Best insulation for crawl spaces: 4 options ranked (October 2026)
Ranked October 2026. Prices, code summaries and program rules checked October 22, 2026; we re-rank monthly.
Closed-cell spray foam on the walls and rim joists is the best insulation for a crawl space you are sealing, and rigid foam board is the cheaper runner-up. For a crawl space that stays vented, neither belongs on the walls: the insulation goes under the floor, and batts are the usual budget choice. Open-cell spray foam ranks last.
The ranking turns on one decision you make first: seal the crawl space and bring it inside the house, or leave it vented and insulate the floor above it. HomeGuide puts it plainly: "Insulate the subfloor and rim joists when the crawl space is vented. Insulate the walls and rim joists when the crawl space is unvented or contains HVAC ducting."
Sources: HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025).
The ranking at a glance
| Rank | Option | Where it goes | HomeGuide price per sq ft | HomeGuide total, 1,000 sq ft crawl | Best for |
|---|---|---|---|---|---|
| 1 | Closed-cell spray foam | Walls and rim joists of a sealed crawl space (HomeGuide prices walls plus subfloor) | $2.00 to $5.00 | $850 to $6,700 | Damp or humid crawl spaces, stone or uneven block walls, ducts under the house |
| 2 | Rigid foam board (EPS, XPS or polyiso) | Walls of a sealed crawl space | $1.20 to $3.70 | $500 to $1,600 | Flat concrete or block walls, DIY-minded owners, tighter budgets |
| 3 | Fiberglass or rock wool batts | Between the floor joists of a vented crawl space | $0.90 to $1.40 | $900 to $1,400 | Dry climates and flood zones where the crawl space stays outside the house |
| 4 | Open-cell spray foam | Walls, if anywhere | No crawl space rate published | Unknown | Rarely the right pick under a house; see why below |
HomeGuide also lists rigid board on the subfloor ($1.20 to $3.70 per sq ft, $1,200 to $3,700) and blown-in under the floor ($1.00 to $2.80, $1,000 to $2,800). Angi's single figure for a spray foam crawl space job is $1,200 to $6,900, and it rates closed-cell as best for "Attics, crawl spaces, and basements."
Sources: HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025); Angi, "How Much Does Spray Foam Insulation Cost? [2026 Data]" (updated Sep 9, 2026).
1. Closed-cell spray foam on the walls and rim joists
Closed-cell spray foam ranks first because every source we read accepts it on both the walls and the rim joists, wet or dry. HomeGuide says closed-cell and rigid foam both "provide insulation and an air and moisture barrier when adequately sealed." DOE's retrofit checklist for converting a vented crawl space says rim joists "shall be insulated with high-density closed-cell foam sprayed directly against the rim joist or with rigid foam that is cut to fit and sealed in place with caulk or canned spray foam." HomeGuide calls closed-cell "preferred for crawl spaces because it has a higher R-value per inch and provides a moisture barrier." FEMA's Technical Bulletin 2 accepts it below the flood elevation: "Sprayed polyurethane foam (SPUF) or closed-cell plastic foams" are acceptable there. It also goes on walls that board can't follow; DOE's guide shows spray foam sealing the stone walls of an unvented crawlspace.
What it costs on a real crawl space: take a 25 by 40 foot footprint, 1,000 square feet, with 3-foot walls. The perimeter is 130 feet, so the walls are 390 square feet.
| Job | Inches | Board feet | R-value by our arithmetic | Angi range |
|---|---|---|---|---|
| Walls only, 390 sq ft | 1.5 | 585 | About R-10.2 to R-10.7 | $760 to $1,810 |
| Walls only, 390 sq ft | 2.2 | 858 | About R-15.0 to R-15.6 | $1,120 to $2,660 |
| Floor above instead, 1,000 sq ft | 2.7 to 2.8 | 2,700 to 2,800 | About R-19 | $3,510 to $8,680 |
The wall job is small. HomeGuide says most spray foam companies have minimum job fees of $1,000 to $2,000 or more, so a crawl-wall-only bid may be priced at the minimum, not by the board foot. Rim joists add area that this math leaves out. Ask each bid to list the rim joist square footage separately.
The catches. DOE's Building America Solution Center notes that "most codes require a thermal barrier (typically ½-in. gypsum board or equivalent) or an ignition barrier (typically ⅜-in. gypsum board or equivalent) over foam insulation," unless the product is tested to do without. The ground still needs a vapor retarder; DOE's checklist calls for "A minimum 6-mil-thick polyethylene sheeting" over the entire ground, extended at least 6 inches up the walls and taped. Foam also hides the wall, which matters for termites (see the code section below).
Sources: DOE/PNNL Building America Solution Center, Home Improvement Expert checklist: Vented to Unvented Crawl Space; HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025); HomeGuide, "How Much Does Spray Foam Insulation Cost? (2026)" (Dec 1, 2025); FEMA, NFIP Technical Bulletin 2, Flood Damage-Resistant Materials Requirements (January 2025); Profoam product price sheet (R-value per inch column); Angi, "How Much Does Spray Foam Insulation Cost? [2026 Data]" (updated Sep 9, 2026); DOE/PNNL Building America Solution Center, Unvented, Insulated Crawlspaces.
2. Rigid foam board on the walls
Rigid foam board is the method DOE's Building America Solution Center presents as "a cost-effective method" for closed crawl space walls, and HomeGuide's price for board on the walls, $500 to $1,600 for its reference crawl space, is the lowest wall option it lists. The guide recommends polyisocyanurate or XPS "because of its low permeance to water and high insulation value per inch," fastened with masonry fasteners and large button-cap washers, with each joist bay of the rim filled with cut foam and sealed with canned spray foam or caulk.
It ranks second for three reasons. DOE's checklist requires every panel seam to be "fully sealed with caulk, foam, mastic, or flashing tape," and panels "fully in contact with the masonry wall, with no gaps." DOE's board guide is written for walls of "solid concrete, or concrete masonry units (CMU)," not stone. And board still needs a code-approved barrier or a product tested without one; the guide says to "Check with local authorities to ensure that the rigid foam selected meets all code requirements." If your walls are flat poured concrete or block and you are handy, the price gap is real.
Sources: DOE/PNNL Building America Solution Center, Unvented, Insulated Crawlspaces; DOE/PNNL Building America Solution Center, Home Improvement Expert checklist: Vented to Unvented Crawl Space; HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025).
3. Batts between the floor joists of a vented crawl space
Floor insulation is right when the crawl space stays outside the house. DOE's guidance names the cases: "If it is not possible to treat the crawlspace as a part of the house, such as in flood zones in coastal areas or in dry climates where it is not necessary," the floor should be air sealed thoroughly with insulation "installed on the underside of the floor in full contact with the floor." Batts are the cheapest way to do that, at $0.90 to $1.40 per square foot in HomeGuide's table.
It ranks third because of what happens in humid regions. DOE's guide says "Vented crawlspaces in humid climates have experienced serious moisture and mold problems," with outside air condensing on cooler floor joists. HomeGuide calls fiberglass batts "the most common but the most problematic in vented crawl spaces," because they come loose and absorb moisture, and it reserves roll and batt for "subfloors in dry, conditioned spaces." FEMA rates fiberglass unacceptable below the flood elevation. ENERGY STAR's floor targets for retrofits run R-13 in zones 1 and 2, R-19 in zones 3 and 4A/4B, R-30 in zones 4C, 5 and 6, and R-38 in zones 7 and 8.
Sources: DOE/PNNL Building America Solution Center, Insulating Crawl Space Floors in Existing Homes; HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025); FEMA, NFIP Technical Bulletin 2, Flood Damage-Resistant Materials Requirements (January 2025); ENERGY STAR, Recommended Home Insulation R-Values (based on 2021 IECC Table R402.1.3).
4. Open-cell spray foam: last, and why
Open-cell foam is the cheaper spray foam ($0.60 to $1.60 per board foot installed, against $1.30 to $3.10 for closed-cell, per Angi), which is why it shows up on crawl space bids. Three sources point away from it under a house. FEMA's Technical Bulletin 2 lists "open-cell plastic foams" as unacceptable below the flood elevation. Angi says open-cell is "not moisture-resistant, so only use it in dry areas." HomeGuide says open-cell "allows moisture in and is better for above-grade applications."
One official source is more permissive. DOE's vented-to-unvented checklist says foundation walls may get "high-density closed-cell or medium-density open-cell foam." So open-cell is not banned, but if a bid uses it in your crawl space, ask the installer to explain the moisture plan in writing. Our open-cell vs closed-cell comparison covers vapor permeance and cost per R-value in more detail.
Sources: Angi, "How Much Does Spray Foam Insulation Cost? [2026 Data]" (updated Sep 9, 2026); FEMA, NFIP Technical Bulletin 2, Flood Damage-Resistant Materials Requirements (January 2025); HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025); DOE/PNNL Building America Solution Center, Home Improvement Expert checklist: Vented to Unvented Crawl Space.
Code rules that change the answer, by edition
Crawl space rules sit in both the energy code and the building code, and states adopt different editions. These are the provisions most likely to show up on a bid or at inspection.
| Rule | What it says | Edition and source |
|---|---|---|
| Walls instead of floor | "insulation of crawl space walls shall be permitted when the crawl space is not vented to the outside." Insulation runs from the floor down to finished grade and then "for at least an additional 24 inches"; exposed earth gets "a continuous Class I vapor retarder" lapped 6 inches | 2009 IRC N1102.2.9 (ICC) |
| Crawl wall R-value | Zones 1 and 2: none required. Zone 3: R-5 continuous or R-13 cavity. Zone 4 except marine: R-10 or R-13. Zone 5, marine 4, and zones 6 to 8: R-15 or R-19 | 2012 and 2015 IECC/IRC, as summarized by DOE/PNNL |
| Crawl wall retrofit targets | Zone 3: R-5 sheathing or R-13 batt. Zones 4A and 4B: R-10 or R-13. Zones 4C and 5 to 8: R-15 or R-19 | ENERGY STAR, based on 2021 IECC Table R402.1.3 |
| Drying the sealed space | A sealed crawl space needs "a drying mechanism such as a continuously operating exhaust fan or conditioned air supply," delivered in DOE's summary at 1 cfm per 50 sq ft of under-floor area or 50 cfm total, whichever is greater | 2009 IRC R408.3, as summarized by DOE/PNNL |
| Termite inspection strip | In areas prone to termites or carpenter ants, "3 in. of bare wall is required" at the top of the foundation wall | DOE/PNNL, with a map adapted from 2021 IRC Figure R301.2(7) |
| Fire | A thermal or ignition barrier over foam, unless the product is tested without one | DOE/PNNL summary of residential codes |
The DOE checklist adds two safety steps that belong on any bid for sealing a crawl space: "a combustion safety test shall be performed if any natural draft combustion equipment (e.g., water heater, furnace) exists in the crawl space," and the homeowner gets "a radon test kit" after the work. Ask whether either is included.
Your city's adopted edition decides which line applies. Our Atlanta cost page walks through Georgia's own crawl space rules, including a 3-inch termite strip and removable rim joist insulation, and the spray foam guide by climate zone lists ENERGY STAR targets for every zone.
Sources: ICC, 2009 International Residential Code, Section N1102.2.9 Crawl space walls; DOE/PNNL Building America Solution Center, Sealing and Insulating Existing Crawl Space Walls, Code Compliance Brief (2009, 2012 and 2015 IECC/IRC summaries); ENERGY STAR, Recommended Home Insulation R-Values (based on 2021 IECC Table R402.1.3); DOE/PNNL Building America Solution Center, Unvented, Insulated Crawlspaces; DOE/PNNL Building America Solution Center, Home Improvement Expert checklist: Vented to Unvented Crawl Space.
Rebates that name crawl spaces
Few programs we track have a crawl space line. Colorado Springs Utilities pays 40% back, up to $1,250, for crawl space insulation, and requires "Crawl space - R15 CI" from "a Colorado contractor"; our Colorado Springs cost page prices it. TVA EnergyRight's rebate list has $300 lines for air sealing, wall insulation and attic insulation but no crawl space line, and every rebate job must be done by a member of its Quality Contractor Network, so Nashville and Huntsville owners should ask TVA which category, if any, applies (Nashville, Huntsville). Check your utility in the 2026 insulation rebate tracker.
Sources: Colorado Springs Utilities, Insulation & air sealing rebate; TVA EnergyRight, Residential Rebates.
Pick by situation
- Humid region, ducts or a furnace under the house: seal it. Closed-cell foam on walls and rim joists, or rigid board if the walls are flat and the budget is tight. Our Houston and Tampa cost pages cover climate zone 2 code levels.
- Below the flood elevation: FEMA accepts closed-cell foam there and rejects open-cell, fiberglass and cellulose. Ask your floodplain office before anything goes in. Where flood vents are required, DOE's guide says to be careful that installed foam "does not inhibit operation of the vent."
- Dry climate, nothing under the house but framing: a vented crawl with an air-sealed, insulated floor is a legitimate choice, and the cheapest one.
- Doing it yourself: rigid board is the realistic DIY route; HomeGuide puts DIY crawl space insulation at $150 to $700. For foam kits, see DIY spray foam kits vs hiring a pro.
Comparing fiber insulation under the floor? Spray foam vs fiberglass vs cellulose sets them side by side. Holding crawl space bids already? How to compare spray foam quotes shows how to put them on the same footing. You can also price a job in the cost calculator or check a bid in the quote checker.
Sources: FEMA, NFIP Technical Bulletin 2, Flood Damage-Resistant Materials Requirements (January 2025); DOE/PNNL Building America Solution Center, Unvented, Insulated Crawlspaces; HomeGuide, "Crawl Space Insulation Cost" (Dec 19, 2025).
Crawl space insulation FAQ
What is the best insulation for a crawl space?
For a crawl space you seal and condition, closed-cell spray foam on the walls and rim joists ranks first on our list, with rigid foam board a cheaper second. For a vented crawl space you keep outside the house, the insulation goes under the floor instead, usually as batts. HomeGuide: insulate "the walls and rim joists when the crawl space is unvented or contains HVAC ducting."
Should I insulate the crawl space walls or the floor above?
It depends on whether the crawl space stays vented. The 2009 IRC permits wall insulation in place of floor insulation only "when the crawl space is not vented to the outside." DOE's Building America Solution Center says conditioned crawlspaces perform better than vented ones on safety, health, comfort, durability and energy use.
How much does it cost to spray foam a crawl space?
HomeGuide puts closed-cell spray foam on the walls and subfloor of a 1,000 sq ft crawl space with 3-foot walls at $850 to $6,700, and Angi puts a crawl space spray foam job at $1,200 to $6,900. Small wall-only jobs can run into HomeGuide's $1,000 to $2,000 minimum charge.
Is open-cell spray foam OK in a crawl space?
It ranks last on our list. FEMA rates "open-cell plastic foams" unacceptable below the flood elevation, Angi says open-cell is not moisture-resistant and belongs in dry areas, and HomeGuide says it "allows moisture in." DOE's retrofit checklist does allow "medium-density open-cell foam" on foundation walls, so ask why a bid uses it.
Do I need a gap in crawl space insulation for termites?
Often. DOE's Building America Solution Center says that in areas prone to termites or carpenter ants, "3 in. of bare wall is required" as an inspection strip at the top of the foundation wall. Your local building department decides whether that applies to your address.