What is a vapor barrier?
A vapor barrier is a layer of polyethylene sheeting placed beneath concrete slabs to block moisture rising from the ground.
Polyethylene sheeting installed under concrete slabs and in basement subgrades serves as a barrier to stop ground moisture from wicking upward into the structure. In the Piedmont region around Charlotte Metro, where clay soils hold moisture and groundwater tables can fluctuate seasonally, vapor barriers become critical for protecting both slabs-on-grade and basement construction from dampness and the mold and structural problems that follow.
The barrier works by interrupting the capillary rise of water vapor through the concrete. Without it, moisture moves through the slab and into the building envelope, causing efflorescence on concrete surfaces, adhesion failures for flooring materials, and conditions favorable to mold growth. This is particularly important in basement spaces and crawlspaces where moisture control directly affects indoor air quality and material longevity.
Standard practice calls for 6-mil polyethylene sheet placed over prepared subgrade, with overlaps sealed and edges brought up to meet grade level. The barrier must be installed before concrete is poured. Proper installation includes removing sharp rocks or debris from the subgrade and ensuring continuous coverage so moisture cannot find gaps.
When moisture control fails, remediation becomes expensive and intrusive. Choosing contractors who understand local soil and water conditions and install barriers to specification makes the difference between a dry basement and persistent dampness. If you are planning a slab or basement project in Charlotte Metro, work with concrete contractors experienced in local moisture conditions.