What’s the Difference: Vapor Barriers and Vapor Retarders?
by Rob Yagid
You don’t have to be a building-science expert to know that trapped moisture is bad for houses. To help slow moisture diffusion through roof, wall, and floor assemblies, many experts-and in some parts of the country, building codes-demand the use of vapor retarders.
Across the building industry, however, the term vapor barrier is commonly used in place of vapor retarder. This misuse raises the question of whether the two terms categorize the same products and whether those products have the same performance traits.
Understanding perm ratings
Permeability, which is the amount of moisture that can pass through a material, is measured in perms. The lower the number, the less permeable the material and the more moisture it will block. Contact the manufacturer to get the perm-rating information for the product you are thinking about using.
Vapor retarders are all-encompassing
The International Residential Code (IRC) defines a vapor retarder as a vapor-resistant material, membrane, or covering with a perm rating of 1 or less. However, the 2007 IRC supplement recognizes some materials that have ratings of 1 perm and higher as vapor retarders. Based on their perm rating, building products fall into one of three classes of vapor retarder.
The IRC does not mention vapor barriers, but some manufacturers and some people in the building-science industry use vapor barrier to distinguish a class-I vapor retarder, or an impermeable material.
Why terminology is important
As building science progresses and influences the way houses are built, attention to detail and accuracy are critical. An assembly that calls for a vapor barrier is designed to stop moisture on one surface-under a concrete slab, for example-while more permeable vapor retarders allow some movement of moisture. If walls, roofs, or floors are configured with the wrong vapor-retarding products, a structure can trap moisture. Misusing these terms leads to confusion in product choice, which ultimately can lead to failure where it matters most-in your home.