Virginia / Fairfax County
Score 20 · moderate · #28 of 95 in VA
Crawl space moisture in Fairfax County, VA
Fairfax County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 43% of acres — it averages 2.1 ft below grade.
- 1Water table 2.1 ft
- 2Wet soils 15%
- 3Moisture score 20
- 4EPA radon zone 1
Reading the Fairfax County soil survey
The score of 20 combines four parts, each from the USDA survey: wet drainage classes contribute 12.8 points of 100 on their own scale, group D soils 27.5, a shallow water table 29.1, and flooding 7.4; the weights are on the methodology page. The dominant hydrologic group is C (53% of acres): slow infiltration (clay layers).
At 2.1 ft the seasonal water table is usually below a crawl-space floor, but the 43% 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.
Fairfax County against the rest of Virginia
Fairfax County ranks 28 of 95 Virginia counties, 8 points above the state median of 12. 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 15%. After well drained, the next most common class is somewhat poorly drained at 12%.
Flooding. 2.2% of acres with a flooding class are rated frequent; 7.8% of acres with a flooding class are rated occasional; 0% 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. Fairfax 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 Fairfax County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area VA059; drainage covers 79% of 259,200 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.