Encapsulating a Crawl Space

Encapsulation is a way of extending a home’s insulation and vapor barrier envelope to include the crawl space. Done properly, encapsulation protects the floor above from moisture, rot, and pest infestation.

Crawl Space Encapsulation

What Exactly Is Crawl Space Encapsulation?

Encapsulation seals the crawl space from the outside environment by cleaning, sealing, adding vapor barrier, and insulating the walls. The result is a cleaner dryer area that resists moisture, mold, mildew, rot, and pests.

Crawl space encapsulation and crawl space insulation are often used interchangeably. They are not the same process. Encapsulation seals the entire area. Insulation only deals with the underside of the floor above.


The benefits of crawl space encapsulation are many and varied.

  • Structure. Prevents rot and insect damage to framing.
  • Moisture. Keeps the entire space dry.
  • Pests. Eliminates pests like termites and rats.
  • Comfort. Living area remains warmer.
  • Mold. Eliminates mold and mildew growth.
  • Energy Costs. Keeps out cold and saves energy costs.
  • Storage. Provides warm dry storage area.
  • Property Value. Increases resale value.


The only significant disadvantage of encapsulation is cost. It is more expensive than crawl space insulation. You may need to hire contractors to deal with electrical, plumbing, or HVAC repairs or changes. Some jurisdictions require a permit to encapsulate a crawl space. Permitting usually relates to eliminating conventional crawl space ventilation and should be less than $250.00.

How to Encapsulate a Crawl Space

Before starting the work, inspect the entire crawl space for rot, mold, water, gaps, cracks, and holes. Plan to address all these issues first.

Clean and Seal

Most crawl space floors are dirt. Not much care is taken to make them level. Often garbage that falls inside is just left there. Homeowners toss stuff inside and forget about it. Clean everything out–including any old insulation from under the floor above. You do not want to insulate the floor when encapsulating the crawl space.

The floor should be as dry as possible before beginning work. If there is a lot of standing water, dig a hole for it to run into and pump it out. If the floor is only damp, set up a fan to dry it. Consider installing a sump pump if the water table is always high or you live in an area prone to floods.

Use low-expansion spray foam in a can and/or caulking to fill and seal every gap and crack you found. Fill all wall and wood subfloor penetrations. Including plumbing, electrical, and venting. Install compression weatherstrip on the access and make sure it seals and latches tightly when closed.

Level the Floor

Make the floor as level as possible. Use a short-handled spade to level off humps and fill in depressions. Installing poly on a smooth and level surface is easier. It seals better and is less likely to tear.

Insulate the Rim Joist

Rim joists are the pieces of wood that close off the ends of floor joists. Usually, it’s OSB or solid wood. The R-value of 1 ½” softwood is R-1.41. Rim joist insulation is essential when encapsulating a crawl space.

Install 2” extruded polystyrene (R-5.0 per inch) against the rim joist. Seal gaps with spray foam in a can or with acoustic caulking. Alternatively, buy a DIY spray foam kit and apply foam to the rim joist and sill plate. Spray foam does a better job of sealing but costs more. It provides an excellent air barrier.

Both types of insulation form a vapor barrier if they are 2” thick–or more. Caulk the sill plate where it meets the concrete foundation.

Install Vapor Barrier On the Floor

Dirt floors are almost invariably a source of moisture in crawl spaces. Clogged gutters, short downpipe runoffs, and improperly sloped landscaping all contribute to water pooling against the foundation and making crawl space floors wet. Make sure to address these problems before installing a vapor barrier on the floor.

Roll out heavy-duty poly on the floor. Six mil poly is a minimum; twelve mil is better. (One mil = one-thousandth of an inch.) Heavy poly lasts longer and resists tearing when walked on. Use landscaping fabric staples to secure the plastic to the dirt floor. Overlap the poly sheets at least 6” and tape them with vapor barrier tape.

Leave 6” of extra poly to extend up all of the walls. Run a heavy bead of acoustic caulking about 5” from the floor and embed the poly into it. The caulking holds the poly in place and forms a seal.

Insulate The Walls

Cut and install 2” extruded polystyrene (XPS) rigid insulation boards. Glue them to the wall using foamboard adhesive. Extend the XPS at least 2” over the floor poly attached to the wall. Four inches is better. A level floor makes wall insulation easier. All pieces can be cut to the same size. Two inch foam acts as a vapor barrier. Seal all gaps between the foam with spray foam. (Acoustic caulking also works well but never dries.) XPS is R5.0 per inch.

Using spray foam to cover the entire wall two inches thick is another excellent option. Using DIY spray kits will increase costs. Hiring a spray foam contractor may cost more because of the size of the job.

Install a Dehumidifier

Regardless of how well a crawl space is encapsulated, humidity will eventually get inside. Humidity increases the chance of rot on the subfloor and framing, It causes condensation on HVAC ducts that can make puddles on the poly floor–possibly inviting mold and mildew growth.

The recommended humidity for the space is 30% – 60%. Install a stand-alone dehumidifier or extend a vent into the crawl space if you already have a whole house system.

Cost of Encapsulating a Crawl Space

The material required to encapsulate a crawl space should cost less than $1500.00–not including a dehumidifier. Encapsulation is a fairly simple but labor-intensive DIY project. Repairing damaged or rotted framing takes more time and money. It also may require a professional.

Contractor encapsulation can cost as little as $1500.00 and as much as $15,000.00. The average cost is about $5500.00. Higher costs usually involve things like framing repairs, mold removal, pest removal, etc.