Crawl space vapor barrier vs. encapsulation

Many homes in North Carolina, Georgia and Texas sit over a crawl space: a short space between the ground and the first floor. In a humid climate, that space can stay damp for much of the year. Damp crawl spaces lead to wood rot, mold, sagging insulation, rusty pipes and pests.

There are two common ways to fight that moisture. A vapor barrier covers the dirt floor with plastic sheeting. Encapsulation goes further: it seals the whole crawl space off from the outdoors and dries it with a mechanical system. This guide explains the difference and how to choose. To price a vapor barrier, see crawl space vapor barrier cost.

Step one for either: fix the water

Neither a vapor barrier nor encapsulation fixes water pouring into a crawl space. The Department of Energy's Building America program is clear that moisture problems must be dealt with before a crawl space is sealed and insulated [1].

Common water sources and fixes [1, 2]:

• Soil that slopes toward the house. Regrade so water runs away from the foundation.
• Gutters and downspouts. Extend downspouts so they drain away from the house, not next to it.
• Groundwater. Footing drains or a sump pit with a sealed lid may be needed. A sump pump alone may not solve water coming through the walls [1].
• A dryer vent that ends in the crawl space. It dumps warm, wet air under the house and has to be rerouted outside [2]. See dryer vent safety.

Signs of a moisture problem include standing water, puddles, water stains, white mineral deposits on the walls, damp insulation, mold or mildew, and rusty pipes and nails [2]. A HomeCloud evaluation notes puddles on the vapor barrier when we see them. Standing water is usually a drainage or grading fix first: see the foundation and structure guides.

What a vapor barrier does

A vapor barrier (also called a vapor retarder or ground cover) is plastic sheeting laid over the dirt floor. Bare soil gives off moisture all the time. The sheeting stops most of it from rising into the crawl space air, where it would soak into the wood framing and insulation.

A good vapor barrier, per Building America guidance [1]:

• Is polyethylene sheeting at least 6 mil thick, covering the entire floor.
• Runs 8 to 12 inches up the foundation walls and piers, or up to the outside ground level.
• Has seams overlapped 6 to 12 inches and taped.
• Is fastened and sealed to the walls and piers so it stays put.

Thicker sheeting holds up better when people crawl over it to service pipes, ducts and wiring.

A vapor barrier on its own leaves the foundation vents open. The crawl space is still connected to the outdoor air. That's the traditional setup in many older Southern homes. Common findings include no vapor barrier at all, bare ground showing and torn sheeting.

What encapsulation adds

Encapsulation (also called a closed, sealed or unvented crawl space) treats the crawl space as part of the house instead of part of the outdoors. A typical job includes [3]:

1. A sealed ground cover that runs up the walls, with seams overlapped and sealed. Building America recommends a 12-inch overlap, more than the 6 inches the model code requires [3].
2. Sealed foundation vents and a weather-stripped access door.
3. Insulation on the walls instead of under the floor. Rigid foam board is common because it handles moisture well. With the walls insulated, the floor above usually doesn't need insulation [3].
4. A drying system. A sealed crawl space still needs a way to remove moisture. Building codes require one, such as a dehumidifier, a small supply of conditioned air from the HVAC system, or an exhaust fan [3, 4].

Building America research found that unvented, conditioned crawl spaces cut crawl space humidity by more than 20 percent and saved 15 to 18 percent on heating and cooling in the homes studied [5]. One reason: in humid weather, outdoor air coming through open vents can be wetter than the crawl space air, and it can condense on cool floor framing [3, 5].

Rules differ by state

North Carolina has its own code section for closed crawl spaces. It requires mechanical drying and lists five ways to do it: a dehumidifier, conditioned supply air from the HVAC system, air from the house, an exhaust fan, or making the crawl space part of the home's conditioned space. State code officials note that the supply-air and conditioned-space methods need a heating contractor with the right state license [4].

Georgia and Texas codes are generally based on the International Residential Code, the national model code. It also requires a drying method for an unvented crawl space [3]. States, cities and counties can amend it.

In termite-prone areas, codes may also require a strip of bare foundation wall left uncovered so a pest company can inspect for termites [3]. Radon and flood rules can apply too.

Vapor barrier vs. encapsulation at a glance

	Vapor barrier	Encapsulation
What it covers	The dirt floor	Floor, walls, vents and access door
Foundation vents	Stay open	Sealed
Insulation	Under the floor, between joists	On the walls
Drying system	None	Required, often a dehumidifier
Ongoing upkeep	Patch tears	Run and maintain the drying system
Cost	Lower	Higher

How to choose

HomeCloud looks at crawl space moisture in three levels, from basic to premium:

1. Vapor barrier. Plastic sheeting over the dirt floor, after any water coming in is fixed. This is where most homes start.
2. Seal and dehumidify. For a crawl space that stays damp: close the vents and other openings to outside air and add a dehumidifier.
3. Full encapsulation. The premium level: the vapor barrier runs over the floor and up the foundation walls with overlapped, taped seams, every foundation vent is closed, the access door is sealed, the walls and rim joist are insulated, and a dehumidifier keeps the air dry, as described above.

A vapor barrier alone may be enough when:

• The crawl space is dry, with no standing water or wet wood.
• The floor insulation and framing are in good shape.
• There's no ductwork or HVAC equipment in the crawl space.

Sealing or full encapsulation is worth pricing when:

• The crawl space feels damp or smells musty in summer, even with a vapor barrier.
• Floor insulation keeps falling or gets wet.
• Ducts or the furnace or air handler sit in the crawl space, where sealing helps protect them and keeps them in a drier space [3].
• You plan to stay in the home long enough to benefit from the comfort and energy savings.

Encapsulation also adds a small ongoing cost. A dehumidifier uses electricity, needs its filter cleaned and needs a working drain line. Building America suggests watching the crawl space for about a year after a conversion, including checking the moisture in the floor framing [1].

If you're keeping a vented crawl space, keep the vents in good repair. See foundation vent repair and crawl space floor insulation. For more on wall and floor insulation types, read types of insulation.

HomeCloud is a licensed general contractor. We evaluate homes and price and do the repairs, including crawl space work.

Sources

1. Building America Solution Center: Water management of existing crawlspace floor
2. Building America Solution Center: Look at crawlspace for moisture issues
3. Building America Solution Center: Unvented, insulated crawlspaces
4. North Carolina Office of State Fire Marshal: Closed crawl space moisture control (2019)
5. U.S. Department of Energy: Unvented, conditioned crawlspaces (Building America top innovation)