Missouri / Sullivan County
Score 42 · elevated · #50 of 113 in MO
Crawl space moisture in Sullivan County, MO
55% of Sullivan County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 67% of acres — it averages 2.4 ft below grade.
- 1Water table 2.4 ft
- 2Wet soils 40%
- 3Moisture score 42
- 4EPA radon zone 2
Reading the Sullivan County soil survey
The score of 42 combines four parts, each from the USDA survey: wet drainage classes contribute 35.8 points of 100 on their own scale, group D soils 63.4, a shallow water table 43.1, and flooding 14.1; the weights are on the methodology page. The dominant hydrologic group is D (49% of acres): very slow infiltration, high runoff (clays, high water table).
At 2.4 ft the seasonal water table is usually below a crawl-space floor, but the 67% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.
Sullivan County against the rest of Missouri
Sullivan County ranks 50 of 113 Missouri counties, 3 points above the state median of 39. Wetter counties exist in Missouri, but the soil here still sits well above the national middle. Statewide, 32% of mapped soil is in the wet drainage classes; here it is 40%. After well drained, the next most common class is somewhat poorly drained at 32%.
Flooding. 13% of acres with a flooding class are rated frequent; 1% of acres with a flooding class are rated occasional; 2.4% of acres with a flooding class are rated rare. 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 an elevated county the soil supplies more moisture than average, but a vented crawl space can work if the ground is covered: the code lets vent area drop from 1 sq ft per 150 to 1 per 1500 with a Class I vapor retarder on the ground, which is an admission of how much moisture comes from below. Grading, gutters and a ground cover usually decide whether a crawl space here stays dry before any decision about sealing it.
Radon. EPA radon zone 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Sullivan County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MO211; drainage covers 100% of 417,158 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.