Iowa / Jefferson County
Score 57 · elevated · #14 of 99 in IA
Crawl space moisture in Jefferson County, IA
53% of Jefferson 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 — 88% of acres — it averages 1.3 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.3 ft
- 2Wet soils 53%
- 3Moisture score 57
- 4EPA radon zone 1
Reading the Jefferson County soil survey
The score of 57 combines four parts, each from the USDA survey: wet drainage classes contribute 57.1 points of 100 on their own scale, group D soils 63.1, a shallow water table 70.9, and flooding 6.3; the weights are on the methodology page. The dominant hydrologic group is D (40% of acres): very slow infiltration, high runoff (clays, high water table).
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 Jefferson County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Jefferson County against the rest of Iowa
Jefferson County ranks 14 of 99 Iowa counties, 10 points above the state median of 47. That puts it in the wettest quarter of the state. Statewide, 44% of mapped soil is in the wet drainage classes; here it is 53%. After moderately well drained, the next most common class is poorly drained at 31%.
Flooding. 1.4% of acres with a flooding class are rated frequent; 7.8% of acres with a flooding class are rated occasional; 1.2% of acres with a flooding class are rated rare. 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 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. Jefferson 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 Jefferson County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA101; drainage covers 99% of 281,300 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.