Iowa / Warren County
Score 50 · elevated · #38 of 99 in IA
Crawl space moisture in Warren County, IA
52% of Warren County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 84% of acres — it averages 1.8 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.8 ft
- 2Wet soils 47%
- 3Moisture score 50
- 4EPA radon zone 1
Reading the Warren County soil survey
The score of 50 combines four parts, each from the USDA survey: wet drainage classes contribute 48.7 points of 100 on their own scale, group D soils 55.9, a shallow water table 60.8, and flooding 12.6; the weights are on the methodology page. The dominant hydrologic group is C (43% of acres): slow infiltration (clay layers).
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 Warren County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Warren County against the rest of Iowa
Warren County ranks 38 of 99 Iowa counties, 3 points above the state median of 47. Wetter counties exist in Iowa, but the soil here still sits well above the national middle. Statewide, 44% of mapped soil is in the wet drainage classes; here it is 47%. After moderately well drained, the next most common class is somewhat poorly drained at 27%.
Flooding. 0.9% of acres with a flooding class are rated frequent; 19.1% of acres with a flooding class are rated occasional; 1.1% 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 an elevated county the soil supplies more moisture than average, but a vented crawl space can work if the ground is covered: the code lets vent area drop from 1 sq ft per 150 to 1 per 1500 with a Class I vapor retarder on the ground, which is an admission of how much moisture comes from below. Grading, gutters and a ground cover usually decide whether a crawl space here stays dry before any decision about sealing it.
Radon. Warren 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 Warren County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA181; drainage covers 99% of 366,900 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.