Illinois / Marshall County
Score 40 · elevated · #67 of 102 in IL
Crawl space moisture in Marshall County, IL
Marshall County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 86% of acres — it averages 2.5 ft below grade.
- 1Water table 2.5 ft
- 2Wet soils 45%
- 3Moisture score 40
- 4EPA radon zone 1
Reading the Marshall County soil survey
The score of 40 combines four parts, each from the USDA survey: wet drainage classes contribute 42.8 points of 100 on their own scale, group D soils 36.3, a shallow water table 53.6, and flooding 6.4; the weights are on the methodology page. The dominant hydrologic group is C (31% of acres): slow infiltration (clay layers).
At 2.5 ft the seasonal water table is usually below a crawl-space floor, but the 86% 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.
Marshall County against the rest of Illinois
Marshall County ranks 67 of 102 Illinois counties, 9 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 45%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After well drained, the next most common class is somewhat poorly drained at 28%.
Flooding. 2.2% of acres with a flooding class are rated frequent; 6.9% of acres with a flooding class are rated occasional; 0.1% 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.
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. Marshall 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 Marshall County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL123; drainage covers 97% of 254,880 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.