The second part of our series on Building Better Walls focuses on air control and the basic requirements for air barriers systems in walls.
The management of airflow is important for several reasons: It controls moisture damage, reduces energy losses, and ensures occupant comfort and health. Airflow through a building enclosure is driven by wind pressure and the stack effect – movement caused by warmer air rising and colder air falling that create pressure differences. These, in turn, can lead to air leakage, unexpected airflows, and indoor air-quality problems. Mechanical air handling equipment such as fans and furnaces also impact air flow. A continuous, strong, stiff, durable and impermeable air barrier system is required between the exterior and interior conditioned space to control airflow driven by these natural phenomenon. Wall air barriers provide critical protection in all buildings, regardless of region or climate. Controlling air leakage within the building envelope also enhances a structure’s energy efficiency. Basic Requirements of Air Barrier Systems for Walls Typically, several different materials, joints and assemblies are combined to provide an uninterrupted plane of primary airflow control. Regardless of how air control is achieved, the following five requirements should be met to achieve a proper air barrier system (ABS): Continuity. Enclosures are 3-D systems! Continuity must be ensured through doors, windows, penetrations, around corners, at floor lines, soffits, etc. Strength. The ABS must be designed to transfer the full design wind load (e.g., the one-in-30-year gust) to the structural system. Fastenings can be critical, especially for flexible non-adhered membrane systems. Durability. The ABS should continue to perform for its service life. Ease of repair and replacement, the imposed stresses and material resistance to movement, fatigue, ambient temperature, etc. should all be considered. Stiffness. The air barrier must reduce or eliminate deflections to control air movement into the enclosure; it must also be stiff enough that deformations do not change the air permeance (e.g., by stretching holes around fasteners) and/or distribute loads through unanticipated load paths. Impermeability. Typical recommended air permeability values are less than about 1.3 x 10-6 m3/m2/Pa. In practice, the ability to achieve continuous insulation is more important to performance; the air permeance of joints, cracks, and penetrations outweighs the air permeance of the solid materials that make up most of the ABS. Hence, a component should have an air leakage rate of less than Q< 0.2 lps/m2 @75 Pa, and the whole building system should leak less than Q< 2.0 lps/m2 @75 Pa. It is important to note that increased airtightness must be matched by an appropriate ventilation system to dilute pollutants, provide fresh air, and control cold weather humidity levels. Good airflow control through and within the building enclosure will bring many benefits including reduced moisture damage, lower maintenance costs, energy savings, and increased health and comfort.