Illinois / Cook County
Score 49 · elevated · #48 of 102 in IL
Crawl space moisture in Cook County, IL
69% of Cook 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 — 76% of acres — it averages 2 ft below grade.
- 1Water table 2 ft
- 2Wet soils 38%
- 3Moisture score 49
- 4EPA radon zone 2
Reading the Cook County soil survey
The score of 49 combines four parts, each from the USDA survey: wet drainage classes contribute 49.3 points of 100 on their own scale, group D soils 62.3, a shallow water table 52.2, and flooding 1.8; the weights are on the methodology page. The dominant hydrologic group is D (32% of acres): very slow infiltration, high runoff (clays, high water table).
At 2 ft the seasonal water table is usually below a crawl-space floor, but the 76% 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.
Cook County against the rest of Illinois
Cook County ranks 48 of 102 Illinois counties, level with 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 38%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After moderately well drained, the next most common class is poorly drained at 18%.
Flooding. 1.8% of acres with a flooding class are rated frequent. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods.
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 Cook County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL031; drainage covers 65% of 616,045 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.