North Carolina / Onslow County
Score 51 · elevated · #25 of 100 in NC
Crawl space moisture in Onslow County, NC
54% of Onslow 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 — 87% of acres — it averages 1.4 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.4 ft
- 2Wet soils 54%
- 3Moisture score 51
- 4EPA radon zone 3
Reading the Onslow County soil survey
The score of 51 combines four parts, each from the USDA survey: wet drainage classes contribute 58.3 points of 100 on their own scale, group D soils 37.3, a shallow water table 67.8, and flooding 9.8; the weights are on the methodology page. The dominant hydrologic group is A/D (32% of acres): D when undrained, A 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 Onslow County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Onslow County against the rest of North Carolina
Onslow County ranks 25 of 100 North Carolina counties, 34 points above the state median of 17. That puts it in the wettest quarter of the state. Statewide, 30% of mapped soil is in the wet drainage classes; here it is 54%, half again the state figure or more. After poorly drained, the next most common class is well drained at 25%.
Flooding. 7.4% of acres with a flooding class are rated frequent; 0.1% of acres with a flooding class are rated occasional; 0.8% of acres with a flooding class are rated rare; 0.1% of acres with a flooding class are rated very 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 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. 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 Onslow County by score
All 100 North Carolina counties
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area NC133; drainage covers 92% of 524,934 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.