Illinois / Williamson County
Score 42 · elevated · #65 of 102 in IL
Crawl space moisture in Williamson County, IL
Williamson County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 73% 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 33%
- 3Moisture score 42
- 4EPA radon zone 2
Reading the Williamson County soil survey
The score of 42 combines four parts, each from the USDA survey: wet drainage classes contribute 40.1 points of 100 on their own scale, group D soils 44.1, a shallow water table 52.2, and flooding 18.7; the weights are on the methodology page. The dominant hydrologic group is C (42% of acres): slow infiltration (clay layers).
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 Williamson County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Williamson County against the rest of Illinois
Williamson County ranks 65 of 102 Illinois counties, 7 points below the state median of 49. Wetter counties exist in Illinois, but the soil here still sits well above the national middle. Statewide, 57% of mapped soil is in the wet drainage classes; here it is 33%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After moderately well drained, the next most common class is somewhat poorly drained at 24%.
Flooding. 18.3% of acres with a flooding class are rated frequent; 0.5% of acres with a flooding class are rated occasional; 0.3% 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 Williamson County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL199; drainage covers 94% of 284,365 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.