Illinois / Greene County
Score 36 · moderate · #68 of 102 in IL
Crawl space moisture in Greene County, IL
50% of Greene 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 — 67% of acres — it averages 2.1 ft below grade.
- 1Water table 2.1 ft
- 2Wet soils 50%
- 3Moisture score 36
- 4EPA radon zone 1
Reading the Greene County soil survey
The score of 36 combines four parts, each from the USDA survey: wet drainage classes contribute 38.8 points of 100 on their own scale, group D soils 30.3, a shallow water table 45.5, and flooding 18.5; the weights are on the methodology page. The dominant hydrologic group is B (50% of acres): moderate infiltration.
At 2.1 ft the seasonal water table is usually below a crawl-space floor, but the 67% 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.
Greene County against the rest of Illinois
Greene County ranks 68 of 102 Illinois counties, 13 points below the state median of 49. That places it in the middle range, nationally and for the state. Statewide, 57% of mapped soil is in the wet drainage classes; here it is 50%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After well drained, the next most common class is somewhat poorly drained at 33%.
Flooding. 11.8% of acres with a flooding class are rated frequent; 10.2% of acres with a flooding class are rated occasional; 2.9% 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 moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those first.
Radon. Greene 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 Greene County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL061; drainage covers 99% of 349,810 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.