An air barrier is a system or material that restricts the movement of air through a building's envelope, which consists of its walls, roof, and foundation. The primary function of an air barrier is to control air leakage, both into and out of a building, thereby maintaining a comfortable indoor environment and minimizing energy loss.

Air barriers can be made from various materials and installed in different ways, but they should always be continuous, durable, and able to withstand external pressures. A well-designed and properly installed air barrier system will reduce drafts, prevent moisture issues, and improve the overall performance of a building's insulation.