Illinois / Lee County
Score 42 · elevated · #63 of 102 in IL
Crawl space moisture in Lee County, IL
Lee 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 1.8 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.8 ft
- 2Wet soils 48%
- 3Moisture score 42
- 4EPA radon zone 1
Reading the Lee County soil survey
The score of 42 combines four parts, each from the USDA survey: wet drainage classes contribute 48.3 points of 100 on their own scale, group D soils 36.3, a shallow water table 53.3, and flooding 7.6; the weights are on the methodology page. The dominant hydrologic group is B/D (34% of acres): D when undrained, B 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 Lee County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Lee County against the rest of Illinois
Lee County ranks 63 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 48%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After poorly drained, the next most common class is well drained at 31%.
Flooding. 0.9% of acres with a flooding class are rated frequent; 11% of acres with a flooding class are rated occasional; 0.5% of acres with a flooding class are rated rare. 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. Lee 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 Lee County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL103; drainage covers 99% of 466,500 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.