Vapor Barrier vs. Full Encapsulation: What's the Difference?

A vapor barrier is a sheet of plastic laid over the dirt floor of a crawl space to slow moisture rising from the soil. Full encapsulation is a complete system: the ground cover is sealed and usually carried up the walls and piers, the foundation vents and air leaks are sealed, water problems are corrected, and humidity is controlled with a dehumidifier or another approved method. In short, a vapor barrier treats one moisture source, while encapsulation turns the crawl space into a closed, dry space. In North Carolina, the second option has its own code section, R409, and requires a permit.

Both have a place. The right choice depends on how wet your crawl space is, where the moisture is coming from, and what you want the space to do for the house.

What a vapor barrier is

Building scientists prefer the term vapor retarder, because plastic sheeting slows water vapor rather than stopping it completely, but most contractors and homeowners call it a vapor barrier. In a crawl space it is usually polyethylene sheeting spread across the soil.

Bare soil gives off moisture continuously, even when it looks dry. Covering it is one of the simplest and most important moisture-control steps under a house. NC State Extension's mold and moisture checklist recommends a vapor barrier of at least 6 to 8 mil plastic over the soil in a crawl space.

A typical vapor barrier job includes:

Removing torn, thin or displaced old plastic.
Laying new sheeting across the full floor, with seams overlapped.
Sometimes taping seams and running the plastic a short way up the piers or walls.

What it does not include: sealing the foundation vents, air-sealing the rim joist area, moving insulation, or adding mechanical drying. The crawl space stays vented, so outdoor air still moves through it.

What full encapsulation is

Full encapsulation creates what North Carolina's code calls a closed crawl space. Advanced Energy, the Raleigh nonprofit whose research shaped the state's rules, describes it as an insulated space with no vents to the outside, designed around moisture management first, plus pest control, combustion safety, fire safety, insulation and radon control.

Under the 2018 North Carolina Residential Code, as quoted by the NC Office of the State Fire Marshal, a closed crawl space must include:

A ground vapor retarder of at least 6-mil polyethylene or equivalent, nominally covering all exposed earth, with joints lapped at least 12 inches (R409.2.1).
Drainage: the crawl space floor graded to one or more low spots, each with a drain to daylight or a sump pump, kept separate from gutter and foundation perimeter drains (R409.2.1).
Sealing to walls and columns: the ground vapor retarder sealed to the walls and support columns, and brought at least 4 inches up the columns (R409.2.2, as clarified by OSFM).
A termite inspection gap: if the liner is carried up a masonry wall, its top must stop 3 inches below the top of the wall, leaving a clear gap of 3 to 4 inches below the wood sill (R409.2.2 and R409.2.2.1).
Moisture control by one of five methods listed in R409.5: a dehumidifier, supply air from the home's HVAC system, house air, an exhaust fan, or treating the crawl space as conditioned space.

On top of those code minimums, a full encapsulation job typically includes sealing the foundation vents, air-sealing the band area and access door, and moving insulation from the floor to the foundation walls. OSFM has also stated that converting a vented crawl space to a closed one requires a permit regardless of the job's cost.

The key differences, side by side

Ground coverage

Vapor barrier: covers the floor, sometimes loosely. Encapsulation: continuous, sealed ground cover tied to walls and piers, often run up the walls as a full liner, with the North Carolina termite inspection gap at the top.

Foundation vents

Vapor barrier: vents stay open. Encapsulation: vents and other openings to the outdoors are sealed.

Humidity control

Vapor barrier: none; humidity follows the outdoor weather. Encapsulation: required, most often a dedicated crawl space dehumidifier.

Water management

Vapor barrier: usually not addressed. Encapsulation: the code calls for grading to low spots with a drain or sump pump.

Insulation

Vapor barrier: stays between the floor joists. Encapsulation: usually moves to the foundation walls.

Permit

Vapor barrier: requirements vary by jurisdiction and scope, so ask your local inspections office. Encapsulation: OSFM says a permit is required for any conversion to a closed crawl space.

Maintenance

Vapor barrier: occasional checks for tears and displacement. Encapsulation: dehumidifier upkeep, drain checks and, ideally, a humidity monitor.

Understanding mil thickness

Liner thickness is measured in mils, and one mil is one-thousandth of an inch. The North Carolina code sets 6 mil as the minimum for the ground vapor retarder in a closed crawl space. Many contractors offer thicker, reinforced liners for encapsulation jobs. The usual reasons are durability rather than vapor control: crawl spaces get crawled through by plumbers, HVAC techs, pest inspectors and home inspectors, and thin plastic tears easily on rocks and debris. A tear lets soil moisture through and is easy to miss.

When you compare quotes, ask for the thickness in mils, whether the material is reinforced, how seams are joined, and how far up the walls and piers the liner runs. Our glossary entry on mil thickness goes into more detail.

When a vapor barrier alone may be enough

A vapor barrier upgrade can be a sensible, modest fix when:

The crawl space has no standing water, and the soil is not wet after storms.
Wood moisture readings on joists and sill plates are in a safe range. Research summarized by the USDA Forest Service shows decay fungi do best when wood moisture is above roughly 25 to 30 percent, and builders commonly treat 20 percent as a safety limit.
There is no mold on framing, no musty odor in the house, and the insulation is dry and in place.
The existing plastic is simply missing, torn or too thin.

In that situation, replacing a poor ground cover with a good one deals with the biggest source of moisture for relatively little disruption. Our vapor barrier service page explains what that work involves.

When full encapsulation is the better choice

A vapor barrier cannot stop humid outdoor air from moving through open vents. In a humid climate, the U.S. Department of Energy notes that outside air is often more moist than the air already in the crawl space, and Advanced Energy links wall vents to condensation, surface mold, high wood moisture and rot in the Southeast. Encapsulation is usually the better choice when:

Summer humidity readings in the crawl space are high. The U.S. EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent, to limit mold.
Ducts or pipes sweat in summer, or insulation is damp or falling down.
Mold is growing on joists or subfloor, or the house has a persistent musty smell.
Hardwood floors are cupping or buckling.
You want ductwork and equipment in the crawl space to sit in a drier, more stable environment.

If you are not sure which group your house falls into, our 7 signs of crawl space moisture article covers the symptoms in more detail.

Avoid half measures

The riskiest outcome is a job that falls between the two options. Common examples:

Sealing the vents without adding humidity control. Moisture from the soil, walls and plumbing still enters, and now it cannot leave. Advanced Energy warns that closing the vents on an existing vented crawl space is not enough on its own.
A thick liner over standing water. If water collects under the plastic, it needs drainage. The code requires low spots to drain to daylight or a sump pump.
A wall liner pulled to the sill. In North Carolina, a liner on a masonry wall must stop short of the top to leave a termite inspection gap.
Reverse-lapped seams. Advanced Energy lists reverse-lapped vapor barrier as a common detailing mistake. Seams should be lapped so water runs toward the drain, not under the plastic.

Whichever way you go, ask the contractor to describe the job in writing so you can see exactly what is and is not included.

Cost differences

A vapor barrier replacement is a much smaller job than a full encapsulation, which adds sealing, insulation changes, drainage, mechanical equipment and a permit. Actual prices depend heavily on square footage, crawl space height and condition. See our North Carolina cost guide for current ranges, then compare written scopes line by line.

How to decide

Get readings: crawl space relative humidity, wood moisture content and any sign of liquid water.
Find the moisture source: soil, outdoor air, plumbing leaks or surface water from gutters and grading.
Match the fix to the source. Soil moisture alone may call for a better vapor barrier; humid air, condensation or mold usually call for encapsulation.
Get more than one opinion and compare written scopes.

You can find contractors serving your area through our city pages, such as crawl space encapsulation in Greensboro.