Where should a 6-mil polyethylene vapor retarder be placed to retard vapor transmission through the floor slab?

• A. Between the base course and subgrade
• B. On top of the slab
• C. Between the base course or subgrade and the floor slab
• D. Under the slab

Answer: (C)

Moisture moving from the soil into a building travels upward toward and through the floor slab. To stop that vapor effectively, you want a barrier right at the boundary where the slab meets the soil layers, so the vapor is blocked before it enters the concrete and any finishes above it. Placing a 6-mil polyethylene vapor retarder between the base course or subgrade and the floor slab creates a continuous, low-permeability layer that intercepts moisture at its source path.

If the barrier were placed on top of the slab, moisture from the soil could still diffuse upward through the slab and into the interior, bypassing the barrier. If it were placed under the slab (below the base or subgrade), it wouldn’t sit in the moisture path that travels upward into the slab, so it wouldn’t protect the slab assembly as effectively. This is why the barrier belongs directly beneath the slab, at the interface with the subgrade or base course.