Crawl space encapsulation

Around here a crawl space is often only part of the house: the low gap under an addition, an enclosed porch or a kitchen extension, next to a full basement. Small as it is, it can keep the rooms above it damp and cold.

Call (973) 555-0118

You will be put through to an independent local waterproofing company. Calls may be recorded.

What encapsulation means

A crawl space in section. The green liner covers the earth, climbs both walls and wraps the foot of each pier, so moisture in the soil (blue drops) stays beneath it. The vent in the left-hand wall has been closed. The dark box on the liner is a dehumidifier, and the wavy lines above it are dry air.

The bare ground and the foundation walls are covered with a heavy plastic liner. Its seams are taped, its edges are fastened to the walls, and it is cut and sealed round every pier and pipe. The vents to outdoors are blocked. Then the air inside is dried, by a dehumidifier or by a small feed of conditioned air from the house.

The result is a space that belongs to the inside of the house, not the outside.

Why the vents are closed

Vents were put in to air the space out. In a humid summer they do the reverse. Warm wet air comes in, meets cool joists and ductwork, and leaves its water there. A crawl space with open vents is often wetter in August than in March.

Closing them only works if the air is then kept dry. A sealed space with no dehumidifier and a damp floor is a terrarium.

What has to be right first

A liner hides whatever is under it. These come before it goes down.

Standing water is drained, usually with a shallow trench and a small sump, and its source outdoors is corrected.
Soft or rotted wood is repaired while it can still be reached.
Old insulation, timber offcuts and rubbish are cleared out.
A furnace or water heater that burns fuel inside the space has its air supply checked by a heating contractor before the vents are shut.
Any sign of termites is dealt with by a pest company.

Ask the installer

The liner

How thick is it, is it reinforced, and is it fastened to the walls or only taped?

The top edge

Is a strip of wall left bare below the wood so that termite tubes can be seen in future inspections?

The dehumidifier

Where does it drain, who changes its filter, and can it be reached without crawling the full length?

Insulation

Will the walls be insulated, and what happens to the old batts between the joists?

The hatch

Will the access door be replaced with one that seals?

Encapsulation questions

Is a vapour barrier the same thing?

No. A vapour barrier is a sheet of plastic laid loose over the soil. It slows moisture from the ground and nothing else. Encapsulation also seals the walls, the piers and the vents, and dries the air.

Will the floors above feel different?

Often they feel less cold and less clammy, because the joists are no longer sitting in damp air. How much depends on the house, and no installer can measure it in advance.

Does it straighten a sagging floor?

No. Encapsulation keeps sound wood dry. Wood that has already sagged or rotted is crawl space repair, and that is done first.