28 March

What is an Air Barrier – Vapor Barrier – Water Resistive Barrier?

What’s an air barrier?

An air barrier material resists air leakage and is designed to form a continuous plane around a building to prevent uncontrolled air movement in and out of the building envelope.

What’s a vapor barrier?

Vapor barriers limit the amount of water vapor diffusing through the wall as a result of different vapor pressures.  A vapor barrier does not have to be continuous, does not have to be sealed, does not have to be free of holes and does not have to be lapped. 

What’s a water resistive barrier?

The function of a water resistive barrier is to keep liquid water from entering the building enclosure. Combined with flashing and other materials, the water resistive barrier ensures that there is a shingled assembly to direct liquid water to the exterior.

Confusion between Vapor barrier and Air barrier

The function of a vapor barrier is to retard the migration of water vapor, which is not typically intended to retard the migration of air. This is the function of air barriers.  A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as an air barrier material restricts the flow of air through a material.  Remember that air leakage – and not vapor diffusion – is the real concern because air leakage accounts for over 200 times the amount of moisture transmitted by diffusion.

A single material can function as all three, an air, vapor and water resistive barrier.  There are also materials that function only as a water resistive barrier that are not air barriers.  Understanding the functions of the materials used is critical, so that the building performs in accordance to the Project requirements.

Contact the air and vapor barrier experts at Carlisle Coatings & Waterproofing for project specific questions, details and applications guidance. They may be reached at 888-229-2199 or [email protected] . 

For straight-forward information about how new wall assemblies containing plastic foam insulation and combustible weather barriers can meet tough new NFPA 285 standards, please visit fireresistccw.com.

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