Crawl Space
Insulation:
Vented or Sealed?

Should my crawl space be vented or sealed?

Seal it if your summers are humid, ducts or an air handler live down there, or the floor batts are already failing. Keep it vented and insulate the floor if the climate is dry, the house is in a flood zone, the foundation is open piers, or the existing floor insulation is in good shape. The tool weighs your answers and lists what each path requires.

When a vented crawl space is still the right call

• Dry summers. Denver, Salt Lake City, Boise and Spokane average July dew points of 43–49°F (NOAA). Vent air there doesn't come close to condensing on a 65°F surface.
• Flood zones. A crawl space below the design flood elevation must keep flood openings (IRC R322.2.2), so it can't be sealed.
• Open pier foundations. There's no wall to insulate. The floor is the only option.
• Good batts, no ducts. If the floor insulation is dry, supported and tight to the subfloor, and nothing mechanical lives below it, cover the ground, air-seal and leave it.
• Heavy termite areas where the pest-control company wants every inch of foundation visible.

When sealing wins

Humid summers first. Of the 47 cities in our table, 18 have a July mean dew point above 65°F, which means their average July air condenses on a 65°F surface. That's most of the South and the lower Midwest, Springfield, Missouri included. Add ducts in the crawl, cold-climate pipes, or a musty first floor, and sealing is the better use of the money. ENERGY STAR's fact sheet sums it up: many professionals now see an unvented crawlspace as "the best option" when moisture is controlled.

Crawl space insulation R-value and code requirements

Under the IECC, a floor over a vented crawl needs R-13 in zones 1–2, R-19 in zones 3–4, R-30 in zones 5–6 and 4C, and R-38 in zones 7–8. Crawl space walls, allowed only when the crawl isn't vented, need nothing in zones 1–2 and R-5 to R-15 continuous insulation in zones 3–8, depending on edition. The IRC then sets the venting or sealing rules.

Notice the gap between the two columns. In zone 4, the floor wants R-19 but the walls want R-10 continuous: about 1.6 in. of closed-cell foam (2 in. in practice), or 2 in. of XPS. That, plus the smaller area, is why HomeGuide's wall figure for a 1,000 sq ft crawl ($420–$1,900) runs well below its floor figure ($1,000–$4,500). Our spray foam R-value guide converts any target to inches.

Vented crawl spaces: IRC R408.2

Vent openings must total at least 1 sq ft of net free area for every 150 sq ft of crawl space. With the ground covered by a Class I vapor retarder, the requirement drops to 1 sq ft per 1,500 sq ft. The IBC's under-floor section (1202.4) uses the same two ratios. Net free area is smaller than a vent's face size once the screen or louvers are counted, so go by the manufacturer's rating.

Sealed crawl spaces: IRC R408.3

Vents aren't required when both of these are provided:

• Ground cover: exposed earth covered with a continuous Class I vapor retarder, joints overlapped 6 in. and sealed or taped, edges run at least 6 in. up the stem wall and attached and sealed to the wall or insulation. ENERGY STAR and PNNL both recommend 12 in. laps.
• One drying method: continuous exhaust at 1 cfm per 50 sq ft of crawl floor, with an air path to the house; or conditioned supply air at 1 cfm per 50 sq ft, with a return path; or, in existing homes, use as a supply plenum under M1601.5; or dehumidification sized at 70 pints a day per 1,000 sq ft. The dehumidifier option appears in newer IRC text, so check which edition your county adopted. The exhaust and supply options also require the perimeter walls to be insulated.

North Carolina writes its own rules for this, in a separate "closed crawl space" section (R409) of its residential code. If you're there, read that instead of the model code.

The 24-inch rule for wall insulation

The energy code's crawl wall section (IECC R402.2.11, IRC N1102.2.11) allows wall insulation only "provided that the crawl space is not vented to the outdoors." The insulation must be permanently fastened to the wall and run from the floor down to the outside finished grade, then at least 24 in. farther, either down the wall or horizontally across the ground. In a shallow crawl, that last part often becomes a strip of foam board laid flat around the perimeter under the liner. The calculator adds it when your walls are too short.

Foam needs an ignition barrier in most crawl spaces

Foam plastic, whether spray foam or board, normally has to be covered. In a crawl space entered only to service utilities, the code accepts a lighter ignition barrier instead of drywall: 1½ in. mineral fiber, ¼ in. wood structural panel, ⅜ in. particleboard, ¼ in. hardboard, ⅜ in. gypsum board or 0.016 in. steel (IRC R316.5.4; R303.5.4 in the 2024 IRC). Unlike the attic list, there's no cellulose option. The exception is a product whose ICC-ES evaluation report allows it exposed, usually with a listed intumescent coating and conditions such as no storage in the space. Rim joists have their own rule: up to 3¼ in. of foam can stay exposed there if it meets density and flame-spread limits (R316.5.11). The closed-cell spray foam guide covers barriers in depth.

Termite inspection gaps

• ENERGY STAR (2000 fact sheet): a continuous termite shield, or a 2- to 4-in. gap at the top of the wall insulation for inspection.
• PNNL: 3 in. of bare wall, painted white, where termites or carpenter ants are common.
• South Carolina: at least 6 in. along the top of the foundation wall and sill plate in crawl spaces, plus a continuous 6-in. strip centered on the sill.
• IRC R318.4 (R305.4 in 2024): in "very heavy" termite areas, no foam on the exterior face of or under below-grade foundation walls.

Flood zones: a crawl below the flood line can't be sealed

IRC R322.2.2 requires enclosed areas below the design flood elevation, "including crawl spaces," to have flood openings: at least 1 sq in. of opening per sq ft of enclosed area (unless engineered), at least two openings on different sides, each at least 3 in. across, with the bottoms no more than 1 ft above grade. That rules out a fully sealed crawl. Insulate the floor, and ask your floodplain administrator which materials are allowed below the flood line.

Radon and combustion appliances

Sealing changes air flow, so two checks come with it. Test for radon; EPA's action level is 4 pCi/L, and ENERGY STAR's radon-resistant detail puts a perforated tee under the sealed poly with a 3-in. vent pipe to the roof. And if a chimney-vented gas or oil furnace or water heater sits in the crawl, ENERGY STAR says to make it sealed-combustion before closing the space.

Best insulation for a crawl space: materials compared

On crawl space walls, use something that doesn't mind damp concrete: closed-cell spray foam or rigid foam board. On the rim joist, closed-cell spray foam, or boards sealed at every edge. In the floor over a vented crawl, supported batts work in dry climates; in humid ones we'd rather see closed-cell under the subfloor, or a sealed crawl. Loose fiberglass on walls or ground is the one choice to skip.

Crawl space walls

Closed-cell foam and foam board both work. Spray foam wins on rough block walls, around piers and pipes, and at the rim, because it fills everything and needs no seam sealing. Board wins on price for a handy owner with straight poured walls. Two inches of XPS gets R-10, zone 4's continuous target, for about $2 a square foot in board ($62.78 a sheet). Either one needs an ignition barrier unless its evaluation report says otherwise. We'd skip fiberglass on masonry crawl walls: it's air-permeable, it holds moisture against cold concrete, and it gives rodents a home.

Rim joist

The rim is where three leaky joints meet (sill to foundation, rim to sill, rim to subfloor), and it gets insulated in both approaches. Closed-cell foam seals the sill, rim and subfloor joints in one pass; cut foam board works if every edge is sealed. Our rim joist insulation guide has the thickness rules, a foam calculator and a condensation check.

The floor over a vented crawl

Batts are the standard, and they work when they're installed the way ENERGY STAR describes: flush against the subfloor, held every 12–18 in., compressed no more than 1 in. In humid climates the underside of that floor meets summer air above its dew point, so a vapor-open, air-permeable layer is the wrong choice there. Closed-cell under the subfloor, or a sealed crawl, handles it better. For the batt debate in general, see spray foam vs. fiberglass.

Crawl space insulation cost: what people actually pay

Crawl space insulation runs about $1.00–$4.50 per square foot installed (HomeGuide, Dec 2025), or roughly $500–$3,700 for typical projects. Insulating the floor of a 1,000 sq ft crawl costs $1,000–$4,500; insulating its walls, $420–$1,900. A sealed crawl adds a liner, vent sealing and a dehumidifier, which is where totals climb into the thousands.

Worked example: a 1,000 sq ft crawl in Springfield, Missouri

A 40 x 25 ft crawl with 3 ft walls, 2x10 floor joists, climate zone 4A, 2021 IECC. Running the calculator above at default settings:

Read that honestly. The vented floor is cheaper up front, and in a dry crawl with no ducts it's a perfectly good answer. You're paying extra for the sealed crawl to fix the summer moisture that Springfield's 67°F July dew point brings in, bring the ducts inside, and cut the insulated area in half. If a rim-only foam job is going to hit a minimum charge anyway, a handy owner can do the rims with foam board for $101–$250 (Family Handyman, 2024) and save most of that line.

What moves the price

• Crawl height. Under about 18 in., everything takes longer.
• Old insulation and debris. Pulling wet batts runs $0.75–$2.00 per sq ft.
• Water. Drainage, a sump or grading work comes first and is priced separately; see our foundation waterproofing guide.
• Minimums. Spray foam crews carry $1,000–$2,000 minimums. Bundling rims, walls and other foam work into one visit makes them disappear.
• Liner grade. 6-mil poly is the lightest liner ENERGY STAR and PNNL describe; 12- to 20-mil liners, or a sprayed polyurea membrane, cost more and stand up better to foot traffic.

How to insulate a vented crawl space: the floor, done right

Cover the ground, air-seal every hole through the floor, insulate the rim, then install batts at your zone's R-value flush against the subfloor, supported every 12–18 in. without crushing them. Insulate ducts and water lines below. That's the ENERGY STAR method, and it's the one that holds up.

1. Cover the ground

   6-mil polyethylene over the dirt, seams lapped 12 in. and taped, run 6 in. up the walls (ENERGY STAR). It also cuts the vent area code requires to 1 per 1,500 sq ft.

2. Air-seal the floor

   Seal plumbing, wiring and duct penetrations, tub drains and the access hatch. Canned foam for gaps from ¼ in. to about 3 in., caulk below that.

3. Seal and insulate the rim

   Closed-cell foam, or foam board cut to fit with every edge sealed. See the rim joist guide.

4. Install batts tight to the subfloor

   ENERGY STAR says "flush against the subfloor to eliminate any gaps." An air gap between the batt and the floor defeats it. Use the batt R-value your zone needs and check that it fits the joist depth.

5. Face the paper the right way

   ENERGY STAR's fact sheet puts the vapor retarder facing up, toward the heated floor, in most regions, and down in the Gulf states and other areas with mild winters and hot summers. Unfaced batts avoid the question.

6. Support it for good

   Insulation hangers or wire every 12–18 in., compressing the batt no more than 1 in. Don't rely on stapled paper flanges alone.

7. Insulate what's below

   Ducts and water lines are now outside the insulation. Seal and insulate the ducts; protect the pipes from freezing.