What is the Purpose of a Vapor Barrier?

What is the Purpose of a Vapor Barrier?A homebuilder’s biggest priority is to create a comfortable and dry home. However, keeping a home dry is not simple: engineers and architects spend much of their building planning on keeping water out, or letting it dry or drain when it comes come in. By focusing on water and its movement, builders can implement a better moisture control strategy. To help, a vapor barrier might be a key component to the overall strategy. But what is the purpose of a vapor barrier, and when and where is it most effective to help control air moisture? Here’s a simple guide to understanding a vapor barrier and its optimal uses.

Definition of a Vapor Barrier

A vapor barrier is essentially a material that helps reduce the rate at which water vapor can move through a material. Also known more accurately as a vapor diffusion barrier, a vapor barrier (or vapor retarder) is defined by three classes, according to the International Residential Code.

Vapor barriers and retarders are classified by permeability, with Class I vapor retarders (known as true vapor barriers) rated at 0.1 permanence or less. Because of its low perm rating, Class I vapor retarders are considered to be impermeable, making it a true barrier against vapor. Class II vapor retarders have a permeability between .1 perms to 1.0 perms, and Class III vapor retarders have a permeability between 1.0 perms and 10.0.
Vapor barriers are used in basements, walls, slab-on-grade foundations, ceilings, attics, floors, and crawlspaces, and come in different materials, such as membranes or coatings. But for them to be completely effective, they must be used properly and as part of an entire system for moisture control.

Using a Vapor Barrier

A vapor barrier’s low permeance retards the movement of water vapor greatly, which can make it a great asset to a home. Depending on the type of materials used to build an exterior wall, and where the vapor barrier is located in the assembly, the vapor barrier can help become a drainage plane and air barrier, which will keep water from condensing. By preventing condensation, especially inside assemblies, a home is less likely to rot or grow mold.

So where should a builder place a vapor barrier? Joseph Lstiburek at Building Science provides in-depth scenarios that can help builders pick vapor barriers and install them in a way to allows for water vapor to dry out according to climate or housing conditions.

Builders must note however that a vapor barrier is just one component of an entire moisture control strategy. Builders must be conscientious about controlling heat transfer and proper air sealing, in order to improve overall moisture control as well. This is especially true for in-wall moisture problems, where condensation is likely to occur depending on the climate conditions pertinent to the property, and where moisture might be building from outside or inside the home.

Improper Uses of a Vapor Barrier

As builders try to prevent moisture-related problems, it can become easy to confuse how  the vapor barrier  is used and install it improperly. When used incorrectly, a vapor barrier can actually create moisture problems since vapor barriers installed to prevent assemblies from getting wet can also make them harder to dry out.

Here is more information from Dr. Joe Lstiburek on the correct use of vapor barriers.
How Vapor Barriers Are Used in Flow Through Assemblies

How Double Vapor Barriers Can Work

As you can see, there are no simple solutions, but builders will not want to rely solely on vapor barriers. You will want to instead look at other classes of vapor retarders, or maybe go without. This is especially important in mixed climates, where the materials used for a vapor retarder can help in summer, where it’s not as humid in the region, but keep moisture out during a cold and snowy winter.


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.