South Carolina / Richland County
Score 26 · moderate · #22 of 46 in SC
Crawl space moisture in Richland County, SC
Richland County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 49% of acres — it averages 1.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.9 ft
- 2Wet soils 24%
- 3Moisture score 26
- 4EPA radon zone 3
Reading the Richland County soil survey
The score of 26 combines four parts, each from the USDA survey: wet drainage classes contribute 24.1 points of 100 on their own scale, group D soils 24.5, a shallow water table 35.4, and flooding 14.6; the weights are on the methodology page. The dominant hydrologic group is B (38% of acres): moderate infiltration.
A water table under two feet is the single most important number here. A crawl-space floor is usually 18 to 36 inches below the joists and often below outside grade, so in a wet season groundwater can stand at or above the crawl-space floor on the soils that report a water table — about 49% of Richland County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Richland County against the rest of South Carolina
Richland County ranks 22 of 46 South Carolina counties, 1 points above the state median of 25. That places it in the middle range, nationally and for the state. Statewide, 34% of mapped soil is in the wet drainage classes; here it is 24%, below the state figure — the score here comes from flooding and ponding rather than drainage class alone. After well drained, the next most common class is moderately well drained at 15%.
Flooding. 7.9% of acres with a flooding class are rated frequent; 11.2% of acres with a flooding class are rated occasional. That is a large flood-prone share; a crawl space in the floodplain needs flood vents under most local floodplain ordinances, which is a separate rule from R408 and can rule out a fully sealed design.
What it means for a crawl space here
In a moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those first.
Radon. EPA radon zone 3. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Richland County by score
All 46 South Carolina counties
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SC079; drainage covers 96% of 493,700 mapped acres); EPA Map of Radon Zones, county table (June 2024 JSON); IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII). Checked 2026-10-23.