The 3 Critical Problems with Generic Poly Plastic Below a Concrete Slab   

One of the most important characteristics of an effective under-slab vapor barrier is durability. If a material deteriorates, it may fail to protect the building envelope. This could happen moments after it is installed (if it is punctured or torn before concrete placement) or beneath the placed slab over time (because the material lacked the necessary performance characteristics to withstand the rugged below-slab environment).

In this article, I’ll review the three main problems with generic poly plastic and why it is not an ideal component for the construction of buildings and homes. You’ll see exactly why selecting high-performance vapor barriers matters to the entire spectrum of building project team members: owners, designers, contractors, and homebuilders.

• Problem #1: Cheap, Recycled Resins Make for Weak, Ineffective Plastic Sheeting
• Problem #2: Lack of Antioxidants
• Problem #3: Lacking Multi-Layer Co-Extrusion

Generic, low-quality polyethylene (often referred to as visqueen, although “Visqueen” is in fact a trade name of British Polythene Limited and sold as a quality brand in the UK) is perhaps the best example of a material now considered to be defective when installed beneath the slab. Visqueen is unlikely to withstand either the rigors of the installation process or avoid degradation over time. Worse, its degradation over time is typically correlated with its poor ability to withstand installation!

Perhaps even more alarming, generic (often 6-mil thick) poly sheeting has been especially common in residential construction to satisfy bare minimum code requirements.

The easiest way to understand why generic poly is defective when installed beneath the slab is to hold it in one hand, while holding in the other a vapor barrier material that meets Class A of the standard specification for plastic water vapor retarders, ASTM E1745. Poly is thin, riddled with inconsistencies (i nodules, variation in thickness), and easy to tear, while a Class A vapor barrier is the opposite.

Since you may not be holding samples of material while reading this article, the next best thing is to gain an understanding of how high-performance plastic sheeting, specifically those designed as under-slab vapor barriers, are engineered.

3 Problems with Generic Poly Plastic

Problem #1: Cheap, Recycled Resins Make for Weak, Ineffective Plastic Sheeting

Prime, Fist-Melt Resins are Critical for Effective Under-Slab Vapor Barriers

The first problem is a complex, material science matter, but even a basic knowledge of polymer chemistry makes it easy to understand. Here we go:

High-performance under-slab vapor barriers primarily utilize polyolefins. “Olefin” refers to class of plastic monomers (e.g. ethylene, propylene) with different atomic structures—they are all hydrocarbons on a basic level— and “poly” denotes that these materials are strung together in large chains. The lengths of these chains and degree of branching along them in large part determine the quality of the resins and many of the physical characteristics of finished goods utilizing them. Chain length and branching tend to correlate well with the density of the bulk plastic; thus, especially when dealing with polyethylene, we tend to classify products by their density (e.g. low-density polyethylene [LDPE], high-density polyethylene [HDPE], etc.).

The “right” chains—with sufficient chain length, branching, sub-branching, etc.—requires a large degree of science and technology, but many of the firms making polymer resins are very skilled at what they do. These resin suppliers can produce an array of materials with varying degrees of conformance to almost any material specification an end-user can dream up (with costs often increasing alongside performance). To achieve the desired properties in an under-slab plastic sheeting as indicated below—prime resins are the key.

• Impact resistance: its ability to withstand punctures
• Tensile strength: its ability to withstand stretching or tearing
• Water vapor permeability: its ability to prevent water vapor diffusion through the membrane

“Prime” means resins meet specific material specifications; thus, using prime resins provides assurance that the right physical properties can be obtained when the pelletized prime resins are compounded, extruded, and blown into plastic sheeting. Aside from prime, resins may fall into other classes, with quality (or at least consistency) suffering as one moves down the hierarchy. Near the bottom is “wide spec” (often called “off-grade” or “off-spec”), the feedstock of most generic plastic sheeting (like those used to make trash bags).

Virgin resins are those that have yet to be made into a finished good. In contrast to virgin, recycled resins lack two key distinctions. First, recycled resins are intended to be cheap; thus, although you can break down a plastic into its base monomers and repolymerize those feedstocks into a high-quality product, this chemical recycling process is in its infancy, and the resulting materials are very expensive. Instead, pre- and post-consumer materials are cleaned, re-ground, perhaps re-pelletized or compounded with other additives, and sent back through the extrusion process to be blown into plastic sheeting. Remelting a plastic introduces additional heat to its “history,” breaking apart some of its complex chemical chains and adversely affecting the strength of the resultant film in the process (in contrast, prime resins are used on a “first-melt” basis).

The more a product is recycled, the more the heat history can impact the plastic (see below related to antioxidants for additional details). Similarly, contamination or residual inks that were not removed can lead to an array of problems, from concentrating pockets of non-melting materials (like carbon) to inducing lensing (voids) or cross-linking, the latter of which can be seen as gels (hard nodules) in the material. Each of these issues can lead to stress concentrations and loss of strength in the finished film.

An under-slab vapor retarder with this problem -- inferior content/composition -- is unlikely to stand up to the demands of construction and construction traffic, leading to subpar protection in the installed system.

Problem #2: Lack of Antioxidants

Oxidation and Its Unforgiving Effect on Plastic Sheeting

Processing (and reprocessing) of plastic resins involves a great deal of heat energy. This added energy can break polymer chains, decreasing the length of chains that is a major factor in the strength of the plastic. It can also increase oxidation along a chain, a process by which electrons are stripped from the polymer. This can lead to cross-linking or undesirable branching, both of which can also adversely affect the properties of the finished good.

In this way, a reprocessed or recycled material is often weaker than the original plastic that provided the raw material and may lack the other desired properties. If this recycling contributes to the reuse of a trash bag or a park bench, we can all see the value of reprocessing and reuse. But when the material is meant to protect a built environment for the life of the building, a recycled or reprocessed material is rarely – if ever – up to the task.

Oxidation, as described above, is an unforgiving process that affects many plastics. It is all the more insidious for three reasons.

1. Oxidation will occur anytime a polymer chain will give up an electron to an oxidizing agent. Oxygen is abundant in our atmosphere, and it is a strong oxidizing agent; even at room temperature, oxidation will happen, albeit somewhat slowly. Contaminants in the soil (e.g. heavy metals), against which many vapor barriers are placed, can also act as oxidizing agents.
2. Sunlight, specifically ultraviolet light, can provide the right amount of targeted energy to break polymer chains and strip electrons from the polymer at key places. Sunlight is ubiquitous on jobsites and essentially impossible to avoid on most projects, even if the wrap is only exposed prior to concrete placement for a matter of days. In other words, the ever-present sunlight coming in contact with visqueen as it sits out on the job site waiting to be installed or waiting for the concrete pour, will accelerate product breakdown and negatively affect the plastic’s long-term performance.
3. Oxidation of polyethylene is an autocatalytic process; in other words, the greater degree of oxidation, the more oxidation will occur in the future. A heavily recycled product is therefore at a great disadvantage in terms of oxidation—and future ill effects as a result— compared to a prime, virgin material.

To counteract this, resin compounders and suppliers can add special ingredients that absorb ultraviolet radiation and neutralize free radicals (molecules with unpaired electrons), limiting their ability to react with oxygen and adversely affect the polymer structure. The recycling process can destroy or use up these antioxidants, can increase the number of free radicals as electrons are stripped during energy-intensive processing, and will leave a material more susceptible to deterioration in future generations. Worse, since visqueen in general is often cheap, the addition of helpful antioxidant compounds may be overlooked for economic reasons.

In some ways, there are three variables in the function of visqueen’s defectiveness; it not only 1) fails to withstand installation and; 2) degrades over time, but chances are that; 3) oxidation has already gone to work on the material before it ever rolls out on the job site.

Problem #3: Lacking Multi-Layer Co-Extrusion

Recycled Plastic Is Only As Good As Its Weakest Link

Not all of the properties necessary for an effective vapor barrier (puncture strength, permeance, chemical resistance, etc.) are gained by a single polymer/density combination. The most effective high-performance under-slab vapor barriers are often multi-layered co-extrusions of different densities (or even different polymers or co-polymers) which isolate the function of each layer from its neighboring counterparts.

For example, materials that are best suited to resist degradation to chemicals found in the soil, or that best hold up to the wear and tear of jobsite traffic, can be located at the outer layers, protecting inner layers of a multi-layer, co-extruded plastic sheeting more effectively, allowing them to fulfill whatever specialized role they may take for the plastic sheeting (e.g. low permeability).

A generic poly plastic made of recycled resins is most often intended to be cheap. Thus, there is likely little specialization in the layers, if it is even co-extruded at all. It is very unlikely that one of these commodity plastic sheets would employ layers for a strategic purpose. You are far more likely to find average properties through and across the plastic. The result: individual properties of the whole plastic sheet, compared to a co-extrusion with specialized layers, tend to suffer – the end product is less than the sum of its parts.

A co-extruded product can be greater than the sum of its parts; generic plastic is likely only as good as its weakest link. This can be especially problematic for properties of the plastic sheeting that depend on uniformity throughout, like water vapor permeance or ability to impede soil-gases, like radon. And similar to the concerns with recycled materials and the contamination that can be introduced, if incompatible plastics are used together, the plastic can lack cohesive strength and further suffer on a jobsite under the loads of foot and machine traffic.

The reason visqueen can still be found on job sites is because it’s cheap! How it’s made explains why but is also a big reason for its defectiveness as an under-slab vapor retarder.

 

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The Choice is Clear and Critical

Your building products need to stand the test of time and maintain their integrity throughout the building’s lifetime. Generic poly fails in this regard, which can lead to a cascade of other problems over the life of the building or home – in the concrete foundation, the indoor air quality, and in the failure of other building components, like flooring.

You only have one chance to place proper vapor protection below your building or home, so why take a chance with building products that are not engineered for long-term protection?

Editor's note: This blog post was initially published in November 2020 and has been revised to stay up-to-date. 

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