Air Barriers • Vapor Barriers • Weather Barriers

By Roof Online Staff • Updated March 24, 2023

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Polyethylene sheeting installed as a vapor barrier on this steel roof deck.
Dupont’s tyvek is one of the most familiar brands of weather barrier in the world.

Pictures of Vapor Barriers

Construction Definition – Air Barrier:

A material used to prevent the flow of air through a building component, typically preventing outside air from flowing in and inside air from flowing out. Not all air barriers are vapor barriers.

Construction Definition – Vapor Barrier:

A material, such as a waterproof coating or a sheet membrane, installed to prevent or reduce the movement of water vapor into a building component, such as a roof or wall assembly, in order to prevent interstitial condensation and consequent damage to the building or other issues, such as mold growth.

Construction Definition – Weather Barrier:

Weather-Resistive Barrier. A component of a wall assembly that functions to prevent air or water from moving from the building exterior into the interior. Although weather barriers are often sheet membranes, engineered panels with sealed joints are also common. A weather barrier is typically not a vapor barrier; a weather barrier is usually designed to allow water vapor to escape from the wall assembly.

1. Book Recommendation: Manual of Low-Slope Roof Systems: Fourth Edition by C.W. Griffin & Richard Fricklas

2. General: A great introductory article on the function of air barrier systems is available on this page at the Whole Building Design Guide site, which is maintained by the National Institute of Building Sciences.

3. General: “All About Water-Resistive Barriers” is an excellent introduction to weather barriers. Available on the Green Building Advisor website.

4. General: The website of the Air Barrier Association of America is an excellent source of technical information and up-to-date code requirements for air barrier systems. All sorts of information is available; the site is well-put-together. 

5. Technical: A short article on the function of air barriers and vapor retarders, “Roof Leaks and Air Infiltration” by roof expert Dick Fricklas, is archived on the website. The article is from 2009. 

6. Technical: “Water Vapor Migration 101 [AIA course]” is a good, readable, and reasonably short introduction to the fundamentals of vapor flow and vapor barriers. “This course will describe how to select vapor retarders to control moisture migration and prevent condensation within the building enclosure.” Available on the Building Design + Construction website.