Looking beyond codes
At a minimum, the barrier should conform to established industry standards. The Air Barrier Association of America (ABAA) has established
a maximum air leakage rate for wall air barrier assemblies at 0.2 L/s/m2 (0.04 cfm/sf) at a pressure difference of 7.7 kg/m2 (1.57 psf) or 0.075 kPa (75 Pa), when tested in accordance with ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies.2 Another testing standard in use is ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization.3 While ASTM E2357 is designed to be performed on a wall specimen in the lab, ASTM E779 is designed to measure air leakage of a built structure in the field, testing the envelope across a range of both positive and negative pressure differentials.
While measuring air leakage is important, it is not the whole story; the adhesive bonding of the air barrier to the wall substrate should also be assessed since it affects the long-term performance of a structure.
Adhesion is critical
The continuity of an air barrier and its ability to resist differences in air pressure are key physical properties to determine its performance. Strong adhesion of the membrane is needed to ensure a continuous barrier and one which resists pressure differentials.
To achieve this, the membrane must:
- Seal to the substrate, creating an aggressive bond across the entire surface.
- Seal to itself, preventing weak laps which allow air intrusion.
- Seal around penetrations, including fasteners, eliminating potential failure points.
Poor adhesion at any point can allow air to migrate behind the membrane and into or out of the structure. Further, the bond must be strong enough to withstand the test of time. Improper installation or a weak adhesive can result in a barrier which appears functional but may separate from the substrate over time, compromising the performance. Worse yet, with siding and other external wall components covering the barrier, building owners may not even be aware of the failure.
Given this risk, understanding the factors impacting air barrier adhesion are essential to make the right choice.
Testing adhesive performance
While manufacturers often have published specifications for adhesion strength, it is important to validate these claims independently. Fortunately, there are standardized test methods for measuring the pull-off strength of an adhered air barrier, including both sheet-applied and fluid-applied membranes.
ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers is designed to measure pull-off strength between an air barrier membrane and a substrate. It is used to assess in-field performance.4 The ABAA has a similar test method, ABAA T002-2019, Standard Test Method for Pull-Off Strength of Adhered Air and Water Resistive Barriers Using an Adhesion Tester.5 With both methods, a disc is adhered to the surface of the air barrier and allowed to cure. A test apparatus is attached to the disc and a pulling force is applied and gradually increased until failure. The pressure at failure is recorded along with the failed layer: the membrane (cohesive failure), the membrane bond (adhesive failure), or the substrate (substrate failure).
