Illinois / Douglas County
Score 67 · high · #2 of 102 in IL
Crawl space moisture in Douglas County, IL
92% of Douglas 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 — 96% of acres — it averages 1 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1 ft
- 2Wet soils 92%
- 3Moisture score 67
- 4EPA radon zone 1
Reading the Douglas County soil survey
The score of 67 combines four parts, each from the USDA survey: wet drainage classes contribute 80.9 points of 100 on their own scale, group D soils 56, a shallow water table 81.3, and flooding 3.1; the weights are on the methodology page. The dominant hydrologic group is B/D (52% of acres): D when undrained, B 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 Douglas County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Douglas County against the rest of Illinois
Douglas County ranks 2 of 102 Illinois counties, 18 points above the state median of 49. Only 1 other county in the state score higher. Statewide, 57% of mapped soil is in the wet drainage classes; here it is 92%, half again the state figure or more. After poorly drained, the next most common class is somewhat poorly drained at 37%.
Flooding. 3.1% of acres with a flooding class are rated frequent; 0.1% 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. Douglas 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 Douglas County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IL041; drainage covers 100% of 266,970 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.