The control of air flow is important for several reasons: to control moisture damage, reduce energy losses, and to ensure occupant comfort and health. Airflow across the building enclosure is driven by wind pressures, stack effect, and mechanical air handling equipment like fans and furnaces. A continuous, strong, stiff, durable and air impermeable air barrier system is required between the exterior and conditions space to control airflow driven by these forces. Air barrier systems should be clearly shown and labelled on all drawings, with continuity demonstrated at all penetrations, transitions, and intersections. In addition, enclosure assemblies and buildings should be vertically and horizontally compartmentalized, may require secondary planes of airtightness (such as those provided by housewraps and sealed rigid sheathing) and may need appropriately air impermeable insulations or insulated sheathing.