What is crawl space encapsulation?

Crawl space encapsulation is sealing the crawl space from the ground and the outside air: a reinforced liner across the floor and up the walls, foundation vents closed, and the space conditioned with a dehumidifier so relative humidity stays controlled year-round. It is a system, not a sheet of plastic.

How much does crawl space encapsulation cost?

In the Wilmington area, $4,200–$14,000, with a typical project near $5,500. Liner and vent sealing are the fixed part; drainage, mold treatment and structural repair are what create the spread.

When is encapsulation the wrong answer?

When the crawl space still takes on groundwater and the budget will not cover drainage. Sealing over active water traps it under the liner where it cannot evaporate. In that case the honest sequence is drainage now, encapsulation later — not a cheaper encapsulation.

Does an encapsulated crawl space still need a dehumidifier?

Nearly always in this climate. Sealing stops moisture entering from soil and outside air, but the space still exchanges some air with the house and holds residual moisture. Without conditioning, relative humidity in a sealed crawl space commonly settles above 60%, which is enough to support fungal growth.

Will encapsulation cause moisture problems by trapping air?

That concern comes from sealing without conditioning. A closed crawl space with no dehumidification can hold moisture, which is why IRC R408.3 treats an unvented crawl space as requiring either conditioned air supply or mechanical dehumidification. Sealed plus conditioned is the tested assembly.

Crawl Space Encapsulation

What the work involves in Wilmington and the Cape Fear region, what it costs, and the situations where it is the wrong thing to buy.

Last updated: 2 August 2026

910 Crawl Space Pros provides crawl space encapsulation across Wilmington and the Cape Fear region. Crawl space encapsulation is the practice of sealing a crawl space from both the soil below and the outside air — a reinforced liner across the floor and up the walls, the foundation vents closed, and the space conditioned so relative humidity stays under control through the year. Done properly it removes the moisture source that drives fungal growth, cupped flooring and softened framing.

The word covers a lot of ground, though, and two quotes using it can describe genuinely different work. What follows is the specification we work to and what each part of it is for.

What we do on the first visit

Nothing is quoted from a photograph. The assessment takes about an hour: a relative humidity reading in the crawl space, moisture content readings in joists and subfloor at several points, a check for standing water and for the high-water mark that shows where it has been, and a look at clearance, access and the condition of any existing liner. You get the photographs and the readings whether or not you go ahead.

Liner specification, and why 6 mil is not enough

The liner is the component most quotes differ on, and it is largely invisible once installed. We use a reinforced liner of 12–20 mil meeting ASTM E1745, the standard covering water vapor retarders used under concrete slabs and in crawl spaces. A Class I vapor retarder is one with a permeance below 0.1 perms — that number is the point of the material.

A 6 mil unreinforced builder sheet meets neither the thickness nor the puncture resistance that a working crawl space needs. It tears the first time anyone crawls across it to service a duct, and a torn vapor retarder is not a vapor retarder. The reinforcement — a scrim layer between plies — is what lets a liner survive being walked on.

Why drainage has to come first

This is the sequence error that produces most of the failed encapsulations we are called to look at. Where groundwater rises into the crawl space, a liner laid over it does not remove the water; it covers it. The water is then under an impermeable membrane, where it cannot evaporate and cannot be seen, and the first sign of it is usually odour or a failed liner seam months later.

On the Cape Fear coast this matters more than it does inland. The regional water table is high, the soil drains quickly at the surface and slowly below it, and roughly 57 inches of rain falls a year. Where the assessment finds water, perimeter drainage and a sump go in first, and the liner follows.

Sealing the vents, and what the code says

Foundation vents exist on the assumption that outside air is drier than crawl space air. For most of the year here it is not, and venting pulls humid air across framing that sits below the dew point. The moisture condenses on the wood.

Closing the vents is therefore part of the system rather than an optional extra — but a closed crawl space has to be conditioned. IRC R408.3 treats an unvented crawl space as requiring either a supply of conditioned air or mechanical dehumidification. Sealing without either is the version of encapsulation that earns the technique its bad reputation.

Conditioning: sizing the dehumidifier properly

A dehumidifier is sized to the cubic footage of the space and the moisture load, not chosen from a shelf. An undersized unit runs continuously, never reaches target, and fails early. The condensate has to go somewhere real — a pump to a discharge point away from the foundation, not a bucket and not the soil under the liner.

We re-measure relative humidity before leaving and record it. Below roughly 60% is the number that matters, because that is where fungal growth stops being supported.

What's included

Reinforced 12–20 mil liner meeting ASTM E1745, not 6 mil builder sheet
Liner run up piers and perimeter walls, mechanically fastened and sealed
All seams overlapped and taped with a seam tape rated for the liner
Foundation vents closed and sealed, not simply covered
Insulated, gasketed access door where the existing one leaks
Dehumidifier sized to actual cubic footage, with condensate pump
Condensate and any sump discharge routed away from the foundation
Post-install relative humidity reading, recorded and left with you

	DIY	Cheapest quote	Done properly
Liner	6 mil builder sheet, laid loose	6–8 mil, taped seams, not fastened to walls	Reinforced 12–20 mil to ASTM E1745, sealed to piers and walls
Groundwater	Not addressed	Sealed over — water ends up under the plastic	Drainage and sump first, liner second
Humidity control	None, or a household dehumidifier on an extension cord	Undersized unit, condensate draining to the soil	Unit sized to cubic footage, pumped to a proper discharge
Vents	Left open, or foam blocks pushed in	Covered, not sealed	Sealed and detailed so the space is genuinely closed
Wood already affected	Bleach from a spray bottle	Surface treated, moisture source left in place	Growth removed, framing treated, dried, then sealed
What it costs	$300 – $900 in materials	$1,500 – $3,000	$4,200 – $14,000
How long it holds	A season or two	Often fails within a few years	Liner life measured in decades when water is handled first

How encapsulation work runs

01
Assess and measure

Relative humidity in the space, moisture content in the framing, standing water, clearance and access. Photographed throughout.
02
Dry it out and clean it up

Drainage where groundwater is present, debris and failed insulation removed, affected framing treated and dried.
03
Line and seal

Reinforced 12–20 mil liner across floor, piers and walls, seams taped, terminated and sealed at the perimeter. Vents closed and sealed.
04
Condition and verify

Dehumidifier sized to the cubic footage, condensate pumped to a real discharge. We re-measure RH before leaving.