Abstract: 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. 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.