Vapor Barrier, Vapor Retarder, and Air Barrier: What Is the Difference?

May 14

“Vapor barrier” and “air barrier” are often used as if they mean the same thing. They do different jobs. Confusing them can lead to a wall that traps moisture or leaks air even when every material was installed.

An air barrier controls moving air

An air barrier limits air leakage through the building enclosure. Air can carry a large amount of moisture through cracks around windows, outlets, rim joists and roof connections. The air-barrier system must be continuous and strong enough to resist pressure.

A taped exterior sheathing, sealed housewrap, spray foam or detailed interior gypsum layer may serve as the air barrier. The specific product matters less than the continuity of the complete system.

A vapor retarder slows diffusion

Water vapor can also move through solid materials. A vapor retarder slows that movement. Materials are grouped by how permeable they are. Polyethylene and foil are very restrictive. Kraft facing, special membranes and painted gypsum can be less restrictive and may allow more drying.

The phrase “vapor barrier” usually refers to a very low-permeance material. That is not automatically the best choice. A wall must control condensation and still be able to dry after construction moisture or a small leak.

One material can perform both jobs

Some membranes, foam products and sealed sheathing systems can control both air and vapor. Others perform only one function. Product labels should be checked for both air-leakage and vapor-permeance information.

Which side should it go on?

There is no safe universal answer. Climate, indoor humidity, insulation location, cladding and existing materials all matter. In a mixed-humid climate such as Long Island, a highly restrictive layer in the wrong place can reduce drying during part of the year.

A renovation also has to account for what remains in the wall. Adding impermeable foam outside and impermeable finishes inside can change an assembly that previously dried in both directions.

Before insulating or replacing siding, have the complete wall reviewed.