The interior polyethylene sheet or sheathing placed above the ceiling gypsum board (and ceiling service space if present) should be detailed as airtight to provide the air barrier for this assembly. This interior air barrier will prevent the flow of air into the attic space from the interior.

A ceiling service cavity can be formed with framing lumber attached to the underside of the attic framing. This strategy is recommended where good ceiling airtightness is required to meet more stringent building airtightness targets.

Note that the air barrier transition at the top of each interfacing wall requires careful attention to achieve continuity of the interior air barrier.