Crawl space encapsulation seals the crawl space into the house instead of venting it to the outdoors. The ground is covered with a Class I liner, every seam lapped and taped, and the liner runs up the foundation walls and is sealed there. The vents are closed, the rim joists air sealed, the walls and rim insulated, and something keeps the air dry: a dehumidifier, a small supply of conditioned air, or another drying method the code lists. A vapor barrier on its own is only the first piece: plastic on the dirt of a crawl space that stays vented.
The code treats the sealed version as a system. In New Jersey, Connecticut and New York alike, a liner and closed vents with nothing drying the space is not a legal unvented crawl space. The order matters too: stop the water, test for radon, check anything down there that burns fuel, then seal, insulate and dry.
Crawl space encapsulation vs. a vapor barrier
“Vapor barrier,” “vapor seal,” “moisture barrier” and “crawl space liner” usually mean the same sheet of plastic; what separates the two jobs is everything around it.
- Vapor barrier only. Plastic covers the ground; the vents stay open and the insulation stays in the floor above. The code does not require a ground cover in a vented crawl space, but it rewards one: with a Class I vapor retarder on the ground, the vent area it demands drops from 1 square foot per 150 square feet of crawl space to 1 per 1,500 (Section R408.2 of the International Residential Code; the IRC sections cited here carry the same number in the 2021 and 2024 editions, and where a number did change between them, both are given). The Department of Energy’s Building America insulation guide says a 6-mil sheet should fully cover the ground of a vented crawl space too.
- Encapsulation. The sheet is sealed at every seam and to the walls or wall insulation, the vents and rim joists are sealed, the walls and rim are insulated, the old floor insulation usually comes out, and a drying method runs year-round. The crawl space is now inside the house.
A ground cover stops moisture rising out of the soil, not the air coming through the vents. Joseph Lstiburek of Building Science Corporation explains that summer ventilation air carries moisture into crawl spaces and deposits it on surfaces colder than its dew point, and describes a vented crawl space with a continuous plastic ground cover, code ventilation and excellent drainage where water still beaded under the floor batts and the joists were rotting. That is why the Department of Energy’s researchers recommend closing, air sealing and insulating the walls of crawl spaces in humid climates outside flood-prone areas. Every county we serve in New Jersey, Connecticut and New York falls in a moist “A” climate zone.
What the code requires for an unvented crawl space
Section R408.3 of the International Residential Code sets the rules, in nearly identical words in each of our states: the 2024 IRC in New Jersey’s one- and two-family dwelling subcode, the 2021 IRC in Connecticut’s 2022 State Building Code, and New York’s 2025 Residential Code, built on the 2024 IRC. An unvented crawl space needs both of these:
- A sealed ground cover. Exposed earth covered by a continuous Class I vapor retarder, joints lapped 6 inches and sealed or taped, edges carried at least 6 inches up the stem wall and attached and sealed to the wall or to the insulation.
- One way to dry the space. An exhaust fan running continuously at 1 cubic foot per minute for every 50 square feet of crawl space floor, with an air path to the house and insulated perimeter walls; conditioned supply air at the same rate, with a return path and insulated walls; the crawl space used as a plenum, allowed only in existing houses; or dehumidification sized to the manufacturer’s specifications.
The energy code sets the wall insulation. It lets crawl space walls go bare only when the crawl space is vented to the outdoors and the floor above is insulated (2024 IECC Section R402.2.11; R402.2.10 in the 2021 edition), so a sealed crawl space gets insulated walls:
- New Jersey, 2024 IECC: R-10 continuous or R-13 cavity insulation in the climate zone 4 counties; R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous in the zone 5 counties (Bergen, Hunterdon, Morris, Passaic, Somerset, Sussex and Warren).
- Connecticut, 2021 IECC, all zone 5, and New York, 2025 energy code, every zone: R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous.
In closed cell spray foam, at R-6.9 to R-7.1 per inch, R-10 takes about 1 1/2 inches and R-15 a little over 2. Foam on crawl space walls can skip the thermal barrier only if the crawl space is entered just for repairs or maintenance and the foam is tested for that use or covered by an ignition barrier (Section R316.5.4 of the 2021 IRC, R303.5.4 of the 2024 IRC). Turn the sealed crawl space into storage and that exception is gone: the foam then needs a thermal barrier over it, 1/2-inch gypsum wallboard, a 23/32-inch wood structural panel, or a material that passes the code’s fire tests (Section R316.4 of the 2021 IRC, R303.4 of the 2024 IRC). More on foam and fire in our guide to whether spray foam is safe.
How thick should a crawl space vapor barrier be?
The code never says. It sets a class, not a mil. Its vapor retarder table (Table R702.7(1), the same in both editions) puts sheet polyethylene, nonperforated aluminum foil and any other approved material rated at 0.1 perm or less in Class I, with no thickness attached. The Department of Energy’s guidance names 6-mil polyethylene, and its Building America Solution Center specifies 6-mil sheeting on the floor and walls of a sealed crawl space.
Heavier liners buy durability, not a better class. One liner maker’s data sheets, both dated June 2026, show it: its 10-mil crawl space liner tests at 0.023 perms and its 15-mil at 0.008, both far inside the Class I limit, while puncture resistance in the same ASTM D4833 test rises from 122 to 198 newtons. Another maker’s reinforced 20-mil liner adds a polyester scrim between its plies, which that maker says greatly enhances tear resistance. So pick 6, 10, 12 or 20 mil by what happens on top of the liner: plain 6-mil where nobody goes after the installers leave, a heavier or reinforced liner where someone kneels to service a furnace or sump pump, or stores boxes.
Seams and edges matter more than the mil number:
- Seams. The code minimum is a 6-inch lap, sealed or taped. The Solution Center recommends 12 inches, with adhesive and tape. Piers and pipe penetrations get wrapped and taped too.
- Edges. The code asks for 6 inches up the wall. The Solution Center carries the liner to within 4 inches of the top of the foundation and holds it with strapping and masonry fasteners. With foam on the walls, the liner can stop lower and seal to the insulation, which the code allows.
First the ground is cleared: the code requires vegetation, organic material, wood forms and construction debris out of the under-floor space (IRC Section R408.5).
Crawl space dehumidifiers: sizing and drainage
Four drying methods are legal, and the choice belongs in the design rather than after the liner is down. A dehumidifier is the common one. Where the house has central air, a small conditioned supply at 1 cubic foot per minute per 50 square feet does the same job, and a continuous exhaust fan at the same rate is the third route. The Department of Energy’s guide notes that the savings from closing a crawl space shrink where local conditions mean supplying warm air to it in winter or running an exhaust fan nonstop, so neither of those two is free. Pick the method first: it decides where a drain line, an outlet or a duct has to go.
Sizing a crawl space dehumidifier
The code sizes the unit by the manufacturer’s specifications, and those specifications differ by maker. One maker rates its 70-pint crawl space model for up to 2,600 square feet tightly sealed, 2,200 moderately sealed and 1,800 loosely sealed; another maker’s 70-pint crawl space manual gives a capacity and an operating temperature range and no floor-area rating at all. Ask which rating a quoted unit was sized on. Capacity itself is measured warm: that 70-pint model removes 70 pints a day at 80°F and 60 percent relative humidity, and 43.7 pints at 65°F. Crawl spaces run cool. ENERGY STAR warns that frost can form on a dehumidifier’s coils below 65°F and suggests a model specified for lower temperatures; the two crawl space units we checked list minimums of 49°F and 50°F.
ENERGY STAR calls 30 to 50 percent relative humidity the optimum and says anything higher may promote mold growth; the EPA says below 60 percent, ideally 30 to 50. The Department of Energy’s guide suggests a wireless humidity sensor in the crawl space paired with an alarm inside the house, so a failed unit gets noticed.
Where the water goes
That 70-pint unit is rated to pull 8.75 gallons a day. ENERGY STAR says a whole-home dehumidifier’s drain should be routed to a floor drain or condensate pump by an HVAC professional during installation. One crawl space unit’s manual runs the line on a constant downward slope to the nearest floor drain or to an outside spot that slopes away from the building, or into a condensate pump with a float switch wired to shut the unit off if water backs up, and it wants a grounded outlet within reach of the cord. Many crawl spaces have neither a drain nor an outlet down there, which is one more reason the drying method gets settled before the liner goes in.
A dehumidifier in a crawl space that is still vented is drying air the vents keep replacing. Seal first, then dry.
Crawl space moisture control starts outside: standing water, grading and gutters
A liner over wet ground keeps the water under the plastic, and a dehumidifier removes vapor, not puddles. Stop the water before anything is sealed.
- Grading. The code wants the ground to fall at least 6 inches in the first 10 feet from the foundation, or drains and swales where that is impossible (IRC Section R401.3, the same in all three states). The EPA’s mold guidance agrees: make sure the ground slopes away from the foundation.
- Gutters. The EPA says to clean and repair roof gutters regularly. A downspout emptying against the foundation feeds the crawl space.
- Groundwater. Where the water table can rise to within 6 inches of the finished floor at the building perimeter, or surface water does not drain from the site, the crawl space grade must be as high as the outside grade unless an approved drainage system is installed (R408.6). The Department of Energy’s guide calls for perimeter drainage, like a basement’s, when the crawl space floor sits below the outside grade.
None of that is insulation work. Who does each piece, and in what order, is set out further down. And once the crawl space is sealed, the Department of Energy’s guide says, keep watching for water.
Radon: why a sealed crawl space changes the picture
A vented crawl space dilutes radon with outdoor air; a sealed one can let it build up. The Department of Energy’s guide puts radon ahead of the work: test first, put in mitigation before closing if the level is high, and test again when the job is done, because sealing and insulating can raise the concentration. The EPA says to fix a home at 4 picocuries per liter or more and to consider fixing between 2 and 4; the Department of Energy’s crawl space advice is stricter, calling for mitigation at 2 or more before closing.
Testing is the cheap part of all this. The Department of Energy’s guide points to low-cost do-it-yourself kits sold in stores, and the EPA says to contact your state radon office for qualified test kits or qualified testers. A zone map does not answer the question for one house: the EPA’s own note on its radon zone maps says the map should not be used to decide whether an individual home needs testing, and that no matter where you live, you should test your home for radon.
Encapsulation and radon mitigation share a membrane. The EPA calls submembrane suction, a high-density plastic sheet over the earth with a vent pipe and fan drawing radon from under it, the most effective way to reduce radon in crawl space homes. What it does not recommend is sealing cracks and openings in the foundation on its own, because sealing by itself has not been shown to lower radon levels significantly or consistently. The Solution Center’s sealed crawl space puts a radon vent under the vapor barrier. So: test; if the number calls for it, a radon mitigation contractor designs the pipe and fan; the liner goes down sealed so the system has something to draw under; test again. Start with your state radon office, the EPA says, since many states license, certify or register radon professionals.
Termites, fuel-burning appliances and flood zones
Termite inspections
New Jersey’s adopted building code rates subterranean termite risk “moderate to heavy” and Connecticut’s “moderate-heavy”; New York leaves the entry to local jurisdictions. R408.3 does not require a termite inspection gap in the liner in any of the three states. The Department of Energy’s guide does say to check with local pest control officials on keeping access for termite inspections, and its Solution Center leaves a strip of bare foundation wall above the wall foam and below the rim joist where the local jurisdiction asks for one: an inspection strip on the wall, not a break in the ground liner. Ask your termite company and the building department before the walls are covered; the 2024 energy code in New Jersey and New York otherwise wants crawl space wall insulation to start at the sill plate. The code’s ban on foam below grade applies only where the termite probability map rates an area “very heavy” (Figure R305.4 in the 2024 IRC; the same map is Figure R318.4 in the 2021 edition Connecticut uses). That band follows the Gulf and the southern coast, and none of our three states is in it.
Furnaces and water heaters
A natural-draft furnace or water heater is more likely to backdraft in a closed crawl space, the Department of Energy’s guide says; it wants oil or gas equipment there to be sealed-combustion units. Gas appliances must also get combustion air by a method in the fuel gas chapter of the residential code (Section G2407, the same number in both editions), and in some houses that air arrives through the crawl space vents. A heating contractor checks the equipment before anything is closed.
Flood zones
In a flood hazard area, walls enclosing an under-floor space must have flood openings, with an exception only for spaces built to FEMA Technical Bulletin 11 (IRC Section R408.7). That works against sealing, and the Department of Energy makes its case for sealing only for homes outside flood-prone areas. Near the shore, a river or low ground, find the flood zone before planning anything else: FEMA’s Flood Map Service Center at msc.fema.gov takes an address, and the town’s building or floodplain office can confirm what the map means for the house.
Encapsulate, or insulate the floor above?
In a humid climate these are not equal choices. The Department of Energy’s guide says a vented crawl space in a humid climate zone is vulnerable to serious moisture problems, and that insulation added to the ceiling of a vented crawl space raises the risk of mold and wood rot: batts between the joists sit cool on the underside, the exposed joist edges cooler still, and below the dew point water condenses. The vented crawl space Building Science Corporation photographed had code ventilation, excellent drainage and a continuous ground cover, and the joists were rotting anyway. Every county we serve is in a moist “A” zone, and the same guide sums up the research this way: closed crawl spaces typically perform better than vented ones for safety, health, comfort, durability and energy use.
So sealing is the default here. Encapsulation earns its cost when:
- ducts, an air handler, a furnace or a water heater sit in the crawl space, because sealing puts them inside the house;
- floor batts keep falling, soaking or growing mold, or joists and ducts sweat in summer;
- the water can be stopped, the house is outside a flood hazard area, and the radon test is done.
Insulating the floor above fits a narrower set of houses, and every one of them carries conditions. A house in a flood hazard area is the clear case: flood openings and a sealed crawl space do not go together. Otherwise the floor route needs a crawl space that stays dry through the summer, with no standing water and no condensation on pipes, ducts or joists, that drains well and holds no heating equipment, with the ground covered and the subfloor, rim joists and penetrations air sealed before any insulation goes back. Where the space runs damp in summer, the Department of Energy’s guide shows two assemblies that hold up where batts do not: closed cell spray foam encasing the floor framing, or foil-faced rigid board across the underside of the joists with every seam foil-taped, both of which it says work in any climate zone. Without those conditions, new floor insulation repeats the failure it replaced. The floor option is covered in our crawl space insulation guide.
What crawl space encapsulation costs
Most of our projects run between $3,500 and $12,500. On an encapsulation that range buys our part of the job: the ground liner, the wall and rim insulation, the air sealing, the vents and access door, and the old insulation and debris that comes out. Within it, the number moves with the floor area and wall height, the liner weight, how much has to be removed, the number of vents and piers, and the headroom. The drying equipment and the other trades named below are priced on top of it.
National cost guides quote the whole job, with several trades folded in:
- HomeAdvisor (updated 20 June 2026): $5,000 to $15,000, averaging $5,500; $2 to $4 per square foot for basic encapsulation and $3 to $10 for more extensive work.
- HomeGuide (updated 14 January 2026): $3 to $10 per square foot, or $3,000 to $15,000; a vapor barrier alone about $2 to $4 per square foot installed; a crawl space dehumidifier $1,000 to $3,000 installed; an interior drain $800 to $3,000. Liner material runs $0.30 to $0.35 a square foot for 6-mil and $0.60 to $0.70 for 20-mil.
State and utility programs exist; see New York and New Jersey insulation rebates and Connecticut insulation rebates. The federal 25C tax credit ended for anything placed in service after 31 December 2025.
How to judge a crawl space encapsulation quote
Whichever company you hire, a good quote reads like the code section. Check that it names:
- The liner: Class I, thickness, reinforced or not, seam lap and tape, how far up the walls it runs and how it is fastened, and how piers are handled.
- The walls and rim joists: the insulation, its R-value against your state’s number, and its ignition barrier or exposure rating.
- Air sealing: rim joists, sill plate, and holes in the subfloor for pipes, wires and ducts. See air sealing.
- Vents and access: every vent closed and sealed, the access door insulated and weatherstripped.
- The drying method: which of the code’s four, who installs it, its sizing, its drain and its power.
- The water: what was found and who fixes it before the liner goes down.
- Radon and combustion: a test before and after, and a check of any fuel-burning appliance.
- What comes out: old floor insulation, debris, and any mold, which goes to a remediation company first.
Two shortcuts to refuse. A quote that lists a liner and a dehumidifier and never mentions the rim joists, the vents or the walls is a vapor barrier job at an encapsulation price. And a plan to seal over mold with a coating ignores the EPA’s advice not to paint or caulk moldy surfaces: clean up the mold and dry the surfaces first.
Common questions about crawl space encapsulation
What is crawl space encapsulation?
It is the sealed, unvented version of a crawl space: the vents close and the space is brought inside the house instead of being treated as outdoors. That takes a continuous Class I liner sealed over the ground and up the foundation walls, air sealing at the rim joists and vents, insulation on the walls and rim instead of the floor above, and a drying method running year-round. Section R408.3 of the International Residential Code, the same in the 2021 and 2024 editions, requires the liner and one of four drying methods together.
What is the difference between crawl space encapsulation and a vapor barrier?
A vapor barrier is a sheet of plastic on the ground of a crawl space that stays vented, with the insulation left in the floor above. Encapsulation seals that sheet at every seam and to the walls, closes the vents, insulates the walls and rim joists, and adds a drying method. The ground cover stops moisture rising from the soil; only the sealed version keeps humid summer air out.
How much does crawl space encapsulation cost?
Most of our projects run between $3,500 and $12,500, and on an encapsulation that range covers our part only: the liner, the wall and rim insulation, the air sealing, the vents and the removal of old insulation. Drying equipment, drainage, electrical, radon and mold work are priced separately by those trades. National guides quote the whole job with the trades included: HomeAdvisor, updated June 2026, gives $5,000 to $15,000 with an average of $5,500, and HomeGuide, updated January 2026, gives $3 to $10 per square foot.
Does an encapsulated crawl space need a dehumidifier?
It needs a drying method, and a dehumidifier is one of four the code accepts; conditioned supply air, a continuous exhaust fan and, in an existing house, a plenum are the others. The code sizes the unit to the manufacturer’s specifications, and one maker rates its 70-pint crawl space model by floor area and how tightly the space is sealed. Choose a model rated for cool temperatures, set it for 30 to 50 percent humidity, and drain it to a floor drain, the outside or a condensate pump.
Is a 20 mil vapor barrier better than 6 mil?
It is tougher, not more code compliant. The code asks for a Class I vapor retarder, 0.1 perm or less, and names no thickness; the Department of Energy’s guidance uses 6-mil polyethylene. Thicker and reinforced liners resist punctures and tears better, which matters where people crawl, service equipment or store things. HomeGuide prices 20-mil material at about twice the cost of 6-mil.
Does crawl space encapsulation help with radon?
Not by itself, and sealing can raise radon levels. A vented crawl space dilutes radon and a sealed one can concentrate it, which is why the Department of Energy wants a test before sealing, mitigation first if the level is high, and a second test afterward. Low-cost do-it-yourself kits are sold in stores, and your state radon office can point you to qualified test kits or testers. The EPA’s most effective crawl space method, submembrane suction, draws radon from under a plastic sheet on the ground, so a sealed liner and a radon system work together.
Can you encapsulate a crawl space with standing water or mold?
Not until the water is stopped. A liner over wet ground holds the water under it, and a dehumidifier handles vapor, not puddles. Grading and drainage go to a waterproofing or excavation contractor, sump pumps to a waterproofing contractor or plumber, leaks to a plumber, and gutters to a gutter or roofing company. Mold on the framing goes to a remediation company; the EPA says not to paint or caulk moldy surfaces, so coating mold is not a fix. Taking out the wet or failed insulation itself is our work.
Should I encapsulate my crawl space or insulate the floor above it?
Encapsulate when ducts or heating equipment sit down there, when floor batts keep failing, and when the water can be stopped and the house is outside a flood hazard area. The floor above suits a house in a flood zone, or a crawl space that stays dry through the summer, drains well and holds no equipment, with the ground covered and the subfloor air sealed, and closed cell foam or foil-faced rigid board instead of batts if it ever runs damp. The Department of Energy warns that in a humid climate insulation in the ceiling of a vented crawl space raises the risk of mold and rot, so those conditions are not optional.
Is basement encapsulation the same thing?
No. The phrase usually describes waterproofing a basement with drainage, a sump, wall panels and a dehumidifier, which is a waterproofing contractor’s trade. Insulating a basement is a different assembly: the residential code exempts basement walls from its vapor retarder rule (Section R702.7 in both editions), and the Department of Energy says an insulated basement wall must be able to dry inward, so no plastic sheeting over fiberglass. We insulate basement walls with closed cell foam or rigid board.
Crawl space encapsulation contractors in New Jersey, Connecticut, the Hudson Valley and Long Island
We do the building-envelope side of an encapsulation with our own crew: removing wet or failed floor insulation under containment into a HEPA insulation vacuum, clearing debris, air sealing the rim joists, sill and subfloor, closing and sealing the vents, insulating the walls and rim with closed cell spray foam or rigid foam board, installing and sealing the ground liner, and insulating the access door. The drying method is part of that design, planned with you from the start.
The rest of the work is contracted by you, not through us: our crew never subcontracts, so the HVAC contractor who installs the dehumidifier or supply duct, the electrician who adds the outlet, the waterproofing contractor or plumber who handles grading, drainage, sump pumps and leaks, the radon mitigation contractor and the mold remediation company are each hired and paid by you directly. What we do give you after the inspection is the order, because it decides whether the job works: stop the water first; test for radon and have mitigation installed before closing if the number calls for it; have a heating contractor check any fuel-burning appliance; then our part, with the liner sealed over the radon piping; then the electrician’s outlet and the HVAC contractor’s dehumidifier or supply duct; then a second radon test.
Before we price a crawl space, we crawl it: the ground, any water and its source, the vents and rim joists, the floor insulation, the ducts, pipes and equipment. The energy audit is free. Call 914-825-6824 or use the contact form, 8am–8pm, seven days. The Home Insulators is family-owned, based in New Rochelle, NY since 2015, with a 100-plus-person in-house crew that never subcontracts. We work across all of New Jersey, all of Connecticut, New York’s Hudson Valley including Westchester, and Long Island; see our service area.