Iowa / Carroll County
Score 37 · moderate · #65 of 99 in IA
Crawl space moisture in Carroll County, IA
Carroll County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 61% of acres — it averages 1.8 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.8 ft
- 2Wet soils 42%
- 3Moisture score 37
- 4EPA radon zone 1
Reading the Carroll County soil survey
The score of 37 combines four parts, each from the USDA survey: wet drainage classes contribute 38.2 points of 100 on their own scale, group D soils 35.9, a shallow water table 44.1, and flooding 13.7; the weights are on the methodology page. The dominant hydrologic group is C (37% 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 Carroll County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Carroll County against the rest of Iowa
Carroll County ranks 65 of 99 Iowa counties, 10 points below the state median of 47. That places it in the middle range, nationally and for the state. Statewide, 44% of mapped soil is in the wet drainage classes; here it is 42%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After well drained, the next most common class is poorly drained at 29%.
Flooding. 0.5% of acres with a flooding class are rated frequent; 21.9% of acres with a flooding class are rated occasional. 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 a moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those first.
Radon. Carroll 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 Carroll County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA027; drainage covers 99% of 364,900 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.