum performance. All seams must be properly lapped and sealed with an appropriate tape meeting the MVT of ASTM E-1745. Prior to placement of fill or concrete over the underslab vapor retarder, the material should be inspected for any tears, punctures or penetrations, and any defects should be repaired with tape. Complete installation procedures are outlined in ASTM E-1643, “Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs”.

Cost Comparison of Concrete Underslab Vapor Retarders*

Product 

Standard Specification Sq.Ft. Cost
Moistop   ASTM E-1745 MVT/Tensile Strength $.065 - $.07
Moistop Plus ASTM E-1745 Class "C" $.153 - $.167
Moistop Ultra ASTM E-1745 Class "B" $.20 - $.213

About the author: Ron Beers is Concrete Products Manager for Fortifiber Building Products Systems. He can be contacted by phone at (800) 773-4777 or by email at rbeers@fortifiber.com


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