Illinois / Logan County
Score 48 · elevated · #53 of 102 in IL
Crawl space moisture in Logan County, IL
62% of Logan 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 — 78% of acres — it averages 1.6 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.6 ft
- 2Wet soils 62%
- 3Moisture score 48
- 4EPA radon zone 1
Reading the Logan County soil survey
The score of 48 combines four parts, each from the USDA survey: wet drainage classes contribute 55.7 points of 100 on their own scale, group D soils 40.1, a shallow water table 59.6, and flooding 10.1; the weights are on the methodology page. The dominant hydrologic group is B/D (37% 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 Logan County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Logan County against the rest of Illinois
Logan County ranks 53 of 102 Illinois counties, 1 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 62%. After poorly drained, the next most common class is well drained at 30%.
Flooding. 9.1% of acres with a flooding class are rated frequent; 1.2% of acres with a flooding class are rated occasional; 1.1% 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. Ponding — water standing at the surface with nowhere to go — affects 4.3% of acres.
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. Logan 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 Logan County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL107; drainage covers 99% of 396,255 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.