Illinois / Adams County
Score 46 · elevated · #59 of 102 in IL
Crawl space moisture in Adams County, IL
54% of Adams 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 — 83% of acres — it averages 2.1 ft below grade.
- 1Water table 2.1 ft
- 2Wet soils 54%
- 3Moisture score 46
- 4EPA radon zone 1
Reading the Adams County soil survey
The score of 46 combines four parts, each from the USDA survey: wet drainage classes contribute 46 points of 100 on their own scale, group D soils 47.8, a shallow water table 56.3, and flooding 15.7; the weights are on the methodology page. The dominant hydrologic group is C/D (38% of acres): D when undrained, C when drained.
At 2.1 ft the seasonal water table is usually below a crawl-space floor, but the 83% 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.
Adams County against the rest of Illinois
Adams County ranks 59 of 102 Illinois counties, 3 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 54%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After somewhat poorly drained, the next most common class is well drained at 29%.
Flooding. 10.4% of acres with a flooding class are rated frequent; 8.6% of acres with a flooding class are rated occasional; 0.7% 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. Adams County is in EPA radon zone 1, the highest predicted potential. A sealed crawl space and a radon system share most of their parts — the liner, the sealed seams, a fan — so if you are encapsulating, test for radon first and design both together. EPA itself says the zone map is not a reason to skip testing in zones 2 and 3.
Near Adams County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL001; drainage covers 98% of 557,470 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.