Air Barriers 101: Controlling the Movement of Air in Exterior Walls
Controlling the movement of air between indoor and outdoor environments is a key concern when building comfortable, energy efficient homes. According to the U.S. Department of Energy, up to 40 percent of the energy used to heat and cool a building is due to uncontrolled air leakage. Therefore, it is important for builders to have a thorough understanding of air barrier materials and installation strategies.
Quite simply, an air barrier is a material or system of materials designed to control air flow between a conditioned space and an unconditioned space. A building material can be defined as an air barrier if its air permeance is equal to or less than 0.02 L/(s-m2) @ 75 Pa when tested in accordance with ASTM E2178. Alternately, an air barrier system can be tested for air leakage, which must be equal to or less than 0.02 L/(s-m2) @ 75 Pa when tested in accordance with ASTM E2178. Air barriers can also be defined through whole building testing.
In 2012, the International Energy Conservation Code (IECC) began requiring continuous air barriers, and a growing list of states now include them in their building codes. Though there are many ways to prevent air movement through a building assembly, not all air barriers are equal. It is important to understand the forces working against an air barrier and the properties that will help it stand up to them. After all, an air barrier’s performance is defined by its weakest link, and it only takes one tear or unsealed connection to compromise the integrity of the entire system.
As the primary boundary between indoor and outdoor air, the air barrier assembly will be subjected to constant air movement, water, ultraviolet light, and surfactant chemicals present in certain cladding materials and cleaning agents. Any one of these elements could potentially cause materials to break down over time, jeopardizing their effectiveness. Likewise, if a material isn’t durable enough to stand up to installation conditions, it won’t be able to do its job.
Though builders have traditionally used wood pulp based building paper when constructing weather resistant barriers, newer plastic housewraps offer greater benefits in durability and ease of installation. The TYPAR Weather Protection System, for example, is surfactant resistant, offers six months ultraviolet light exposure resistance, and has a tear strength almost five times greater than competitive wraps.
Equally important as selecting the right material, proper installation is critical in order for an air barrier system to work as intended. Sealing all laps and penetrations with the proper tape can improve the building wrap’s performance by 20 percent. Even if the primary air barrier material meets ASTM requirements, continuity of the air barrier system can be compromised by incompatible tapes and flashing or improper installation.
The simplest way to be sure the entire system will work together effectively and meet all code requirements is to specify wrap, tapes and flashing from a single manufacturer. Doing so provides added assurance that each component will work together seamlessly, and often the system will be covered by the manufacturer’s warranty.
Understanding how air barrier materials and systems are evaluated and knowing their performance characteristics will help you find the right solution for your project needs. Approaching air barriers from a holistic view and evaluating the entire air barrier system—rather than just an individual material—will result in a tighter, more durable enclosure that can stand the test of time.
