A Sealed Liner Is the Floor of the Crawl Space Envelope

Crawl Space Encapsulation Liner: how it works and when it's the right call.

Installation starts with clearing debris from the crawl space floor and identifying any active water source that needs to be addressed with drainage before the liner goes down. The liner, a reinforced material well beyond the thickness of basic construction plastic, is then laid across the floor with each sheet overlapped and the seams sealed, so there is no open edge for ground moisture to work around. The wall run comes next: the liner is carried up the foundation walls and held with termination bars or mechanical fasteners, which closes the gap at the base of the wall that a loose barrier always leaves open. Every pier and support post gets the same treatment, with the liner cut to fit, wrapped, and sealed to the pier so the soil around it is covered rather than exposed. Sealing the crawl space vents happens as part of the same project, since an open vent lets in the same humid air the liner is meant to keep out, just from a different direction. Once the floor, walls, piers, and vents are sealed, a dehumidifier sized to the space handles whatever humidity remains, which is what makes it a complete crawl space encapsulation rather than a sheet of plastic. Stopping at the liner alone, with the vents open and no dehumidifier, tends to produce condensation on the framing above the liner instead of on the dirt below it, which moves the moisture problem rather than solving it.