Crawl Space Insulation in Charleston

The floor or the walls: two ways to insulate a crawl space

Insulate the floor and keep the vents open. South Carolina's energy standard, which is the 2009 IECC written into state law (S.C. Code 6-10-30), lists R-19 for a floor in climate zone 3, and the state compliance guide says floor insulation must stay in contact with the underside of the subfloor. ENERGY STAR recommends the same R19. Ducts, pipes and sometimes an air handler stay in the outdoor climate.

Seal the crawl space and insulate the walls. The 2009 table lists crawl space walls at 5/13: R-5 of continuous board or R-13 in a framed cavity. ENERGY STAR's zone 3 advice for that wall is R5 insulative sheathing or an R13 batt. Ducts under the floor end up inside the conditioned envelope.

What South Carolina's code asks of a sealed crawl space

South Carolina writes its own version of the residential code's unvented crawl space section (S.C. Code of Regulations 8-1221, the state's R408.3). A sealed crawl space needs:

• A ground cover: a continuous vapor retarder over the exposed earth, its seams overlapping by 6 inches under tape or sealant, and the sheet turned up the wall at least 6 inches and sealed there or to the wall insulation.
• One way to keep the air dry: an exhaust fan running nonstop at 1 cubic foot per minute for every 50 square feet, with an air path to the house; conditioned supply air at that rate, with a return path; in an existing house, use of the space as a plenum; or a dehumidifier sized to its maker's specifications. A 1,500-square-foot crawl space works out to 30 cubic feet per minute.
• Insulated perimeter walls wherever the fan or supply-air route is used.

Foam board left uncovered in a crawl space entered only for repairs has to be a product tested for that use or sit behind an ignition barrier such as 1/4-inch plywood (R316.5.4). Kiawah Island requires a contractor-installed dehumidifier to be commercial grade, drained to the outside. And equipment needs power: NOAA's hurricane center dates the Atlantic season June 1 to November 30, so look in on a closed crawl space after any long outage.

Termites, foam board and the 6-inch inspection strip

The City of Charleston's design criteria table rates termite damage here as very heavy, and South Carolina's version of the code (S.C. Code of Regulations 8-1216 and 8-1217) sets these limits on foam plastic insulation:

• None on the outside face of foundation walls or slabs below grade, and none underneath them.
• Above grade, a gap of 6 inches or more between the foam and bare soil.
• In a crawl space, an inspection gap of at least 6 inches along the top of the foundation wall and the sill plate, with a continuous 6-inch strip centered on the sill left open.

So foam board on a crawl space wall stops short of the dirt and short of the sill. The rim joist can still be sealed and insulated, but the band along the sill stays bare. If the house has a termite bond, tell the pest control company before the job starts.

Flood zones: where a sealed crawl space and the flood rules collide

In a Special Flood Hazard Area, the A and V zones on FEMA's flood map, a sealed crawl space and the flood rules want opposite things. Sealing closes the foundation vents. The flood rules keep openings in those same walls so water can flow in and out by itself instead of pushing the walls in. The flood rule wins.

FEMA's Technical Bulletin 1 (March 2020) says sealed, mechanically ventilated crawl spaces, the kind sold as conditioned or encapsulated, still need flood openings in a flood hazard area. Charleston County's floodplain page gives the minimum for enclosed spaces below the base flood elevation, crawl spaces and garages included: two or more openings on different walls, the bottom of each within 12 inches of grade, 1 square inch of net open area per square foot of enclosed area, open to water at all times. South Carolina's residential code requires the same of under-floor spaces in flood hazard areas (R408.7).

FEMA does not count an ordinary vent that can be shut by hand unless it is permanently fixed open, a vent with a solid cover put on by hand, a temperature-operated vent not built to pass floodwater, or an access door with no opening in it. So closing the vents, the usual first step of sealing, is the step a flood-zone crawl space may not take when they are its required openings. FEMA also warns that altering an enclosed area can leave a building out of compliance and raise its flood insurance rate.

What does work under a house in a flood zone

• Vented, with flood-tolerant floor insulation. FEMA's Technical Bulletin 2 (January 2025) rates closed-cell foam and sprayed polyurethane foam acceptable below the base flood elevation, and fiberglass, mineral wool, cellulose and open-cell foam unacceptable. Its crawl space bulletin, Technical Bulletin 11, recommends keeping all insulation above that elevation where possible, and notes that soaked insulation turns heavy and pulls away from the joists.
• Closed, with engineered flood openings that stay shut to air and open for water. FEMA treats any opening with moving parts as an engineered opening, backed by a design professional's statement or an evaluation report. The floodplain administrator decides at the permit.
• No crawl space at all in a V zone. FEMA does not allow them; houses there stand on open pilings. Older houses on open brick piers are in the same position anywhere: with no wall to seal, the floor is the only place insulation can go, with a solid cover under the joists to hold it and keep animals out.

Two paperwork points. In AE and VE zones the City of Charleston requires a permit for all work on an existing home, interior work included. And improvement costs are totaled for the flood rule that applies at 50% of the building's value: one year of permits in the city, three on Sullivan's Island, whose worksheet counts insulation, and five in unincorporated Charleston County, Edisto Island included.

Ducts, rim joists and the holes in the floor

Air sealing comes first under the floor, as it does in the attic: the rim joist and sill, the plumbing and wiring holes, the duct boots and the hatch are sealed before insulation goes up, so the insulation is not left filtering moving air (see air sealing; closed-cell foam at the rim is on the spray foam page).

Ducts in a vented crawl space are outdoors. The 2009 IECC wants ducts outside the attic sealed and insulated to at least R-6. Dominion Energy South Carolina pays $300 for duct sealing that cuts leakage by 50% or 150 CFM on a duct-blaster test, or $300 for duct insulation to R-8, done by a licensed mechanical contractor and paid to the account holder. Santee Cooper pays $500 toward whole-home duct replacement, in a program year ending November 30, 2026. In a flood zone, FEMA wants ductwork above the base flood elevation or sealed against floodwater, and Charleston County bars it below two feet above that line in new and substantially improved homes.

What sets the price, and who helps pay

We print no price, because no public source gives one for this market that we trust. The factors: square footage and headroom; whether fallen or wet batts come out first (see insulation removal); standing water, or a yard that drains toward the house; the number of vents or flood openings; the ducts; a dehumidifier and its drain line; the permit.

• Dominion's limited-time Check-up Plus offer, 90% up to $2,880 for work finished by March 31, 2027, is limited to attic insulation, air sealing and duct sealing in electric-heat homes, and the work is done by the program's own contractor, not by us. Crawl space insulation is not on the list.
• Berkeley Electric's HomeAdvantage loan: 5%, up to $20,000, repaid on the power bill, after an energy audit, with an approved contractor and a lien on the property. Coastal Electric offers on-bill loans of up to $10,000 over as long as five years.
• Free weatherization through Palmetto Community Action Partnership, for income-qualified households in all three counties, can include floor insulation.
• The federal 25C credit is gone for this work: it does not reach projects put in service after 2025.

How a crawl space job goes

1. Flood zone first. FEMA's flood map or the elevation certificate settles it. Charleston County's building office may have one on file for a house in the unincorporated county or a town it serves built from the late 1970s on (it charges a research fee).
2. The free home assessment. The R-value under the floor is measured, the ground, vents, ducts and sill are checked, and everything is photographed. The photographs are yours either way.
3. One line-itemed written scope, with the rebate and loan question answered before any quote.
4. The work: old material out, air sealing first, then the ground cover and insulation, with the termite strip left open.
5. The permit: on us. Charleston County's sub-trade permit form names insulation, and Mount Pleasant skips the permit for jobs worth $1,000 or less in labor and materials, unless the work needs an inspection.

Common questions about crawl space insulation in Charleston

Where we work

Crawl space insulation across greater Charleston, from the peninsula and the sea islands out to Summerville, Goose Creek and Moncks Corner. Each city has its own page with the permit rule and code edition that apply there: