The polyethylene sheet bonded to the cast concrete slab is the air barrier in this assembly and is usually the most straightforward to make continuous with adjacent building enclosure assemblies such as the concrete foundation wall.

The polyethylene sheet should be taped and sealed at all laps and penetrations. Besides providing a robust air barrier, the polyethylene functions as the vapour barrier, and as part of the soil gas mitigation system where required. A high-density polyethylene (HDPE) sheet is recommended, as it should be robust enough to withstand exposure to foot traffic and abrasion (i.e., remain continuous) prior to and after the concrete slab installation.