Masonry Magazine August 2016 Page. 24
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Important considerations for material selection and code compliance
By Tom Gangl
MASONRY-CLAD BUILDINGS LEAK. They leak air, vapor, and water. These leaks place an environmental load on the building envelope, structural components, mechanical systems, and even building occupants. If left unchecked, these loads can result in building component and system failures. To prevent such failures, barrier systems are used to stop or at least control the leaks. While it is logical to use the correct barrier system for the job at hand, sometimes we don't.
Air barriers first appeared in cold climates, where vapor barriers were employed in wall systems. By combining the air and vapor control, the air-vapor barrier (AVB) was born. Ofcourse, a vapor barrier is really a Class-I vapor retarder, considering that even the quintessential "vapor barrier" �6-mil poly-has some degree, albeit low, ofmoisture vapor permeance.
As their benefits became widely recognized, air barri�ers moved beyond cold climates and into more temperate zones. Along with this migration came the adaptation of vapor-permeable air barriers. Interestingly, the term AVB stuck. Some people still refer to the air barrier -even the vapor-permeable variety -as an AVB. This habit under�scores the trap that designers and constructors can fall into, thinking that all barriers are the same. Let me be clear: not all barriers are created equal. To avoid the trap, it may be best to consider the functions ofeach barrier, exclusive from one another, before combining barrier systems to gain de�sign and construction efficiencies on the wall.
While the different physical forms that barriers take -mechanically fastened sheets (wraps), sheet-applied membranes (SAs), fluid-applied membranes (FAs), board�stock (BS), and spray polyurethane foam (SPF)-maybe obvious, their capabilities are often less clear. The inaccu�rate reference to vapor-permeable air barriers as AVBs not�withstanding, the ambiguity around barriers is particularly evident when it comes to air and water-resistive barriers (WRBs ). Air barriers are not the same as WRBs.
Consider this for a moment: air barriers are addressed in the International Energy Conservation Code (IECC), while vapor barriers and water-resistive barriers are addressed independent of one another in the International Building Code (IBC). These two different codes address functionally different barriers. Makes sense, doesn't it?
To state the obvious: air barriers stop -or, more pre�cisely, retard -air. To achieve an acceptable level of whole-building air tightness, air barrier materials defined in the 2012 IECC sec. c402.4.1.2.1 as having an air permeance ofs0.004 cfm/ft2 @ 0.3-inch w.g. when tested per ASTM E2178 are integrated using accessory materials to create as�semblies. These assemblies, per the IECC, shall have average air leakage not to exceed 0.04 cfrn/ft2 @ 0.3-inch w.g. when tested per ASTM E 2357, ASTM E 1677, or ASTM E 283.
Another requirement of air barriers is continuity throughout the thermal envelope. Joining these assemblies together in a continuous, air-tight manner is essential and means that contractors don't install an air barrier system. Rather, they create one using air barrier materials. The air barrier system, when properly constructed in the field, re�sists air flow in both directions and works to keep condi�tioned air inside the environmental separator and prevent outside, unconditioned air from leaking past it. The benefits of a properly designed and constructed air barrier system include greater occupant comfort, improved indoor air quality (IAQ), reduced HVAC cycling, and efficient (low�er-cost) equipment performance, not to mention code compliance. In addition, because air is a highly effective transport mechanism for water vapor (but not the only mechanism), a second benefit can be realized: keeping air�borne water vapor out of wall assemblies. This is a good
RIGHT:
The secondary benefit of air barriers is actually the primary role of the water-resistive barrier (WRB): keeping wall assemblies dry.
22 I MASONRY � August 2016 � www.masoncontractors.org The Voice of the Masonry Industry