California / Napa County
Score 21 · moderate · #19 of 48 in CA
Crawl space moisture in Napa County, CA
Napa County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 9% of acres — it averages 3 ft below grade.
- 1Water table 3 ft
- 2Wet soils 11%
- 3Moisture score 21
- 4EPA radon zone 3
Reading the Napa County soil survey
The score of 21 combines four parts, each from the USDA survey: wet drainage classes contribute 9.8 points of 100 on their own scale, group D soils 59.7, a shallow water table 5, and flooding 4.6; the weights are on the methodology page. The dominant hydrologic group is C (45% of acres): slow infiltration (clay layers).
At 3 ft the seasonal water table is usually below a crawl-space floor, but the 9% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.
Napa County against the rest of California
Napa County ranks 19 of 48 California counties, 3 points above the state median of 18. That places it in the middle range, nationally and for the state. Statewide, 11% of mapped soil is in the wet drainage classes; here it is 11%. After well drained, the next most common class is excessively drained at 13%.
Flooding. 2.9% of acres with a flooding class are rated frequent; 0.3% of acres with a flooding class are rated occasional; 8% 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 moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those 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 Napa County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area CA055; drainage covers 89% of 505,821 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.