Sealing damp crawl spaces under split-levels

Where crawl spaces turn up

Split-level houses often have a basement under one half and a crawl space under the other. Ranches and colonials get them under later additions, sunrooms and family rooms. Many of these spaces have bare soil or a thin, torn plastic sheet for a floor and open vents in the rim. In humid summers, that ground breathes moisture straight into the framing and, through gaps in the subfloor, into the rooms above.

Signs it is happening: a musty smell in the room over the crawl, cupping in wood floors, sweating ductwork, or a white or dark bloom on the joists.

Liner, sealed vents and a dehumidifier

Encapsulation treats the crawl space as part of the conditioned house instead of part of the outdoors.

• A thick reinforced liner covers the ground, runs up the foundation walls and is sealed at the seams and the top edge.
• Vents and other openings to the outside are closed so humid summer air stops coming in.
• A dehumidifier sized for the space holds the humidity down and drains to a pump or a floor drain.
• Insulation can go on the walls once the space is sealed, where it stays dry.

When water collects, a sump goes in

Humidity is one problem; standing water is another. If the crawl floor puddles after storms, or groundwater rises into it when the Millstone or a nearby brook is high, a liner alone will just float on top. In that case a small pit and sump pump go in at the low point under the liner, and sometimes a short run of drain along the worst wall feeds it.

Getting in and checking later

Access decides a lot. A crawl space reached through a small hatch in a closet takes longer than one with a full-height door off the basement. Low clearance, old debris, abandoned ductwork or pest damage gets noted at the visit, because it has to be dealt with before the liner goes down.

Once it is sealed, the space should stay clean enough to crawl through. Look in a couple of times a year: check the dehumidifier is running and draining, and look for tears in the liner where anyone has worked on pipes or wires.