Crawl space encapsulation: what it is and when it's worth it

What encapsulation includes

When I think about encapsulating a crawl space, I start with what needs to happen before any plastic goes down. A complete job has three parts: find and fix the source of the moisture, seal the crawl space, and keep it dry.

Inspection and prep

• Check for water and measure humidity, including moisture readings in the wood framing.
• Check plumbing and HVAC condensate lines for leaks.
• Look for rot, mold and termite damage.
• Remove wet insulation and debris.
• Fix any liquid water first. See water in the crawl space.

Sealing the space

• A continuous liner over the entire floor, with seams overlapped and sealed. See vapor barriers.
• Sealed around piers, pipes and other penetrations.
• Run up the foundation walls and fastened to them.
• A strip of wall left visible below the sill plate so the wood can be checked for termites.
• Vents sealed once the design becomes a closed crawl space. Flood vents are the exception; more on that below.
• An access door that seals but can still be opened for service.

Keeping it dry

• A drying system, which is usually a dedicated dehumidifier. See crawl space dehumidifiers.
• A permanent drain for the dehumidifier, never a bucket.
• Final humidity and wood moisture readings, so you have a baseline.

1. No vents in the foundation walls
2. Insulation on the walls instead of the floor
3. Liner over the ground and up the walls, sealed at the seams
4. Dehumidifier
5. Conditioned air from the house

Is crawl space encapsulation worth it?

This will depend on your current situation. Here are the questions to ask, in order. Stop at the first one that applies.

1. Is there standing water, or water after heavy rain? Fix the water first. Encapsulating a wet crawl space hides the problem behind the liner.
2. Is the house in a flood zone? Talk to your local building department before closing anything. The Department of Energy’s Building America Solution Center says non-vented, conditioned crawl spaces should not be used in flood-prone areas.
3. Does the crawl space stay dry, with humidity under control and water managed around the house? A good ground cover with working vents may be all you need. Encapsulation isn’t required just because it’s popular.
4. Does humidity stay high for long stretches, is the floor bare dirt, or does condensation keep coming back on ducts and pipes? This is where encapsulation makes the most sense, especially in humid climates and when ductwork runs through the crawl space.

Studies by the U.S. Department of Energy’s Building America program have shown that in humid climates, closed, conditioned crawl spaces used 15% to 18% less energy for heating and cooling than vented ones and reduced humidity by more than 20%.

Does it add value to a home? We haven’t found any studies that show how much encapsulation increases a home’s value, so we can’t give you a number. But it can help with issues that come up in a home inspection, such as a musty odor, high humidity and wet insulation.

What building codes require

Most states base their residential code on the International Residential Code (IRC), but each one has its own version and amendments, so contact your local building department. As an example, here’s what the 2021 Virginia Residential Code, in effect since January 18, 2024, requires for an unvented crawl space (Section R408.3):

• Ground cover: exposed earth covered with a continuous Class I vapor retarder. Seams overlap by 6 inches and are sealed or taped, and the edges run at least 6 inches up the wall and are sealed to it.
• A way to keep the space dry, one of these:
  • continuous exhaust ventilation of 1 cubic foot per minute for each 50 square feet of floor, with an air path to the house and insulated walls;
  • conditioned air supplied at the same rate, with a return path to the house and insulated walls;
  • in existing buildings, using the crawl space as a plenum under specific conditions;
  • dehumidification sized at 70 pints of water a day for every 1,000 square feet of crawl space floor.
• Termite inspection gap: an unobstructed strip of 1 to 2 inches below the sill plate along the top of any interior wall covering, with some exceptions (R408.3.1).
• Flood hazard areas: the walls need flood openings that let water in and out (R408.7).

For a 1,500-square-foot crawl space, the dehumidification option works out to at least 105 pints a day.

Before you seal it

Always address water first. Then here’s what I look for before encapsulating a crawl space:

• Slope of the ground outside, gutters and downspouts
• Water table and any history of flooding
• FEMA flood zone
• Plumbing and HVAC condensate drains
• Moisture in the wood, and any structural damage
• Termites
• Radon
• Electrical wiring in the crawl space
• The sump pump, if there is one
• Combustion appliances

Two of these deserve a closer look.

Gas or oil appliances in the crawl space. If you have a water heater or furnace with a natural draft vent in your crawl space, don’t seal it until combustion and combustion air have been checked. According to ENERGY STAR, sealing your home can cause naturally vented combustion appliances to backdraft, pulling exhaust gases back into the house.

Flood zones. I like to be extra cautious with homes in a Special Flood Hazard Area. If your crawl space is below the Base Flood Elevation, it may need openings that let floodwater in and out to equalize the pressure on the walls. These aren’t ventilation vents, and you can’t just seal them.

Testing for radon before you encapsulate is also a good idea, so you know where you stand. Radon is a gas that moves up through the ground and into homes through cracks and other openings in the foundation, and the EPA notes that any home may have a radon problem, whether it’s new or old, drafty or well sealed.

Common mistakes

Apart from encapsulating before fixing the water, these are the problems I find most often in other companies’ work:

• A liner that’s too thin for people to walk and work on for years
• Seams that aren’t sealed, or plastic just leaning against the wall
• Tape stuck to block that was damp or dusty
• Piers left unsealed
• Vents closed with no drying system added
• Wet fiberglass insulation left in place, or damp wood covered before it dried
• The termite inspection strip covered up
• A dehumidifier that’s too small for the space
• Condensate draining back into the crawl space
• No access left to service the sump pump or dehumidifier
• An open sump pit ignored
• A home in a flood zone encapsulated as if it weren’t

DIY or hire it out?

If you’re handy, you can install a ground cover yourself; see our guide on how to install a vapor barrier. A full encapsulation is more involved. You’ll need to seal vents and air leaks, have any combustion appliances checked, and install and drain a dehumidifier, which often means electrical work by a licensed electrician. If you’re going to do some of this yourself, don’t seal all of your vents until you know how you’re going to keep the crawl space dry.

Reading a quote

When you get a quote for encapsulation, you should see each line item, not just “Encapsulate crawl space: $9,000.” If one company says “encapsulation” and another lists a dozen line items, you can’t compare prices until you know what the first one includes. Ask for:

• Square footage covered
• Prep and debris removal, and removal of old insulation
• Any mold, drainage or sump pump work
• Liner type and thickness, and whether walls and piers are included
• Sealing of vents and penetrations
• Wall insulation
• Dehumidifier model and capacity, its drain and any electrical work
• Warranties and maintenance

For typical prices, see crawl space encapsulation and repair costs.

After encapsulation: maintenance

Your crawl space will still need some maintenance once it’s encapsulated, just not as much.

First year: take a look at each change of season and after heavy storms.

After that, twice a year:

• Check the humidity reading
• Look for water on top of the liner
• Check the seams and where the liner is fastened to the walls
• Look for signs of pests
• Test the sump pump, if there is one
• Make sure the dehumidifier is draining
• Clean or replace the dehumidifier filter

Have the dehumidifier coils serviced once a year. I also recommend a remote hygrometer you can read from inside the house. That way you know something is going on before you smell it.