Understanding the Difference Between Vapor Barriers and Vapor Retarders

Vapor Barrier vs Vapor Retarder

All building materials, no matter how solid, allow water vapor to pass through. It may be at a very fast rate or an excruciatingly slow rate, but water vapor passes nonetheless. The process is referred to as the material’s permeability or its permeance. Materials used to create vapor barriers or vapor retarders are given a rating system based on permeability and permeance. The rating is known as a ‘perm’. A low perm rating means that water vapor has a difficult time getting through the material so the material is an effective barrier.  The higher the perm rating, the faster the rate at which water vapor passes through.

Perhaps it is the similarity in name, but builders, architects and designers often become confused over the difference between vapor barriers and vapor retarders. The definition of the differences is illustrated by the Building Science Corporation — a vapor barrier is defined as a layer with a permeance rating of 0.1 perm or less. A vapor retarder is defined as a layer with a permeance greater than 0.1 perm but less than or equal to 1 perm. This includes materials such as materials as polyethylene sheeting, aluminum foil, kraft paper facing, and vapor-retarding paint.

Many building professionals and manufacturers and some people use the term ‘vapor barrier’ to classify between a class-I vapor retarder and an impermeable material. Based on their perm rating, building products fall into one of three classes of vapor retarder. According to the 2009 IRC and the U.S. Department of Energy, the vapor retarder classifications are as follows:

Difference Between Vapor Barriers and Vapor Retarders

This chart and more information on choosing the proper air and moisture barrier can be found in the DELTA-VENT SA Technical Guide

Although many areas of the country strictly follow the IRC definition of the differences between vapor barriers and vapor retarders, some areas of the United States still adhere to the 2006 International Energy Conservation Code (IECC) which states that a vapor retarder is a material that has a permeance of 1 perm or less and does not acknowledge vapor barriers at all. The definition clearly includes vapor-retarding paint, polyethylene sheeting, aluminum foil and kraft paper facing.

Even though different areas of the country follow varying guidelines, there remains a general consensus that class I vapor retarders are generally considered to be vapor barriers and are normally referred to as such instead of vapor retarders. The accepted nationwide definition of a vapor barrier remains that it is not a permeable material, unlike a vapor retarder, which effectively stops the movement of water vapor.

Any time a vapor barrier or a vapor retarder must be used, the deciding factor of which material is implemented remains the climate zone it is being installed within. Various climates require various moisture control considerations. In areas of the United States with exceptionally warm and humid climates, e.g. Florida, it is recommended to install a vapor retarding layer near the exterior side of wall assemblies, according to the North American Insulation Manufacturers Association.

Difference Between Vapor Barriers and Vapor Retarders 2

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This is chart is another tool found in the the DELTA-VENT SA Technical Guide.

At Cosella Dorken, our knowledgeable staff is here to answer any questions you might have pertaining to vapor barriers and vapor retarders in commercial construction. We have been in the construction industry for over a hundred years and we still continue to take a strong leadership role as the manufacturer of premium quality products.


Cosella-Dörken delivers innovative, high-performance air and moisture barriers for commercial and residential construction sold under the DELTA® brand name. A North American manufacturer based out of Beamsville, Ontario, Cosella-Dörken Products, Inc. is a subsidiary of Ewald Dörken AG, a leading European developer and manufacturer of waterproofing and drainage products sold worldwide. Cosella-Dörken is known for delivering premium products while providing educational programs and full technical support. For more information, call 1-888-4DELTA4 (433-5824) or visit www.cosella-dorken.com.