What Is the Difference Between Vapor Barriers and Vapor Retarders?

Along with similar-sounding names, vapor barriers and vapor retarders both are used to manage the movement of vapor in a wall or roof system. But specifying the wrong one can lead to moisture issues in the wall cavity.

In short, a vapor barrier blocks vapor diffusion movement while a vapor retarder significantly slows the diffusion movement through an assembly. Vapor retarders come in a range of permeance levels to regulate moisture moving through the cavity. 

When to Use a Vapor Barrier 

To know when to specify a vapor barrier or a vapor retarder, you first have to understand what you’re trying to control. An area with an extreme moisture load will require a vapor barrier. The most common place for a vapor barrier is the basement with foundation walls in the ground. When the ground is wet, there is a large amount of moisture that wants to move into the dry house. To stop moisture from getting to the foundation, we must use a vapor barrier. Under the concrete slab is another common area. It’s rare to need a vapor barrier in an above-ground wall system; a room with an indoor pool, which generates a tremendous amount of moisture that wants to move from inside to outside, is one occasion.

Vapor barriers aren’t limited to sheet products—closed-cell foam insulation, for example, is a vapor barrier because it blocks moisture transfer. It’s also important to note that vapor barrier installation has to be perfect. Any cracks or gaps can allow moisture to get through; the interior materials can now get wet but cannot dry out. 

When to Use a Vapor Retarder

In areas with typical quantities of moisture, such as the home’s walls, it’s safer to use a vapor retarder. A vapor barrier will stop all drying potential—but in a wall system, we want the moisture to be able to move and not become trapped so that the cavity can dry out; a vapor retarder allows this.

For more information on Benjamin Obdyke products, visit www.benjaminobdyke.com