Illinois / Grundy County
Score 62 · high · #12 of 102 in IL
Crawl space moisture in Grundy County, IL
84% of Grundy 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 — 87% of acres — it averages 1.1 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.1 ft
- 2Wet soils 84%
- 3Moisture score 62
- 4EPA radon zone 1
Reading the Grundy County soil survey
The score of 62 combines four parts, each from the USDA survey: wet drainage classes contribute 73.2 points of 100 on their own scale, group D soils 54.9, a shallow water table 72.9, and flooding 4.8; the weights are on the methodology page. The dominant hydrologic group is C/D (46% 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 Grundy County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Grundy County against the rest of Illinois
Grundy County ranks 12 of 102 Illinois counties, 13 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 84%. After poorly drained, the next most common class is somewhat poorly drained at 35%.
Flooding. 3% of acres with a flooding class are rated frequent; 3.1% of acres with a flooding class are rated occasional. 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 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. Grundy 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 Grundy County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL063; drainage covers 97% of 275,355 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.