Air Barrier
A continuous layer that stops uncontrolled air from moving through the envelope.
Quick Answer
An air barrier is a system of materials and joints that forms a continuous layer across a building's envelope to limit uncontrolled air leakage. It reduces energy loss, moisture transport, and drafts. To work, it must be continuous across walls, roofs, and floors, including transitions, penetrations, and joints, and it must resist wind loads.
The Full Picture
Air leaking through a building envelope carries heat and moisture with it. Air leakage can drive a significant share of heating and cooling loss and can deposit water vapor inside walls, where it condenses. An air barrier system is the layer, or set of connected layers, designed to stop that movement.
The air barrier is a system, not a product. Materials such as fluid-applied membranes, self-adhered sheets, sheathing with taped joints, spray foam, and some structural elements can all serve, provided they are sufficiently airtight and are joined to each other. What matters is continuity at every transition: wall to roof, wall to foundation, around windows and doors, and around pipes and ducts.
Placement varies. The air barrier can be on the exterior, within the wall, or on the interior face, and one layer can also serve as the water-resistive barrier or the vapor retarder. Designers should state which layer is intended to do which job, since the functions are related but different.
Performance is checked through mockup testing, material and assembly tests, and whole-building air leakage tests, often by pressurizing the building with fans. Energy codes, including the IECC, require air barrier continuity or testing for many building types, so inspection of the barrier before it is covered is a common quality step.
Real Examples
Common Misconceptions
People assume: An air barrier and a vapor barrier are the same thing.
Actually: An air barrier stops air movement. A vapor retarder slows diffusion of water vapor. A single material can serve both purposes, but they are different functions.
People assume: Housewrap alone gives you an air barrier.
Actually: Only if the material, laps, and attachments are designed and tested to be airtight and connected to adjacent components. Many wraps are mainly water-resistive barriers.
Frequently Asked Questions
What does an air barrier do?
It limits uncontrolled air flow through the envelope, which reduces energy use, drafts, and moisture carried into assemblies by moving air.
Where must an air barrier be continuous?
Across all walls, roofs, and floors of the conditioned enclosure, including joints, transitions between systems, and penetrations such as windows, pipes, and ducts.
How is air barrier performance verified?
Through material testing, mockups, visual inspection before covering, and whole-building fan pressurization tests, depending on code and project requirements.
Can the air barrier also be the water barrier?
Often yes. Some membranes serve as air barrier, water-resistive barrier, and vapor retarder together, but the design should clearly identify each function.