Iowa / Madison County
Score 44 · elevated · #56 of 99 in IA
Crawl space moisture in Madison County, IA
Madison County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 80% of acres — it averages 2.2 ft below grade.
- 1Water table 2.2 ft
- 2Wet soils 35%
- 3Moisture score 44
- 4EPA radon zone 1
Reading the Madison County soil survey
The score of 44 combines four parts, each from the USDA survey: wet drainage classes contribute 43 points of 100 on their own scale, group D soils 48.3, a shallow water table 53.2, and flooding 11; the weights are on the methodology page. The dominant hydrologic group is C (59% of acres): slow infiltration (clay layers).
At 2.2 ft the seasonal water table is usually below a crawl-space floor, but the 80% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.
Madison County against the rest of Iowa
Madison County ranks 56 of 99 Iowa counties, 3 points below 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 35%, 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 well drained at 22%.
Flooding. 5.2% of acres with a flooding class are rated frequent; 9.5% of acres with a flooding class are rated occasional; 0.6% 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. Madison 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 Madison County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA121; drainage covers 97% of 360,400 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.