Illinois / Hamilton County
Score 57 · elevated · #24 of 102 in IL
Crawl space moisture in Hamilton County, IL
65% of Hamilton County's mapped soil is somewhat poorly to very poorly drained, which means ground that stays wet near the surface for significant periods. Where a seasonal water table is reported — 96% of acres — it averages 1.6 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.6 ft
- 2Wet soils 65%
- 3Moisture score 57
- 4EPA radon zone 2
Reading the Hamilton County soil survey
The score of 57 combines four parts, each from the USDA survey: wet drainage classes contribute 61.4 points of 100 on their own scale, group D soils 46.1, a shallow water table 73.2, and flooding 21.1; the weights are on the methodology page. The dominant hydrologic group is C/D (50% of acres): D when undrained, C when drained.
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 Hamilton County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Hamilton County against the rest of Illinois
Hamilton County ranks 24 of 102 Illinois counties, 8 points above the state median of 49. That puts it in the wettest quarter of the state. Statewide, 57% of mapped soil is in the wet drainage classes; here it is 65%. After somewhat poorly drained, the next most common class is moderately well drained at 31%.
Flooding. 12% of acres with a flooding class are rated frequent; 14.8% of acres with a flooding class are rated occasional; 1.2% 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 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Hamilton County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL065; drainage covers 100% of 279,070 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.