Air barrier systems are usually two conventional types:

exterior air barrier systems, with the primary airtight elements placed at the exterior side of the enclosure, and interior air barrier systems, with the primary airtight elements installed at the interior side of the enclosure.

Within these systems there are various approaches and components used to achieve the air barrier.

Exterior air barrier approaches use an airtight layer, usually a dedicated membrane, installed over the

exterior face of the building structure, and made continuous with tapes, membranes, and sealants over joints, transitions and penetrations. The interior approaches use an airtight layer applied from the interior of the enclosure, interfacing with the various interior elements, transitions, and penetrations. In general, the exterior approach is simpler, because it does not interface with numerous interior elements like framing or service penetrations for electrical and plumbing. Also, because the components of the exterior air barrier are often also used as the water resistive barrier (for example spun-bonded polyolefin on walls), the effort and care required to achieve a continuous layer to resist moisture intrusion also contributes to the overall continuity of the air barrier.

However, the exterior air barrier still must interface with interruptions at the outside of the building, such as balconies, canopies, and some service penetrations.

Interior approaches, often using polyethylene sheet or interior finishes as the primary airtight element, must account for the numerous interruptions at the interior. The detailing and effort required to make the interior surfaces airtight across these elements is more difficult compared with using an exterior approach. This guide assumes an exterior air barrier approach is used.

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