Illinois / McLean County
Score 52 · elevated · #37 of 102 in IL
Crawl space moisture in McLean County, IL
59% of McLean 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 — 91% of acres — it averages 1.7 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.7 ft
- 2Wet soils 59%
- 3Moisture score 52
- 4EPA radon zone 1
Reading the McLean County soil survey
The score of 52 combines four parts, each from the USDA survey: wet drainage classes contribute 58.9 points of 100 on their own scale, group D soils 45.2, a shallow water table 67.8, and flooding 2.8; the weights are on the methodology page. The dominant hydrologic group is B/D (35% 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 McLean County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
McLean County against the rest of Illinois
McLean County ranks 37 of 102 Illinois counties, 3 points above 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 59%. After poorly drained, the next most common class is somewhat poorly drained at 27%.
Flooding. 0.1% of acres with a flooding class are rated frequent; 4.7% of acres with a flooding class are rated occasional. 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. McLean 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 McLean County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL113; drainage covers 99% of 759,795 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.