NASCLA Accredited Commercial General ContractorConcreteEasy
A concrete contractor is pouring a 6-inch thick concrete slab for a warehouse floor. The specifications require the use of a vapor barrier directly beneath the slab. What is the primary reason for installing a vapor barrier in this application?
- ATo prevent radon gas from entering the building.
- BTo increase the overall structural strength of the concrete slab.
- CTo provide a smooth, level surface for the concrete pour.
- DTo prevent moisture vapor from migrating through the slab into the building interior.
Show answer & explanationAnswer & explanation
Correct answer: D. To prevent moisture vapor from migrating through the slab into the building interior.
The primary purpose of a vapor barrier (or vapor retarder) beneath a concrete slab on grade is to prevent moisture vapor from the ground from passing through the concrete slab. This protects floor coverings, stored goods, and the interior environment from moisture damage.
Why the other options are wrong
- A. While some vapor barriers can also act as radon barriers, their primary function is moisture control.
- B. Vapor barriers do not contribute to the structural strength of the concrete slab.
- C. A vapor barrier is a thin membrane and does not provide a smooth or level surface; the subgrade does that.
Vapor Barrier (Slab)
A material, typically a plastic sheet, placed under a concrete slab on grade to impede the movement of moisture vapor from the ground into the building.
- Protects interior finishes and contents from moisture.
- Reduces humidity within the building.
- Must be durable and properly sealed.
Memory trick: Barrier for vapor, keeps the floor drier.