Ohio / Marion County
Score 69 · high · #16 of 88 in OH
Crawl space moisture in Marion County, OH
81% of Marion 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 — 97% of acres — it averages 0.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.9 ft
- 2Wet soils 81%
- 3Moisture score 69
- 4EPA radon zone 1
Reading the Marion County soil survey
The score of 69 combines four parts, each from the USDA survey: wet drainage classes contribute 72.9 points of 100 on their own scale, group D soils 73.1, a shallow water table 83.4, and flooding 3.4; the weights are on the methodology page. The dominant hydrologic group is C/D (46% of acres): D when undrained, C when drained.
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 Marion County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Marion County against the rest of Ohio
Marion County ranks 16 of 88 Ohio counties, 23 points above the state median of 46. That puts it in the wettest quarter of the state. Statewide, 44% of mapped soil is in the wet drainage classes; here it is 81%, half again the state figure or more. After somewhat poorly drained, the next most common class is very poorly drained at 28%.
Flooding. 1.4% of acres with a flooding class are rated frequent; 3.1% of acres with a flooding class are rated occasional; 0.7% of acres with a flooding class are rated rare. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods.
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. Marion 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 Marion County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area OH101; drainage covers 98% of 258,336 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.