If the roof membrane is monolithic (i.e., a membrane roof) it serves as an air retarder. However, separate air barriers are sometimes incorporated into roof systems. When air barriers are incorporated into wall systems, they are normally included to control air movement, control moisture and/or reduce energy consumption, or to prevent pumping due to wind, which can cause uplift with mechanically fastened membranes. When an air barrier other than the roof membrane is incorporated into a roof system, it is normally included to address wind performance issues as discussed in Wind Safety, or to address a building code requirement. To reduce the potential of interior air being pumped into the roofing system an air barrier should be located at the roof deck level under the roof insulation, sealing all roof deck voids.
The deck itself can be a barrier if it is monolithic, such as cast-in-place concrete. When the deck is used as an air barrier, deck penetrations such as plumbing vents should be sealed, and the deck should be sealed at parapets. However, a separate sheet material such as 6-mil polyethylene, approved housewrap, a two-ply built-up membrane or a one-ply modified bitumen sheet is typically used to create an air barrier. Membranes used as air barriers, must be made airtight at all penetrations. Air barriers are further discussed in A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs, which is available from the National Research Council Canada.
Requirements for air barriers are included in some building codes and widely adopted standards such as International Energy Conservation Code (IECC), 2012 edition and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1–۲۰۱۰ and –۲۰۱۳—Energy Standard for Buildings Except Low-Rise Residential Buildings.
Also refer to The NRCA Roofing Manual: Architectural Metal Flashing, Condensation and Air Leakage Control, and Reroofing for air barrier and vapor retarder requirements.