Nebraska / Johnson County
Score 45 · elevated · #2 of 93 in NE
Crawl space moisture in Johnson County, NE
65% of Johnson County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 78% of acres — it averages 1.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.9 ft
- 2Wet soils 5%
- 3Moisture score 45
- 4EPA radon zone 1
Reading the Johnson County soil survey
The score of 45 combines four parts, each from the USDA survey: wet drainage classes contribute 29.5 points of 100 on their own scale, group D soils 73.8, a shallow water table 55.2, and flooding 10.7; the weights are on the methodology page. The dominant hydrologic group is D (65% 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 Johnson County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Johnson County against the rest of Nebraska
Johnson County ranks 2 of 93 Nebraska counties, 35 points above the state median of 10. Only 1 other county in the state score higher. Statewide, 9% of mapped soil is in the wet drainage classes; here it is 5%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After moderately well drained, the next most common class is well drained at 21%.
Flooding. 2.8% of acres with a flooding class are rated frequent; 12% of acres with a flooding class are rated occasional; 3.9% 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. Ponding — water standing at the surface with nowhere to go — affects 2.7% of acres.
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. Johnson 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 Johnson County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area NE097; drainage covers 99% of 241,101 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.