States / Iowa
99 counties · median score 47
Crawl space moisture in Iowa
Most of Iowa sits on soil that holds water: 61 of its 99 counties score high or elevated, led by Pocahontas County at 66. In much of the state a vented crawl space over bare ground is working against the soil it was built on.
- 1Wettest county Pocahontas County
- 2High or elevated 61 of 99
- 3Median score 47
- 4Driest county Crawford County
Iowa's soil, taken as a whole
Across the 36,016,200 acres mapped in these 99 counties, 44% of soil with a drainage class is somewhat poorly, poorly or very poorly drained, and 66% of acres report a seasonal water table within about two metres. That is a wet state by any measure; a third or more of its land holds water near the surface for part of the year. 99 of the 99 counties have mapped soils subject to occasional or frequent flooding.
| Drainage class | Share of mapped acres |
|---|---|
| Excessively drained | 1.1% |
| Somewhat excessively drained | 0.6% |
| Well drained | 36.8% |
| Moderately well drained | 17.7% |
| Somewhat poorly drained | 18.3% |
| Poorly drained | 23.8% |
| Very poorly drained | 1.8% |
Radon. By EPA zone, 99 Iowa counties here are zone 1 (highest potential), 0 zone 2 and 0 zone 3. With most of the state in zone 1, any crawl-space sealing project here should start with a radon test and be designed to take a radon fan.
Every Iowa county
Sorted wettest first. High and elevated counties link to their own page with the full soil breakdown.
What drives the Iowa numbers
The wettest county, Pocahontas County, is 55% poorly drained by dominant condition, with a seasonal water table averaging 0.9 ft below grade across the 97% of acres that report one, and sits in EPA radon zone 1. The driest, Crawford County, is 92% well drained. 12 Iowa counties reach the high tier, where poorly drained soils, group D clays and a shallow water table occur together.
The score is a guide to the soil, not to a house: a well-graded lot with working gutters in a high county can have a dry crawl space, and a bad downspout can flood one in a low county. The R408 calculator sizes venting or encapsulation for any footprint.
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted; EPA Map of Radon Zones, county table (June 2024 JSON). Checked 2026-10-23.