Vapor retarders long have been a common part of low- and steep-slope roofs in many U.S. climates. Increasingly, air barriers also have become a routine requirement for all types of roof systems. Starting with Massachusetts' building code in 2001, several other state building codes now require an air barrier as part of any exterior assembly. This trend will continue nationally when building codes and LEED® requirements adopt the ASHRAE 90.1—2010, "Energy Standard for Buildings Except Low-Rise Residential Buildings," which requires air barriers.
However, roof systems continue to fail because of a lack of reliable vapor retarders and/or poorly detailed air barriers.
Some roof system designers, roofing contractors and manufacturers remain unclear about the differences between these two components—when they are needed, their intended functions and the requirements for effectively installing each. Often, the same material layer is used to provide both functions, adding to the confusion.
It's important to be aware of the differences between air barriers and vapor retarders and the problems that can occur when they are improperly designed or installed.