Iowa / Washington County
Score 48 · elevated · #42 of 99 in IA
Crawl space moisture in Washington County, IA
Washington County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 91% of acres — it averages 1.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.9 ft
- 2Wet soils 38%
- 3Moisture score 48
- 4EPA radon zone 1
Reading the Washington County soil survey
The score of 48 combines four parts, each from the USDA survey: wet drainage classes contribute 49.3 points of 100 on their own scale, group D soils 45.3, a shallow water table 64.8, and flooding 11.9; the weights are on the methodology page. The dominant hydrologic group is C (56% 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 Washington County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Washington County against the rest of Iowa
Washington County ranks 42 of 99 Iowa counties, 1 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 38%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After moderately well drained, the next most common class is poorly drained at 21%.
Flooding. 6.4% of acres with a flooding class are rated frequent; 7.6% of acres with a flooding class are rated occasional; 5% 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. Washington 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 Washington County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA183; drainage covers 99% of 365,200 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.