DESIGN/AIR BARRIERS
3.1 AIR BARRIERS
The purpose of an air barrier is too prevent airflow through the building envelope. This includes
both the prevention of outdoor air from entering the building through walls, roofs and foundations,
and the prevention of indoor air from exfiltrating through the building envelope to the outside. The
inclusion of an air barrier system in the envelope design is essential to controlling air leakage and
achieving good thermal envelope performance. Air leakage leads to excessive energy
consumption, poor thermal comfort and indoor air quality, condensation within the envelope and
the associated degradation of envelope materials, and interference with the proper operation of
mechanical ventilation and smoke control equipment.
Even if an air barrier is not specified in the envelope, those elements which are most impermeable
to airflow will be subjected to the envelope pressure differences. They will then "act" as the air
barrier, most likely a poor one. The material experiencing the pressure difference, and its means of
attachment, will probably not be adequate to withstand the pressure and it will be displaced. For
example, rigid insulation board may be forced out of position by wind pressures when there is no air
barrier system in the wall and the insulation attachment is not designed to withstand the wind
pressures.
The air barrier system must be designed with full recognition that envelope materials are not
dimensionally stable and that differential movements occur due to temperature effects and
structural loads. The elements of the air barrier at locations where differential movement is
expected to occur must be capable of accommodating this movement using systems and materials
that will retain the essential performance requirements of the overall air barrier system.
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