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Air Barriers: Air Barrier and Flashing Compatibility Verification

Words: Jeremy DouglasQ. It seems every project I work on now requires me to provide confirmation that flashing materials are compatible with the air/vapor barriers. This has never been an issue before, so what’s the problem?

A. You are absolutely right. I assume that any project today with an air barrier and flashing interface is going to require verification — from one or both manufacturers of those products — that the two can be used together. This has become an issue because the majority of the air/vapor barrier products and systems manufactured today are polymer based, and trying to utilize those in conjunction with old flashing technology is not viable. Let’s look at why.

When we started installing these air/vapor barrier systems with any regularity, nearly 20 years ago, the products comparable to today’s fluid applied systems were primarily asphaltic in composition. Likewise, the most popular flashing materials available at the time were asphaltic as well. There was little or no concern in using these products together because of their similar composition. As time went on, the rubberized asphalt self-adhering flashings and their corresponding mastics and primers also became popular as air and vapor barrier systems.

Over time, in both fluid and sheet applied systems, those asphaltic components have given way to high-performance polymers to allow for more dynamic properties, including weather and UV resistance, elongation, permeability and the ability to bridge cracking, just to name a few. These polymer-based systems also allow for the use of better sealants than the mastics previously used for lapping and patching. The use of silicone, polyurethane or polyether sealants has become quite prevalent in today’s masonry wall systems, as they offer much better performance and longevity than an asphalt-based mastic.

That is also where the flashing compatibility issues begin, as those types of sealants will not stick to any asphaltic components, which then becomes problematic when one needs to seal the flashing and air barrier together. Conversely, many asphaltic composite metal and rubberized asphalt flashings have mastics, sealants and primers that contain petroleum components that will be damaging to the polymer-based fluid or sheet applied systems. This is why more and more of the non-asphaltic flashings have become available over the years.

So, because of the incompatibility which arose as the air and vapor barrier systems evolved, the necessity for verification became more prevalent as well. It’s not uncommon to find system incompatibility even between components from the same manufacturing firm — for example, a polymer-based, fluid applied system and an asphalt peel-and-stick flashing membrane that also utilizes a solvent-based primer applied to the cured air barrier prior to flashing installation.

Because this consternation will certainly continue, I highly recommend getting in front of it and knowing whether or not your interfacing manufacturers have an understanding of compatibility on all levels. Be sure the wall will operate as a system, rather than several products simply installed in sequence.
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