What is the purpose of a vapor retarder under a slab-on-grade?

• A. To improve surface finish of the slab
• B. To add thermal insulation
• C. To prevent moisture from migrating from the subgrade into the slab.
• D. To provide electrical grounding

Answer: (C)

Moisture control under a slab-on-grade is the key idea. A vapor retarder (typically a plastic sheet) sits between the soil and the concrete to stop or slow water vapor moving from the subgrade into the slab. By limiting that moisture, it protects floor coverings and adhesives from moisture damage, helps ensure a proper concrete cure, and reduces issues like staining, efflorescence, and mold. It isn’t used to improve surface finish, provide meaningful thermal insulation, or serve as electrical grounding.