What characteristic defines an air barrier?

An air barrier is primarily defined by its ability to create a continuous, unbroken seal across the building enclosure. This continuity is crucial because it prevents air leakage, which can lead to energy inefficiencies and affect indoor air quality. For an air barrier to effectively reduce air movement between the inside and outside of a building, it must extend fully over all surfaces and interfaces, ensuring that there are no gaps or compromised areas that would allow air to pass through.

While being impermeable to water flow, flexible in structure, and easy to install are beneficial characteristics in certain contexts, they do not embody the fundamental purpose of an air barrier. The effectiveness of an air barrier hinges upon its consistency and lack of discontinuity in the building envelope, making the requirement for it to be continuous the most defining characteristic.