Crawl space and basement encapsulation

Ground moisture you cannot see

Soil and concrete give off water vapor constantly. In a crawl space with a dirt floor, that vapor rises into the framing. In a basement, it moves through the slab and walls. Add humid summer air coming in through vents, and wood can stay damp enough for mold and rot. Many older houses in the river villages have additions or porches over crawl spaces, and some have partial basements with a dirt crawl area behind a knee wall.

How encapsulation works

The contractor removes debris and old, wet insulation, deals with any standing water through drainage or a small sump, then lays a thick polyethylene liner over the floor and up the walls. Seams are overlapped and taped, and the liner is fastened and sealed at the top. Vents and gaps to the outside are closed. A dehumidifier sized for the space keeps humidity down, and it should drain to a sump or outside rather than into a bucket.

• Clean out debris and wet insulation
• Fix standing water first
• Heavy liner on floor and walls
• Taped seams and sealed edges
• Vents and gaps closed
• Dehumidifier with a permanent drain

Safety with fuel-burning equipment

If a furnace, boiler or water heater in or next to the space burns gas, oil or propane, sealing it changes the air supply that equipment depends on. The contractor must make sure combustion air and venting stay safe, and a licensed heating professional may need to check the setup. Never seal around fuel-burning equipment yourself. The EPA also recommends testing every home for radon, and a sealed space can help with radon control when designed for it.

Basement floors and walls

In a full basement, a vapor barrier is often part of finishing: a membrane over the slab under new flooring, and moisture-tolerant insulation on the walls. These steps reduce condensation and musty smells but do not stop liquid water. Fix leaks with drainage first, then control vapor.

After the work

Check the liner once or twice a year for tears, and keep stored items off it. Clean the dehumidifier filter on schedule and make sure its drain line stays clear. Water on top of the liner after a storm means outside drainage still needs attention, so call the contractor back.

Questions for the contractor

Ask how thick the liner is, whether it is fastened and sealed to the walls, how seams are taped, which dehumidifier they use and where it drains, and how they will handle any mold already present.

Typical encapsulation costs

National figures. Not quotes.

National figures, rounded, not quotes. Your price depends on size, clearance, drainage and existing mold or debris.

Sources: Bob Vila, crawl space encapsulation; Fixr, basement waterproofing