Without a vapor retarder under a slab, which outcome is most likely?

• A. Moisture migration is unlikely
• B. The slab will become waterproof
• C. The slab will cure too quickly
• D. Moisture migration from subgrade into the slab is more likely

Answer: (D)

The key idea here is that a vapor retarder serves as a barrier to moisture moving from the soil beneath the slab up into the concrete. Subgrade soils often have significant moisture, and concrete is porous, so both liquid water and water vapor can travel through it by diffusion and capillary action.

When a vapor retarder is not used, that vapor movement isn’t blocked. The moisture from the damp subgrade easily migrates into the slab, raising its moisture content. This can create problems for any finish or flooring placed on the slab—adhesives may fail, wood or laminate flooring can swell or warp, and surface finishes can be disrupted by excess moisture.

It’s important to remember that this isn’t the same as waterproofing the slab; a vapor retarder reduces moisture vapor transmission, not all water from rainfall or surface leaks. It also doesn’t imply the slab will cure too quickly—the issue is ongoing moisture migration from below, not the curing rate.