August 17, 2022

How to Measure the Mil Thickness of an Air Barrier

Last year, we highlighted the importance of measuring the mil thickness after application of a deck coating. We believe that no matter the size of the project, one of the most important aspects of the job is quality control through application testing. At any level of application, proper testing methods ensure product success, structural endurance, and most importantly, a happy customer. We talked of various tests that can be conducted across all three of our product offerings (sealants, deck coatings, air barriers), some include: proper adhesion testing, moisture content of concrete test, equipment testing, and pull off adhesion strength. For this week’s blog post, we want to return to the discussion of quality control, but this time we explore how to measure the mil thickness of a fluid-applied, air, vapor, and water resistive barrier.

In manufacturing, mil thickness is a common measurement unit to describe the thickness of materials in thousands of an inch (1 mil=0.001 inch). Measuring the mil thickness of your air barrier is important to ensure quality control of the application process. This will allow for adjustment of the coating if it is too heavy or too light ( In order to measure the mil thickness, we must first apply the product.

Pecora offers a range of air barrier systems for various project application specifications. Selecting a particular system may rely on the project’s need for permeable vs. non-permeable protection which help to control the movement of water, vapor, and air through the buildings envelop. Pecora offers three air, vapor, and water resistive barriers: XL-Perm ULTRA VP Fluid Applied, Permeable STPU, XL-Perm ULTRA NP Fluid Applied, Non-Permeable STPU, and ProPerm VP Fluid Applied, Permeable, Water-Based Barrier. What makes our air barriers stand out from the competition is that they are applied in a single, thin mil coat. Below is a list of some reasons why a STPU thin mil air barrier might be a better choice than thick mil barriers:


  1. Solids ContentHigh (~100%) solid content in most thin mils make them significantly better choices than thick mils. Higher solid content essentially means more materials, and less water/ solvent evaporation during curing processes, which translates to reduced shrinkage.
  2. Elastomeric propertiesThin mil systems with STPE/STPU backbones ensure excellent elasticity, making them high performing and durable products. The ability of air barrier membranes to accommodate the expansion and contraction during typical exposure conditions is characterized by elongation at break (ASTM D412). Higher elongation automatically does not mean better elastic properties. Rather, a combination of physical properties, including %elongation and %recovery determine the “elastomeric behavior” of a coating/sealant. Dupont (2017).
  3. Crack BridgingThin mils (STPE/STPU backbone) have excellent low temperature crack bridging capability (in accordance to ASTM C1305), wherein thick mils (rubberized asphalt chemistry) have been associated with some problems including cracking, wrinkling, melting and blistering. Saul et al. (2015).
  4. Compatibility with other building materialsSTPU/ STPE chemistries in thin mil systems allow enhanced compatibility with a diverse selection of wall components. Bleeding in rubberized asphalt membranes (present in many thick mil barriers) has been a concern in the industry as its incompatibility with other materials may result in coatings to peel off the substrate and flow out of the wall system. In such cases, an additional bleed blocker primer or other barrier film or sealant may be needed to solve the problem. When asphaltic materials come in contact with soft PVC or plasticizer in a sealant or coating formula, the asphaltic materials are solubilized and convert into a flowable liquid. It is advised that bituminous air/vapor or waterproofing membrane is substituted with a membrane that does not contain asphalt. Metzler (2008). Most thin mil coatings are compatible and approved to be used with asphaltic self-adhered membranes, adhesives, coatings and sealants.


After ensuring the surface is clean, dry, and frost-free of contaminants such as dirt, dust residue, oil and grease, penetrations and fasteners should be sealed with flashing. We recommend using Pecora XL-Flash. Once cured, apply the specified air barrier to the surface. It is important to remember to follow all manufacturing application guidelines when applying an air barrier or any product. These guidelines will provide the appropriate mil thickness to use when applying the product.

To conduct the test, simply take a wet mil gauge and place it into the wet coating that was just applied. When you lift the mil gauge you will see the teeth of the gauge, which display varying levels of mil thickness, covered in the coating. Depending on what the test garners, this will determine if adjustments need to be made to the coating if applied too thin or too thick. The test is that simple. Pecora offers free mil gauges for customers, please reach out to our Marketing Department to request a few


For more information, or the right air barrier application recommendations for your project, we urge you to contact our Pecora Corporation Technical Services Group. Our expert team is highly qualified to recommend the proper solution and perform testing between Pecora’s products and jobsite samples. Our Technical Services staff offers training in product technology and use, provides the technical assistance you require in the planning and implementation of your project. They can be reached by email or by phone (800)-523-6688.


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