The best insulation for a crawl space is closed cell spray foam or rigid foam board on the walls and rim joists of a sealed crawl space, or batts held tight to the subfloor above a vented one that stays dry. A dirt floor doesn’t change that: nothing goes on the dirt, which gets covered first, and in a sealed crawl space the cover must be a continuous, sealed vapor retarder.
Which plan fits depends on a decision you make first: where the insulation goes. In a vented crawl space it goes in the floor above, between the joists, and the crawl space stays outside the house. In a sealed, unvented crawl space the vents are closed and the insulation goes on the foundation walls and rim joists, so the crawl space becomes part of the house.
Moisture comes before the material and the plan. Fiberglass batts between the joists of a vented crawl space that runs damp in summer are the assembly the Department of Energy warns about, and the same guidance says closing crawl space vents without air sealing is likely to make moisture levels higher.
Does a crawl space need insulation?
For new construction, yes, in one of two places. The energy code requires crawl space walls to be insulated unless the crawl space is vented to the outdoors and the floor above is insulated instead (Section R402.2.11 of the 2024 IECC, R402.2.10 in the 2021 edition). The floor or the walls, never neither.
An older house isn’t forced to add it, but an uninsulated crawl space means cold floors, heat lost downward all winter, and pipes and ducts at outdoor temperature. In an older house it is worth checking for batts lying on the dirt, or nothing in the floor at all.
Vented or sealed: where crawl space insulation goes
Insulate the floor above a vented crawl space when
- The crawl space stays dry, with no standing water and no summer condensation on pipes and ducts.
- No ducts, air handler, furnace or water heater sits in it, and the water pipes can be insulated.
- The house is in a flood hazard area, where the code requires flood openings in the walls around an under-floor space (IRC Section R408.7).
Seal the crawl space and insulate the walls when
- Ducts, an air handler or a water heater are down there. Sealing brings them inside the house.
- Floor batts have gotten wet, moldy or fallen before, or hardwood floors above buckle and warp.
- The house is outside a flood zone and the ground can be kept dry.
The Department of Energy’s Building America researchers lean toward sealing in humid climates. Their guidance says a vented crawl space in a humid climate is vulnerable to serious moisture problems, that adding insulation to the ceiling of a vented crawl space increases the risk of mold and wood rot, and that outside flood-prone areas the vents should be closed, the crawl space air sealed, and the walls insulated. Their research partners found that 40 to 50 percent of the air in a typical home has been in the crawl space at one time. Every county we work in is in an “A”, or moist, climate zone.
Sealing is not automatically right. It costs more, needs a sealed liner and a drying mechanism, and changes how radon and fuel-burning appliances behave. A dry, vented crawl space with an air-sealed, well-insulated floor is a sound job.
How to insulate a crawl space with a dirt floor
Insulation doesn’t go on the dirt, spray foam included. The dirt gets covered, and the insulation goes in the floor above a vented crawl space or on the walls and rim joists of a sealed one. The code rewards covering it: put a Class I vapor retarder over the ground of a vented crawl space and the vent area it requires drops to one tenth (IRC Section R408.2). The order of the work matters more than the material:
- Stop the water. Standing water or wet soil gets fixed before anything else, because a liner over wet ground holds the water under it. The EPA says to keep gutters clean and repaired and the ground sloping away from the foundation, and where the crawl space floor sits below the outside grade the Department of Energy calls for perimeter drainage, just like a basement. Grading and drainage are work for a waterproofing contractor and leaks are a plumber’s, hired by you directly before our work starts.
- Choose vented or sealed, using the tests above.
- Test for radon before sealing. The Department of Energy says to test before closing a crawl space, install mitigation first if the level is high, and test again after the work.
- Take out what has failed. Wet, failed or mouse-soiled insulation comes out first. Taking it out is our work, under containment and into a HEPA insulation vacuum; mold on the framing goes to a remediation company, and rotted framing to a carpenter.
- Cover the dirt, as described just below. We install the liner ourselves.
- Air seal the rim joists and sill plate, the holes in the subfloor and the access hatch, as described under air sealing below.
- Insulate. In a vented crawl space, the floor above, to R-19 or R-30 depending on where you live, in full contact with the subfloor. In a sealed one, closed cell spray foam or rigid foam board on the walls from the sill plate down, and on the rim joists.
- Keep a sealed crawl space dry, and watch it. A sealed crawl space needs one of the code’s drying methods: a supply register or exhaust fan is HVAC work, and a dehumidifier is equipment we can walk you through; see crawl space encapsulation. The Department of Energy says to monitor for water after the work and suggests a wireless humidity sensor paired with an alarm inside the house. In a vented crawl space, look again after the first humid summer, even if it was dry before the batts went in: damp batts or water on the pipes and ducts mean the floor needs a different assembly.
Covering the dirt: the crawl space vapor barrier
- Sealed crawl space: exposed earth covered with a continuous Class I vapor retarder, joints overlapped 6 inches and sealed or taped, edges run at least 6 inches up the stem wall and sealed to the wall or the insulation (IRC Section R408.3).
- Vented crawl space: the IRC doesn’t require a ground cover, but it rewards one. Without it, vents must total at least 1 square foot for every 150 square feet of crawl space, with an opening within 3 feet of each corner. With a Class I vapor retarder on the ground, vent area can drop to 1/1,500 of the floor area (IRC Section R408.2). The Department of Energy’s guidance calls for 6-mil polyethylene covering the ground even in a vented crawl space.
How thick should a crawl space vapor barrier be? Section R408.3 specifies a class, not a thickness; the 6-mil polyethylene above is the Department of Energy’s figure. Heavier reinforced liners hold up better where people crawl over them. We choose the weight to suit how the crawl space gets used.
What holds up in a damp crawl space, and what fails
- Holds up: closed cell spray foam on the walls and rim joists of a sealed crawl space, or under the floor of a vented one when it is sprayed to encase the joists, not just the subfloor between them. It air seals as it goes on. The Department of Energy’s guidance says the foam should completely encase the floor framing, and that at least 3 inches of high-density closed cell should provide an adequate vapor retarder under any floor covering. Building Science Corporation says foam only between the joists works only where the crawl space is open to the outside.
- Holds up: rigid foam board. Extruded or expanded polystyrene on the walls with the seams sealed, or foil-faced board across the bottom of the joists with every seam taped, an assembly the Department of Energy says will work in any climate zone.
- Dry crawl spaces only: batts and blown-in cellulose in the floor above. Mineral wool is water resistant, and one maker says stone wool does not rot or promote mildew or mold; it is still a batt between the joists, so the same rule applies.
- Fails: fiberglass batts between the joists of a damp, vented crawl space. The Department of Energy explains that the batts are cool on the underside, the bottom edges of the joists cooler still, and below the dew point water condenses: the damp batts insulate poorly, mold can grow and the joists can rot. A ground cover alone doesn’t prevent it. Building Science Corporation published a photograph of water droplets on the underside of fiberglass batts in a crawl space with code-specified ventilation, a continuous ground cover and excellent drainage, and blamed the floor insulation itself: with batts in place, the underside of the batts and the exposed bottom edges of the joists sit below the dew point of the summer air coming through the vents.
- Fails: fiberglass or cellulose against the walls. The Department of Energy says fiber insulation should not be used against a crawl space wall because it collects and holds moisture.
- Misplaced: plastic stapled across the bottom of the joists. A batt’s vapor retarder goes on the warm side, up against the subfloor in a vented crawl space, and the ground cover goes on the ground. Foil-faced rigid board in the same spot is a different assembly: Building Science Corporation explains that the board keeps the wood warm and therefore dry, while the foil handles the vapor. Plain plastic adds no insulation.
What the code requires for an unvented crawl space
An unvented crawl space is allowed under Section R408.3 of the International Residential Code, in the 2021 edition Connecticut uses and the 2024 edition New Jersey and New York adopted. It requires:
- A ground cover: the continuous, sealed Class I vapor retarder described above.
- One drying mechanism: continuous exhaust ventilation at 1 cubic foot per minute for each 50 square feet of crawl space floor, with an air pathway to the house and insulated perimeter walls; a conditioned air supply at the same rate, with a return pathway and insulated walls; the crawl space used as a plenum in an existing house; or dehumidification sized to the manufacturer’s specifications.
The energy code sets the wall insulation. Under the 2024 IECC in New Jersey, crawl space walls need R-10 continuous or R-13 cavity insulation in climate zone 4, and R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous in zone 5. Connecticut (all zone 5 under the 2021 IECC) and New York (every zone under its 2025 code) use the zone 5 numbers. The 2024 IECC wants the insulation fastened to the wall from the sill plate down to the top of the footing, or down to the crawl floor when it is on the inside and the floor is more than 24 inches below outside grade (Section R402.2.11.1).
Fire code for foam in a crawl space
No thermal barrier is needed over foam in a crawl space where the crawl space is entered only for repairs or maintenance and the foam is either tested for the use or covered by an ignition barrier, such as 1 1/2-inch mineral fiber insulation, 1/4-inch wood structural panel, 3/8-inch gypsum board or 1/4-inch fiber-cement board (Section R316.5.4 of the 2021 IRC, R303.5.4 of the 2024 IRC). A crawl space used for storage doesn’t fit that exception. At the rim joist, foam up to 3 1/4 inches thick that meets the code’s density and flame-spread limits can stay exposed (R316.5.11, R303.5.11).
Insulating the floor above a crawl space
Should you insulate between the floor joists in a crawl space? Yes, if it is vented. The floor follows the code numbers for any floor over unheated space: under the 2024 IECC, R-19 in New Jersey’s zone 4 counties (Atlantic, Burlington, Camden, Cape May, Cumberland, Essex, Gloucester, Hudson, Mercer, Middlesex, Monmouth, Ocean, Salem and Union) and R-30 in the rest of the state; R-30 across Connecticut; and R-30 in every zone under New York’s 2025 code. ENERGY STAR’s recommended levels for existing homes, which it bases on the 2021 IECC, are R-19 for floors in zone 4A, which takes in New Jersey’s zone 4 counties, Westchester and Long Island, and R-30 in zones 5 and 6; New York’s 2025 code asks for R-30 on new work in Westchester and on Long Island anyway. Is R-19 good for a crawl space? It meets code in New Jersey’s zone 4 counties; everywhere else we work, the target is R-30, or R-19 plus R-7.5 continuous insulation in New Jersey and New York.
- Air seal first. Seal the holes around plumbing, wiring and ducts in the subfloor, and the rim joists. The code requires the floor framing to be air sealed, and the Department of Energy’s installer checklist verifies air sealing before insulation goes in.
- Full contact with the subfloor. Batts between the joists have to stay in permanent contact with the underside of the subfloor, with no gaps or voids, unless they rest on sheathing or rigid board fastened under the joists and fill the full height of the perimeter framing, another method 2024 IECC Section R402.2.8 allows.
- Run it to the rim. Batts go to the end of each joist bay so they touch the band joist, with cut pieces in the narrow bays along the walls.
- Support and enclose it. Wire supports or metal staves, nylon banding, netting or landscape cloth, or rigid board under the joists. Don’t compress the batts; that reduces their R-value. Where the insulation is exposed to pests, the Department of Energy’s weatherization guide calls for a rigid barrier over it. For new work in New Jersey and New York, the energy code (the 2024 IECC in New Jersey, New York’s 2025 code) also requires air-permeable floor cavity insulation, such as batts, to be enclosed (Table R402.5.1.1); rigid board fastened under the joists with the seams sealed is one way to do it.
- Protect what stays below. Pipes and ducts under the insulation are now outside the house. Insulate the water pipes, seal and insulate the ducts, and insulate and air seal the access hatch or door.
Faced or unfaced, paper up or down
Unfaced batts work fine in a crawl space floor. With kraft-faced batts, the facing goes up, against the subfloor: the Department of Energy’s checklist says the vapor retarder faces the conditioned space, and Owens Corning’s instructions say the warm side in a vented crawl space is usually up. Insulation installed upside down, paper toward the dirt, puts the vapor retarder on the cold side and leaves flammable facing exposed, which the code allows only in a concealed space with the facing in substantial contact with the underside of the floor (IRC Section R302.10.1).
Spray foam or rigid board under the floor
In a crawl space that runs damp in summer, batts are the weak point, for the reasons above. The two assemblies that handle it better are closed cell spray foam sprayed to encase the floor framing, which air seals and insulates in one pass, and foil-faced rigid board across the bottom of the joists, every seam taped, which keeps the joists warm while the foil acts as a Class I vapor retarder. Under vinyl flooring, the Department of Energy says to avoid low-density foam or a thinner layer of high-density foam. Both assemblies are foam, so the ignition barrier rule applies.
Wet, sagging or fallen crawl space insulation
Wet insulation in a crawl space is a moisture problem showing up in the insulation: humid summer air condensing on the cool subfloor, joists and batts, ground moisture from an uncovered dirt floor, or water from poor grading, missing drainage or a leak. Damp batts insulate poorly, and the Department of Energy notes that mold can grow and damp joists can rot. Wet batts also get heavy and fall, so sagging and wet insulation are often the same problem. The fix depends on the condition:
- Dry and clean, just fallen: push it back into full contact with the subfloor and support it properly.
- Wet, failed or fouled by rodents: it comes out. Taking out the insulation itself is our work, under containment and into a HEPA insulation vacuum; mold on the framing goes to a remediation company, and any extermination to a pest control company. The EPA says moldy porous materials may have to be thrown away, and that a homeowner can usually handle a moldy area smaller than about 10 square feet; for anything larger it points to its professional remediation guidance. Fix the water before anything new goes in.
- Vermiculite or old pipe wrap: don’t touch it. Vermiculite is a pebble-like loose fill, usually gray-brown or silver-gold; the EPA says to assume it contains asbestos and leave it undisturbed, without relying on a test to clear it. Have old pipe insulation sampled by a trained and accredited asbestos professional before anyone disturbs it, and leave removal of either to licensed abatement.
New batts in the same wet crawl space repeat the failure. Fix the water, then choose: the floor above with an assembly that tolerates the conditions, or a sealed crawl space with insulated walls.
The best insulation for a crawl space, spot by spot
Crawl space walls
Closed cell spray foam is our first choice on the walls of a sealed crawl space. At R-6.9 to R-7.1 per inch it reaches R-15 in just over 2 inches, air seals rough block and stone, and acts as a vapor retarder; the Department of Energy’s guide lists 3 inches of closed cell for crawl space walls. Rigid foam board is the other proven choice: the same guide lists up to 2 inches of unfaced extruded polystyrene or 4 inches of expanded polystyrene, seams sealed. Open cell foam, R-3.9 to R-4.2 per inch, appears in that guide at 10 inches, so it is rarely the answer there. Fiberglass and cellulose don’t belong against a crawl space wall.
The floor above a vented crawl space
Fiberglass batts, about R-3.1 to R-4.3 per inch, are the economical choice in a crawl space that stays dry. Mineral wool batts, about R-4 per inch, are denser and water resistant. Closed cell spray foam that encases the joists, or foil-faced rigid board under them, suits a crawl space that runs damp. Blown-in cellulose, about R-3.2 to R-3.8 per inch, can fill joist bays behind netting, but it too needs a dry crawl space.
Rim joists
Closed cell spray foam, or rigid foam board cut to each bay and sealed at the edges. The 2024 IECC requires an air barrier at the rim joist, sealed joints to the sill plate and subfloor, and insulation in contact with the rim board.
Air sealing comes first
Insulation resists heat flow; it doesn’t stop air. In a crawl space the leaks are the rim joist, the sill plate, holes in the subfloor for pipes, wires and ducts, the access hatch, and the vents. The Department of Energy’s steps for closing a crawl space seal all of them, and seal ductwork with mastic rather than ordinary duct tape. We close vents only as part of a crawl space built to the unvented code. See air sealing.
If a quote specifies a depth of insulation and never mentions air sealing, it is half a job.
Flood zones, radon, termites and fuel-burning equipment
- Flood zones. In a flood hazard area, IRC Section R408.7 requires flood openings in the walls around an under-floor space, and the Department of Energy’s seal-the-crawl-space advice is for homes outside flood-prone areas. In shore towns and on low ground, the floor above is usually the plan.
- Radon. A vented crawl space dilutes radon; sealing one can concentrate it, which is why the test comes before the work. The EPA says to fix a home at 4 picocuries per liter or more and to consider it between 2 and 4; the Department of Energy’s crawl space guidance calls for mitigation at 2 or more before closing. The EPA says submembrane suction, a plastic sheet over the earth with a pipe and fan drawing radon from under it, is, when properly applied, the most effective way to reduce radon in crawl space homes, so a sealed ground liner and a radon system work together. A radon mitigation contractor designs that system.
- Termites. New Jersey’s adopted building code rates subterranean termite risk “moderate to heavy” and Connecticut’s “moderate-heavy”. The Department of Energy says to check with local pest control officials on keeping access for termite inspections, and the Building America Solution Center leaves a strip of bare wall above the wall foam and below the rim joist where the local jurisdiction requires one. Ask your termite company and the building department before the walls are covered.
- Fuel-burning equipment. The Department of Energy warns that natural-draft furnaces and water heaters in a closed crawl space are more likely to backdraft, and recommends sealed-combustion units. Have a heating contractor test them for proper draft before the vents are closed and again when the work is done, and replace them if needed.
What crawl space insulation costs
Most of our projects run between $3,500 and $12,500. Within that range, a crawl space moves on the strategy (floor insulation is priced on floor area, a sealed crawl space on wall area and rim), the material (fiberglass batts cost much less than foam, and open cell less than closed cell), the height and access, obstructions such as ducts, pipes and piers, and the air sealing. The liner is a line item of ours; drainage and a dehumidifier are separate costs.
Rebates exist through state and utility programs; see New York and New Jersey insulation rebates and Connecticut insulation rebates. There is no federal crawl space insulation tax credit on current work: the 25C credit ended for anything placed in service after 31 December 2025. For foam, see spray foam insulation cost.
Common questions about crawl space insulation
What is the best insulation for a crawl space?
It depends on where it goes. On the walls of a sealed crawl space: closed cell spray foam or rigid foam board, never fiberglass. In the floor above a dry, vented crawl space: fiberglass or mineral wool batts held tight to the subfloor, or closed cell foam encasing the joists if it runs damp. At the rim joists: closed cell foam or sealed rigid board.
What is the best insulation for a crawl space with a dirt floor?
Cover the dirt first; the insulation doesn’t go on it. Then, in a sealed crawl space, closed cell spray foam or rigid foam board on the walls and rim joists. In a vented crawl space that stays dry, fiberglass or mineral wool batts held tight to the subfloor; in one that runs damp in summer, closed cell foam sprayed to encase the joists, or foil-faced rigid board under them, instead of batts. A sealed crawl space must have a continuous Class I vapor retarder with sealed 6-inch overlaps, run up and sealed to the walls.
How do you insulate a crawl space with a dirt floor?
In order: stop any water getting in, decide whether the crawl space stays vented or gets sealed, test for radon before sealing, take out failed insulation, cover the dirt, air seal the rim joists and subfloor, then insulate. A vented crawl space gets insulation in the floor above, R-19 or R-30 depending on where you live. A sealed one gets closed cell foam or rigid board on the walls and rim joists, plus a drying method.
Can you spray foam a crawl space with a dirt floor?
Yes, on the walls and rim joists of a sealed crawl space, or under the floor of a vented one if the foam encases the joists, but not on the dirt. The ground gets a sheet vapor retarder; in a sealed crawl space the code requires a continuous Class I vapor retarder over the exposed earth, sealed at the seams and run up the walls. Foam left exposed in a crawl space entered only for repairs needs an ignition barrier or a product tested for that use.
Should I insulate the floor joists in my crawl space?
Yes, if the crawl space is vented. That is where the code puts the insulation: R-19 in New Jersey’s zone 4 counties, and R-30 in its zone 5 counties, Connecticut and New York. If the crawl space is sealed, with vents closed and the ground covered, insulate the walls and rim joists instead.
What R-value insulation do I need for a crawl space?
For the floor above: R-19 in New Jersey’s zone 4 counties under the 2024 IECC, and R-30 in its zone 5 counties, Connecticut, and New York under its 2025 code. For the walls of a sealed crawl space: R-10 continuous or R-13 cavity in New Jersey’s zone 4, and R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous elsewhere.
Do I need a vapor barrier in my crawl space?
In a sealed crawl space, yes: IRC Section R408.3 requires a continuous Class I vapor retarder over the exposed earth, joints overlapped 6 inches and sealed, edges run at least 6 inches up the wall. In a vented crawl space the IRC doesn’t require one, but the Department of Energy’s guidance calls for 6-mil polyethylene over the ground.
What should I do about wet or sagging insulation in my crawl space?
Find the water first: humid air through the vents, an uncovered dirt floor, poor grading or a leak. Dry, clean batts that have fallen can be pushed back against the subfloor and supported. Wet or failed insulation has to come out; taking it out is our work, under containment and into a HEPA insulation vacuum, and mold on the framing goes to a remediation company.
What is the difference between crawl space encapsulation and insulation?
Encapsulation is the sealing half: a ground liner run up the walls, vents closed, and a dehumidifier or conditioned air. Insulation is the thermal half: foam or board on the walls and rim joists of a sealed crawl space, or insulation in the floor of a vented one. An encapsulated crawl space still needs insulated walls, because the energy code requires them unless the crawl space is vented with an insulated floor above. We do both halves: the air sealing and insulation, and the ground liner sealed up the walls. A dehumidifier is equipment we can walk you through.
How much does crawl space insulation cost?
Most of our projects run between $3,500 and $12,500. Where a crawl space lands depends on whether the floor or the walls get insulated, the material, the crawl space height, obstructions, and the air sealing. Removal and the ground liner are line items of ours; a dehumidifier is priced separately.
Crawl space insulation in New Jersey, Connecticut, the Hudson Valley and Long Island
We go into the crawl space before we quote it: the vents and the ground, any water, what is in the floor now, where the ducts and pipes run, and the rim joist. Then we tell you whether the floor or the walls is the right plan, what has to happen first and who does it, and what our part costs. The energy audit is free.
Call 914-554-2289 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.