Illinois / Mercer County
Score 29 · moderate · #89 of 102 in IL
Crawl space moisture in Mercer County, IL
Mercer County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 63% of acres — it averages 2.5 ft below grade.
- 1Water table 2.5 ft
- 2Wet soils 38%
- 3Moisture score 29
- 4EPA radon zone 1
Reading the Mercer County soil survey
The score of 29 combines four parts, each from the USDA survey: wet drainage classes contribute 30.2 points of 100 on their own scale, group D soils 26.1, a shallow water table 38.7, and flooding 10.7; the weights are on the methodology page. The dominant hydrologic group is B (53% of acres): moderate infiltration.
At 2.5 ft the seasonal water table is usually below a crawl-space floor, but the 63% 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.
Mercer County against the rest of Illinois
Mercer County ranks 89 of 102 Illinois counties, 20 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 38%, below the state figure — the score here comes from flooding and ponding rather than drainage class alone. After well drained, the next most common class is somewhat poorly drained at 27%.
Flooding. 9.5% of acres with a flooding class are rated frequent; 6% 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. Ponding — water standing at the surface with nowhere to go — affects 1% of acres.
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. Mercer 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 Mercer County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL131; drainage covers 98% of 363,610 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.