States / Illinois
102 counties · median score 49
Crawl space moisture in Illinois
Most of Illinois sits on soil that holds water: 67 of its 102 counties score high or elevated, led by Ford County at 71. In much of the state a vented crawl space over bare ground is working against the soil it was built on.
- 1Wettest county Ford County
- 2High or elevated 67 of 102
- 3Median score 49
- 4Driest county Carroll County
Illinois's soil, taken as a whole
Across the 36,060,825 acres mapped in these 102 counties, 57% of soil with a drainage class is somewhat poorly, poorly or very poorly drained, and 77% of acres report a seasonal water table within about two metres. That is a wet state by any measure; a third or more of its land holds water near the surface for part of the year. 102 of the 102 counties have mapped soils subject to occasional or frequent flooding.
| Drainage class | Share of mapped acres |
|---|---|
| Excessively drained | 1% |
| Somewhat excessively drained | 0.5% |
| Well drained | 27.6% |
| Moderately well drained | 14.2% |
| Somewhat poorly drained | 31.1% |
| Poorly drained | 24.5% |
| Very poorly drained | 1% |
Radon. By EPA zone, 55 Illinois counties here are zone 1 (highest potential), 43 zone 2 and 3 zone 3; 1 are not in the EPA table. With most of the state in zone 1, any crawl-space sealing project here should start with a radon test and be designed to take a radon fan.
Every Illinois county
Sorted wettest first. High and elevated counties link to their own page with the full soil breakdown.
What drives the Illinois numbers
The wettest county, Ford County, is 61% poorly drained by dominant condition, with a seasonal water table averaging 0.9 ft below grade across the 99% of acres that report one, and sits in EPA radon zone 1. The driest, Carroll County, is 81% well drained. 14 Illinois counties reach the high tier, where poorly drained soils, group D clays and a shallow water table occur together.
The score is a guide to the soil, not to a house: a well-graded lot with working gutters in a high county can have a dry crawl space, and a bad downspout can flood one in a low county. The R408 calculator sizes venting or encapsulation for any footprint.
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted; EPA Map of Radon Zones, county table (June 2024 JSON). Checked 2026-10-23.