Iowa / Davis County
Score 60 · high · #11 of 99 in IA
Crawl space moisture in Davis County, IA
70% of Davis 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 — 83% of acres — it averages 0.8 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.8 ft
- 2Wet soils 70%
- 3Moisture score 60
- 4EPA radon zone 1
Reading the Davis County soil survey
The score of 60 combines four parts, each from the USDA survey: wet drainage classes contribute 59.7 points of 100 on their own scale, group D soils 69.5, a shallow water table 72.4, and flooding 7.8; the weights are on the methodology page. The dominant hydrologic group is D (56% 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 Davis County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Davis County against the rest of Iowa
Davis County ranks 11 of 99 Iowa counties, 13 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 70%, half again the state figure or more. After somewhat poorly drained, the next most common class is poorly drained at 31%.
Flooding. 0.7% of acres with a flooding class are rated frequent; 11.6% of acres with a flooding class are rated occasional; 0.8% 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 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. Davis 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 Davis County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA051; drainage covers 99% of 323,100 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.