Air control layers are three-dimensional systems of materials designed, constructed, and/or acting to control air flow across a building enclosure, or between a conditioned space and an unconditioned space. In multi-unit/townhouse/apartment construction an air control layer system should also separate the conditioned air from any given unit and adjacent units. The pressure boundary of the enclosure should, by definition, be coincident with the plane of a functional air control layer system. In multi-unit/townhouse/apartment construction the air control layer system may also be the fire barrier and smoke barrier between units. In such assemblies the air control layer system must also meet the specific fire resistance rating requirement for the given separation.

Air control layer systems are assembled from materials incorporated in assemblies (or components such as windows) that are interconnected to create enclosures. Each of these three elements has measurable resistance to air flow. The maximum air permeances for the three components are listed as follows: 

•    Material        0.02 l/(s-m2)@75 Pa (0.004 cfm/ft2  @ 0.3” WC)
•    Assembly    0.20 l/(s-m2)@75 Pa (0.04 cfm/ft2  @ 0.3” WC)
•    Enclosure    2.00 l/(s-m2)@75 Pa (0.4 cfm/ft2  @ 0.3” WC)

Materials and assemblies that meet these performance requirements are said to be air control layer materials and air control layer assemblies. Air control layer materials incorporated in air control layer assemblies that in turn are interconnected to create enclosures are called air control layer systems.