Vapor retarder
A plastic sheet installed under a slab on grade to block ground moisture vapor from migrating up through the concrete and damaging floor coverings.
What does vapor retarder mean?
Ground under a building stays wet, and a slab without a vapor retarder acts as a wick, delivering moisture into the building indefinitely. Effective products combine low vapor permeance with enough tensile and puncture strength to survive rebar placement and foot traffic. Whether the retarder is placed in direct contact with the slab or under a layer of granular fill is a long-running debate with real consequences for curling and drying.
In practice
Flooring adhesive failures years after occupancy trace back to this sheet more often than to any other single decision.