Illinois / Iroquois County
Score 67 · high · #3 of 102 in IL
Crawl space moisture in Iroquois County, IL
90% of Iroquois 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 — 93% of acres — it averages 0.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.9 ft
- 2Wet soils 90%
- 3Moisture score 67
- 4EPA radon zone 1
Reading the Iroquois County soil survey
The score of 67 combines four parts, each from the USDA survey: wet drainage classes contribute 80.7 points of 100 on their own scale, group D soils 58.3, a shallow water table 80.6, and flooding 2.9; the weights are on the methodology page. The dominant hydrologic group is C/D (54% of acres): D when undrained, C 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 Iroquois County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Iroquois County against the rest of Illinois
Iroquois County ranks 3 of 102 Illinois counties, 18 points above the state median of 49. Only 2 other counties in the state score higher. Statewide, 57% of mapped soil is in the wet drainage classes; here it is 90%, half again the state figure or more. After poorly drained, the next most common class is somewhat poorly drained at 32%.
Flooding. 2.9% 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. Ponding — water standing at the surface with nowhere to go — affects 1.1% of acres.
What it means for a crawl space here
In a high county, the model-code default of a vented crawl space over bare earth works against the soil: outside air brings humidity in all summer, and the ground underneath keeps supplying moisture from below. The R408 alternative — an unvented crawl space with a sealed vapor retarder lapped 6 inches and turned 6 inches up the stem wall, plus conditioned or exhaust air at 1 cfm per 50 sq ft or a dehumidifier — is built for exactly this. If the water table is shallow, drainage comes first.
Radon. Iroquois 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 Iroquois County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL075; drainage covers 100% of 715,925 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.