The best insulation for a crawl space 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, the ground is covered with a sealed vapor retarder, and the insulation goes on the foundation walls and rim joists, so the crawl space becomes part of the house.
Half-measures cause the trouble: the Department of Energy warns that closing crawl space vents without air sealing is likely to make moisture levels higher. Material comes second, and moisture comes before both.
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. We regularly find crawl spaces with nothing in the floor, or batts lying on the dirt.
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 batts or cellulose in the ceiling of a vented crawl space raise 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.
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: 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.
- 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).
Who does what in a sealed crawl space
- Water: grading that slopes away, perimeter drainage where the crawl floor is below outside grade, plumbing leaks fixed. A waterproofing contractor and a plumber.
- Radon: the Department of Energy says to test before closing a crawl space and install mitigation first if levels are high.
- Old insulation and damage: wet or failed insulation removed and rotted wood replaced. The removal is ours, under containment and into a HEPA insulation vacuum; a carpenter replaces rotted framing.
- Air sealing and insulation: rim joists, penetrations and vents sealed, walls and rim insulated with closed cell foam or rigid board. Our part, coordinated so the liner seals to the wall or insulation.
- Ground liner: ours as well, run up and sealed to the wall insulation.
- Drying: a supply register, exhaust fan or dehumidifier. We can walk you through the dehumidifier option; a register or fan is HVAC work.
Dirt floor crawl spaces and vapor barriers
A dirt floor is a constant source of water vapor. The insulation, spray foam included, doesn’t go on the dirt: it goes in the floor above or on the walls, and the dirt gets covered.
- Sealed crawl space: the code requires the continuous, sealed Class I vapor retarder described above.
- Vented crawl space: a ground cover is not required, but the code 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? The code specifies a class, not a thickness, and the Department of Energy’s guides name 6-mil polyethylene. Heavier reinforced liners hold up better where people crawl over them. We install the liner ourselves and choose the weight to suit how the crawl space gets used.
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. 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. Floor insulation has to stay in permanent contact with the underside of the subfloor (2024 IECC Section R402.2.8), with no gaps or voids.
- 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 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.
- 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 when facing is in substantial contact with 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: the Department of Energy explains that batts between the joists of a vented crawl space are cool on the underside, the joist edges cooler still, and below the dew point, water condenses. Two assemblies handle it better. Closed cell spray foam on the underside of the subfloor air seals and insulates in one pass, and the Department of Energy’s guidance says at least 3 inches of high-density closed cell is an adequate vapor retarder under any floor covering, while thinner or low-density foam should not go under vinyl. Or foil-faced rigid board across the bottom of the joists, every seam taped, keeps the joists warm, with the foil acting as a Class I vapor retarder. Both 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, moldy or fouled by rodents: it comes out. We do the removal, under containment and into a HEPA insulation vacuum; mold remediation of the structure itself, and any extermination, belong to those trades. The EPA says moldy porous materials may have to be thrown away, and that a homeowner can usually handle only a moldy area smaller than about 10 square feet. 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, and the EPA says to leave it undisturbed. Treat it and old pipe insulation as asbestos until a licensed professional tests them, and leave removal 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 Department of Energy says fiber insulation collects and holds moisture.
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, and one maker says stone wool does not rot or promote mildew or mold. Closed cell spray foam under the subfloor 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 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 unless it meets FEMA Technical Bulletin 11, 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. Sealing a crawl space can concentrate radon. Test before closing it, mitigate first if levels are high, and test again afterward.
- 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.
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 if it runs damp. At the rim joists: closed cell foam or sealed rigid board.
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 is the best insulation for a crawl space with a dirt floor?
Insulation doesn’t go on the dirt. Cover it, then insulate the floor above a vented crawl space or the walls and rim joists of a sealed one. A sealed crawl space must have a continuous Class I vapor retarder with sealed 6-inch overlaps, run up and sealed to the walls. In a vented crawl space a ground cover isn’t required, but it reduces the vent area the code calls for.
Should crawl space insulation be faced or unfaced, and which way does the paper go?
Either works in the floor above a vented crawl space. If faced, the paper goes up, against the subfloor, toward the heated rooms. Paper facing down puts the vapor retarder on the cold side and leaves flammable kraft paper exposed. On crawl space walls, don’t use fiberglass at all.
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 it isn’t required, 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 moldy insulation has to come out first, and we do that removal, under containment and into a HEPA insulation vacuum.
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. 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.
Does an encapsulated crawl space need insulation?
Yes. The energy code requires crawl space walls to be insulated unless the crawl space is vented to the outdoors with an insulated floor above, so a sealed crawl space needs insulated walls. Two of the drying options in IRC Section R408.3 also call for insulated perimeter walls.
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 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 914-825-6824 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.