Missouri / Adair County
Score 60 · high · #15 of 113 in MO
Crawl space moisture in Adair County, MO
56% of Adair 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 — 96% of acres — it averages 1.7 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.7 ft
- 2Wet soils 56%
- 3Moisture score 60
- 4EPA radon zone 2
Reading the Adair County soil survey
The score of 60 combines four parts, each from the USDA survey: wet drainage classes contribute 55.9 points of 100 on their own scale, group D soils 76, a shallow water table 71, and flooding 10.4; the weights are on the methodology page. The dominant hydrologic group is D (54% 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 Adair County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Adair County against the rest of Missouri
Adair County ranks 15 of 113 Missouri counties, 21 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 56%, half again the state figure or more. After somewhat poorly drained, the next most common class is moderately well drained at 40%.
Flooding. 2.5% of acres with a flooding class are rated frequent; 12.7% of acres with a flooding class are rated occasional; 1.4% 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 Adair County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MO001; drainage covers 100% of 364,448 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.