What material should be placed beneath a pressure-treated sill and the concrete slab?

The correct material to place beneath a pressure-treated sill and the concrete slab is generally a vapor barrier. A vapor barrier serves a critical role in construction by preventing moisture from the ground from rising into the building structure. This is especially important when dealing with materials like concrete that can absorb moisture and affect the integrity of the wood above.

In the context of this question, although 30-pound felt is sometimes used in construction to act as a moisture barrier, it is not as effective as a dedicated vapor barrier. The primary role of a vapor barrier is to stop moisture diffusion, which is essential in areas where a concrete slab is utilized.

Using a vapor barrier prevents potential health issues related to mold and wood rot that can arise from excessive moisture exposure. While other materials like plywood or plastic sheeting might provide some level of moisture protection, they do not specifically fulfill the same function as a dedicated vapor barrier in the context of protecting building materials that are sensitive to moisture. Thus, it is crucial to use a material designed specifically for that purpose, which validates the choice of a vapor barrier as the appropriate option for installation beneath a pressure-treated sill and concrete slab.