Tennessee / Obion County
Score 43 · elevated · #8 of 88 in TN
Crawl space moisture in Obion County, TN
53% of Obion 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 — 68% of acres — it averages 1.5 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.5 ft
- 2Wet soils 39%
- 3Moisture score 43
- 4EPA radon zone 3
Reading the Obion County soil survey
The score of 43 combines four parts, each from the USDA survey: wet drainage classes contribute 42.6 points of 100 on their own scale, group D soils 41.8, a shallow water table 52.5, and flooding 22.2; the weights are on the methodology page. The dominant hydrologic group is B (31% 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 across much of Obion County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Obion County against the rest of Tennessee
Obion County ranks 8 of 88 Tennessee counties, 31 points above the state median of 12. That puts it in the wettest quarter of the state. Statewide, 9% of mapped soil is in the wet drainage classes; here it is 39%, half again the state figure or more. After moderately well drained, the next most common class is well drained at 30%.
Flooding. 8.7% of acres with a flooding class are rated frequent; 22.5% of acres with a flooding class are rated occasional; 0.2% 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 3. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Obion County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area TN131; drainage covers 96% of 355,620 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.