Missouri / Knox County
Score 65 · high · #9 of 113 in MO
Crawl space moisture in Knox County, MO
82% of Knox 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 — 85% of acres — it averages 1.3 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.3 ft
- 2Wet soils 82%
- 3Moisture score 65
- 4EPA radon zone 2
Reading the Knox County soil survey
The score of 65 combines four parts, each from the USDA survey: wet drainage classes contribute 74.4 points of 100 on their own scale, group D soils 68.1, a shallow water table 68.5, and flooding 12.1; the weights are on the methodology page. The dominant hydrologic group is D (37% 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 Knox County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Knox County against the rest of Missouri
Knox County ranks 9 of 113 Missouri counties, 26 points above the state median of 39. That puts it in the wettest quarter of the state. Statewide, 32% of mapped soil is in the wet drainage classes; here it is 82%, half again the state figure or more. After poorly drained, the next most common class is somewhat poorly drained at 26%.
Flooding. 4.1% of acres with a flooding class are rated frequent; 12.4% of acres with a flooding class are rated occasional; 2.8% 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 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. EPA radon zone 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Knox County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MO103; drainage covers 100% of 324,262 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.