Not All Mil Thicknesses Are Created Equal: Understanding Underslab Vapor Barriers

In the construction industry, underslab vapor barriers play a pivotal role in protecting buildings from moisture intrusion, soil gases, and other external elements that can compromise the integrity of the structure. With a plethora of product choices, it can feel daunting to know which product will best fit your project. The mil thickness of a vapor barrier is often considered one of the primary indicators of its performance and durability. However, not all mil thicknesses are created equal, especially when comparing different product’s engineering, such as if polar or non-polar molecules were used, like EVOH (Ethylene Vinyl Alcohol) resin or Polymer-type of resins. This article aims to demystify the nuances associated with mil thickness and provides insights into how the material composition affects the effectiveness of underslab vapor barriers.

Mil Thickness: Beyond the Numbers

The term “mil” refers to one thousandth of an inch, a unit used to measure the thickness of various construction materials, including vapor barriers. While traditionally, the construction industry has leaned towards the thickness of a vapor barrier as a measure of its effectiveness and durability, the narrative is evolving. It is not merely about halting the intrusion of moisture but extending the barrier’s efficacy to counter soil gases and chemicals, especially in brownfield sites.

Polar Vs. Non-Polar

• POLAR MOLECULES: EVOH resin is made up of polar molecules and thus a strong barrier to non-polar contaminants, such as VOC’s (volatile organic compounds). EVOH vapor barriers are renowned for their exceptional resistance to gas and moisture permeation, owing to the molecular structure of EVOH. A 20-mil EVOH vapor barrier offers advanced protection against soil gases, including radon, and provides a robust defense against moisture intrusion. The complex composition of EVOH is what requires a 20-mil thickness, NOT that a 20-mil thickness offers the protection of an EVOH composition.
• NON-POLAR MOLECULES: Polymer resins are made of non-polar molecules that make great barriers to polar molecules, such as water. polyethylene, polyolefin, polypropylene, PVC, and many other make up the polymer family. Conversely, polymer vapor barriers, even with a comparable mil thickness, may not offer the same level of protection. Polymers, although effective as a moisture barrier, is not as resilient against soil gases. A 20-mil polymer barrier might effectively mitigate moisture intrusion but fall short in providing comprehensive protection against various soil gases.

Unifying Material and Thickness

The distinct performance attributes between non-polar molecule polymers and polar molecule EVOH underline the imperative to assess both material composition and mil thickness synergistically. For architects, builders, and stakeholders, this comprehension is elemental to aligning the choice of vapor barriers with the structural and environmental requisites of the project.

Diving into ASTM E1745 Standards

The ASTM E1745 standard serves as a benchmark in the construction industry, delineating the specifications for water vapor retarders in contact with soil or granular fill under concrete slabs. Commercial construction mandates Class A ratings within this standard, emphasizing both strength and permeance. Importantly, mil thickness isn’t the main criterion.

Multiple underslab vapor barrier products might claim alignment with this classification. A simple verification test is the falling dart test. If a free-falling dart punctures the product, it’s not Class A-compliant. Scrutinizing the product’s data sheet can further reveal whether it surpasses or merely meets Class A requirements.

However, a thicker product is not necessarily better. The mil thickness might enhance strength, but if it lacks a polar molecule-type resin, it won’t fend off gases and VOCs.

Choosing the Right Product

So, with all this information, how do you know which mil thickness you require? Several factors influence the choice of mil thickness:

1. COST: Naturally, thicker barriers, especially those engineered with EVOH, tend to be costlier. Considering your budget is part of the equation. However, also consider the extra cost of repairs if you end up with a product that doesn’t fit your project’s needs and environment.
2. SUBSTRATE CONDITION: The state of the substrate, especially if it contains sharp objects like rocks, can determine the required thickness for puncture resistance. The more uneven, rocky, or unknown the substrate is, the more likely you will need to go with a higher thickness.
3. PROTECTION NEEDS: Last but certainly not least, is considering if the project is on a brownfield site or has any known issues with gasses or VOC contaminates. If so, then a EVOH resin vapor barrier will be required. If not, then a polymer resin will be more than sufficient.

The Pitfall of Inadequate Barriers in Brownfield Sites

A 20-mil polymer resin engineered vapor barrier might appear robust, but its non-polar nature is its Achilles’ heel in brownfield sites riddled with non-polar contaminants like VOCs. Without the defense of a polar molecule structure, as seen in EVOH, the barrier is rendered ineffective, leading to potential VOC infiltration, indoor air quality deterioration, and an array of health and structural hazards and challenges.  The ramifications extend beyond immediate physical effects, spiraling into possible legal disputes, increased insurance costs, and a multitude of other unforeseen complications, underscoring the critical importance of a barrier’s molecular integrity in these environments.

Evaluating Performance Metrics

When delineating the efficiency of underslab vapor barriers, criteria such as permeability and chemical resistance become paramount. EVOH, characterized by low permeability and heightened chemical resistance, emerges as a preferred option in protecting against rigorous gas and vapor containments.

The choice of an underslab vapor barrier isn’t a linear decision tethered to mil thickness. It’s a multidimensional evaluation encompassing material engineering, environmental challenges, and project-specific requisites. Both non-polar molecule polymers and polar molecule engineered underslab vapor barrier products have their distinct utilities and limitations. A well-informed selection is rooted in a holistic appraisal, ensuring the barrier not only meets, but exceeds the anticipations of durability, safety, and performance.

As the construction industry continues to evolve, integrating scientific assessments and practical evaluations will be the cornerstone of erecting structures that are not just built to last but are also epitomes of safety and ecological compatibility.

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