Why the Distinction Matters in Building Science
Air barriers and vapor barriers are both vital for protecting a building’s envelope, but they are not interchangeable. Confusing the two can lead to poor performance, moisture problems, and expensive repairs. To design or retrofit an energy-efficient, durable structure, it’s essential to know how each works—and where they belong.
What Is an Air Barrier?
An air barrier controls the movement of air into and out of the building envelope. Air leakage can transport moisture, reduce insulation effectiveness, and cause drafts that lead to discomfort and higher energy bills. Air barriers are continuous systems that seal all the joints, seams, and penetrations in a building’s structure.
Common air barrier materials include:
- Self-adhered membranes
- Fluid-applied coatings
- Spray foam insulation
- Exterior sheathing with sealed joints
Proper air barriers improve indoor air quality, reduce HVAC loads, and help buildings pass energy code compliance tests like the blower door test.
What Is a Vapor Barrier?
A vapor barrier (or vapor retarder) limits the movement of water vapor through a building assembly. Unlike air, water vapor moves via diffusion and can pass through many materials—even if air cannot. If this vapor hits a cold surface inside a wall or ceiling, it can condense into liquid water, leading to mold, decay, and reduced insulation performance.
Vapor barrier materials typically include:
- Polyethylene plastic sheeting
- Foil-faced insulation
- Kraft-faced batts
- Vapor-retardant paints
Their permeability is measured in perms. Materials with a perm rating of 1.0 or less are considered vapor retarders.
Core Difference: Air Movement vs. Vapor Diffusion
The main distinction is how each barrier controls moisture. Air barriers stop bulk airflow, which can carry large amounts of moisture. Vapor barriers slow vapor diffusion, which is a slower process driven by humidity and temperature differences.
Some materials—like closed-cell spray foam—can serve as both an air and vapor barrier, depending on thickness and application. But most require different products and placements.
Why Proper Placement Is Critical
The placement of each barrier depends on climate zone, wall assembly, and indoor humidity levels. In cold climates, vapor barriers usually go on the interior side of insulation to prevent warm indoor moisture from reaching cold wall surfaces. In hot, humid climates, they may belong on the exterior side to stop outdoor vapor from entering air-conditioned spaces.
Air barriers, however, should completely wrap the building envelope without breaks—regardless of climate—because they affect both moisture control and energy loss.
Common Mistakes to Avoid
- Installing a vapor barrier where only an air barrier is needed, potentially trapping moisture inside walls.
- Using non-continuous air barrier materials, which allow leaks at seams and joints.
- Double vapor barriers, which can trap moisture between layers.
- Ignoring climate-specific design, leading to condensation inside wall assemblies.
Misapplying either barrier type can do more harm than good. It’s vital to consult with building scientists or experienced professionals when specifying barrier materials for your project.
Final Thoughts
Air and vapor barriers play distinct yet complementary roles in building performance. Air barriers stop drafts, control energy loss, and prevent moisture-laden air from infiltrating your space. Vapor barriers slow the movement of water vapor to reduce condensation risks. Knowing the difference—and applying them correctly—can help ensure your structure stays dry, energy-efficient, and code-compliant for decades to come.
