Florida / Nassau County
Score 75 · high · #7 of 67 in FL
Crawl space moisture in Nassau County, FL
96% of Nassau 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 — 95% of acres — it averages 0.7 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.7 ft
- 2Wet soils 96%
- 3Moisture score 75
- 4EPA radon zone 3
Reading the Nassau County soil survey
The score of 75 combines four parts, each from the USDA survey: wet drainage classes contribute 93.5 points of 100 on their own scale, group D soils 55.9, a shallow water table 85, and flooding 23.4; the weights are on the methodology page. The dominant hydrologic group is B/D (43% of acres): D when undrained, B 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 Nassau County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Nassau County against the rest of Florida
Nassau County ranks 7 of 67 Florida counties, 13 points above the state median of 62. That puts it in the wettest quarter of the state. Statewide, 74% of mapped soil is in the wet drainage classes; here it is 96%. After poorly drained, the next most common class is very poorly drained at 36%.
Flooding. 16.2% of acres with a flooding class are rated frequent; 0.4% of acres with a flooding class are rated occasional; 1.8% 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. Ponding — water standing at the surface with nowhere to go — affects 18.4% of acres.
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 3. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Nassau County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area FL089; drainage covers 96% of 428,375 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.