subslab

Subslab refers to the space beneath a concrete floor slab, typically in a slab-on-grade foundation. It comprises the native soil or compacted fill, sometimes augmented with layers designed to manage moisture and drainage before the slab is poured.

Common subslab components include a moisture or vapor barrier (often polyethylene), a layer of granular fill such as crushed stone or gravel, insulation in some cold climates, and a drainage network of perforated pipes that directs water to a sump or exterior drainage. In many homes, the subslab system is designed to limit moisture migration from the ground into the interior and to provide a stable base for the finished floor.

Radon mitigation systems may also use the subslab area, using suction under the slab to draw gas from beneath the building and vent it outdoors.

Design and installation depend on soil conditions, climate, and local building codes. Improper subslab design can lead to moisture problems, mold growth, or radon intrusion; regular inspection and proper sealing of joints between vapor barrier and penetrations are important.