Continuity
To ensure continuity, each component (a wall, window assembly, foundation, or roof) must be interconnected to prevent air leakage at the joints between materials, components, assemblies, systems, and penetrations through them, such as conduits and pipes.
Structural support
Every component of a building’s air barrier system must withstand the stresses and loads placed on it by the forces of wind, the building’s size and shape, or operating systems, and designed to transfer these loads to the structure itself. The design of the system must also account for the fact the balance of air moving in and out of the structure will not compromise structural integrity.
Air impermeability
Materials that are too air-permeable (i.e. they allow an excess of air molecules to pass through a membrane under pressure) must be avoided. The air permeance is measured using ASTM E2178, Standard Test Method for Air Permeance of Building Materials, which requires air leakage be less than 0.02 L/s∙m² @ 75 Pa (0.004 cfm/sf @ 1.57 lb/sf). (More information on the permeance of building materials can be found in “Air Permeance of Building Materials.”) Other industry groups may require tighter standards, such as the U.S. Army Corps of Engineers (USACE), the Naval Facilities Command (NAVFAC), or the Air Barrier Association of America (ABAA). Section 5.4.3.1.3, Acceptable Materials and Assemblies of ASHRAE 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, has the same requirement as ASTM E2178.
Durability
Materials chosen for the air barrier system must be long-lasting and fulfill their purpose for the expected life of the structure. Alternatively, the product must be accessible for periodic maintenance.
Material selection and specifications
The correct layer of material for the wall assembly must be chosen to form the basis of the air barrier system. Popular choices include interior gypsum or foam, sprayed polyurethane foam (SPF) insulation, concrete, oriented strand board (OSB), or plywood deck. The accessory materials must ensure the durable continuity of the air barrier.
One must also know all the specifications detailing methods for ensuring the air barrier continuity, especially at:
- penetrations, corners, and edges (e.g. rough openings for windows, doors, pipes, shafts, and conduits);
- at transitions between one material and another (e.g. wall-ceiling intersections and wall-floor intersections); and
- where the air barrier must pass around structural elements (e.g. heavy vertical steel posts or horizontal beams).
The ‘Pen Test’ is an easy way to gauge the effectiveness of the planned moisture-control elements. This is a proven way to verify the continuity of the insulation, air barrier, and WRB. It consists of color-coding the ‘barrier’ on a detail from the roof to the basement around the entire structure. When following the line, the pen must not come off the paper; the line (and therefore the air barrier) must be continuous. This visual measure makes it easy for all the trades involved in the construction and maintenance of the building to see where the barrier is and how to handle the detailing. It is the fastest and easiest way for designers and applicators to make sure they’re going to achieve a tight envelope on a given project. (Related drawings are online at buildingscienceeducation.net/wp-content/uploads/2014/09/Pen-Test-Moisture-Control-Guidance-for-Building-Design-Construction-and-Maintenance.pdf.)
For additional guidance, ABAA offers excellent support for the design of durable walls by providing open access to a library of master specifications for the various types of air barriers. ABAA defines a successful air barrier as one that:
- reduces required HVAC system size while improving efficiency;
- manages air pressure differentials properly;
- enables water vapor to escape from wall cavities;
- enhances IAQ; and
- improves occupant comfort through reduced drafts.
Conclusion
Air barriers are increasingly being combined by manufacturers into integrated air and moisture barrier systems, which address the demands of today’s more stringent, energy-efficient building codes.
In addition to enhancing IAQ, the comfort of building occupants, and energy savings, there are real economic benefits to controlling air leakage throughout the building envelope. In fact, research has shown air barriers can play a larger role in enhancing energy efficiency than increasing the thickness of standard insulation. High-performance air barriers can not only save on operational energy costs and often offset some construction or retrofit costs, but also open the door to reductions in the size of heating and cooling units.
It is important to remember controlling moisture means controlling airflow—one should never underestimate the value of an effective air barrier system.
Karine Galla is product manager for Sto Corp. She has more than 16 years of experience in product marketing regarding EIFS, stucco, air and moisture barriers, and other materials. Galla has a master’s degree from the University of Lyon, France. She holds the Association of the Wall and Ceiling Industry’s (AWCI’s) EIFS Doing it Right and Building Envelope Doing it Right certifications, as well as the ISO Internal Lead Auditor certification from Georgia Tech. She can be reached at KGalla@StoCorp.com.
