Continuous Air Barrier in Exterior Walls

The air barrier is a layer within a wall assembly that is impermeable to air, made of any durable material that blocks airflow. This can be OSB, drywall, rigid foam, or many other materials. The air barrier layer or layers prevent the unwanted entry of outside air and escape of inside air. Code requires, and best practice dictates, that the home’s thermal layer of insulation be fully aligned with (in full continuous contact with) the home's continuous air barrier (see 2009, 2012, and 2015 IRC). While ENERGY STAR recommends rigid air barriers, flexible air barriers such as house wrap are acceptable if they are fully sealed at all seams and edges and supported using approved fasteners. For the air barrier to be continuous, any seams between sheets of material, or joints between one material and another, or holes must be sealed with a long-lasting air-sealing material. The air barrier may be installed on the interior side of the insulation, the exterior side of the insulation, or both, depending on the building component and the climate.  Regarding floors, the air barrier should be aligned with the exterior vertical surface of the insulation (at the rim joists) in all climate zones and if the floor is over unconditioned space, the subfloor must be aligned with the interior horizontal surface of the floor insulation (i.e., the insulation must be touching the subfloor above it, for example by installing batt insulation with metal staves or twine that will keep the batts up against the floor above).

Key Points:

  • Identify on house plans what materials will constitute the air barrier in all components of the home’s thermal envelope 

  • Install the continuous air barrier which could consist of one or a combination of any of the following air barrier materials: 

    • rigid materials like foam board insulation, drywall, plywood, or OSB

    • flexible materials like house wrap, with all seams and edges sealed and with the house wrap supported using approved fasteners (don't use kraft paper or other materials that tear easily) 

    • fluid-applied membranes like liquid membranes, which are applied with a paint brush, roller, or sprayer over the sheathing

    • spray foam – if used as the air barrier it should be at least 5.5 inches thick if open-cell or at least 1.5 inches thick if closed-cell spray foam insulation.  

  • Seal all seams, gaps, and holes in the air barrier.

ENERGY STAR requires that an air barrier be installed at the exterior vertical surface of the wall insulation in all climate zones and also that an air barrier be installed at the interior vertical surface in Climate Zones 4-8. For ceilings, ENERGY STAR permits the air barrier to be at the interior or exterior horizontal surface in IECC Climate Zones 1-3 and at the interior horizontal surface in Climate Zones 4-8 (ENERGY STAR 2015).