Illinois / Jasper County
Score 63 · high · #10 of 102 in IL
Crawl space moisture in Jasper County, IL
82% of Jasper 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 — 89% of acres — it averages 1 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1 ft
- 2Wet soils 82%
- 3Moisture score 63
- 4EPA radon zone 2
Reading the Jasper County soil survey
The score of 63 combines four parts, each from the USDA survey: wet drainage classes contribute 71.9 points of 100 on their own scale, group D soils 54.5, a shallow water table 76.1, and flooding 12.8; the weights are on the methodology page. The dominant hydrologic group is C/D (68% 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 Jasper County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Jasper County against the rest of Illinois
Jasper County ranks 10 of 102 Illinois counties, 14 points above the state median of 49. That puts it in the wettest quarter of the state. Statewide, 57% of mapped soil is in the wet drainage classes; here it is 82%. After poorly drained, the next most common class is somewhat poorly drained at 37%.
Flooding. 12.2% of acres with a flooding class are rated frequent; 0.2% of acres with a flooding class are rated occasional; 2.4% of acres with a flooding class are rated rare. That is a large flood-prone share; a crawl space in the floodplain needs flood vents under most local floodplain ordinances, which is a separate rule from R408 and can rule out a fully sealed design.
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. EPA radon zone 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Jasper County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL079; drainage covers 99% of 319,100 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.