Crawl Space Map

2,700 counties · USDA soil survey · IRC R408

How wet is the ground under your house?

A crawl space sits in the soil, and the soil decides most of what happens in it. USDA has mapped every county's drainage, water-table depth and flooding; this site reads those surveys and scores each county for crawl-space moisture, then sizes vents or encapsulation the way the building code does.

Look up my county Size vents or encapsulation

A dug soil profile showing dark topsoil over pale, layered subsoil horizons, with a knife for scale
What a soil survey describes: horizons that hold or shed water at different depths. Photo: This image is the personal work of Richard Hartnup…, Public domain, via Wikimedia Commons (resized)
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  1. 1Counties scored 2,700
  2. 2High-moisture counties 257
  3. 3Vent rule, IRC R408 1 per 150 sq ft
  4. 4With an EPA radon zone 2,680
The same idea under a house: the water table drawn to scale below a crawl space.

County soil lookup

Open county page

257High534Elevated843Moderate1066Low
All 2,700 counties scored: counties by soil moisture tier, columns to scale.

IRC R408 calculator

Vented crawl spaces need net vent area; unvented (encapsulated) ones need a continuous sealed vapor retarder over the ground and a conditioning method. The code text below is R408 as adopted in one state; your state may word or amend it differently.

What the soil record tells you

Four things in a county's soil survey matter for a crawl space. Drainage class describes how fast water leaves the soil after rain; a crawl space over "poorly drained" soil sits on ground that stays wet for long periods. Hydrologic soil group D marks clays and high water tables that shed water slowly. The seasonal water table is the shallowest depth groundwater reaches in a normal year, and when it is within a metre or two of grade it can rise into a footing trench. Flooding frequency is the obvious one. The score on each page weights those four, and the methodology page shows exactly how.

Of 2,700 counties scored, 257 come out high, 534 elevated and 843 moderate — those have their own pages. The other 1,066 are listed on their state page.

Vented or encapsulated?

Vented vs unventedWhat R408 allows, what each needs, and why wet-soil counties push toward sealing. Reading a soil surveyDrainage class, hydrologic group and water table in plain terms. Radon and crawl spacesEPA zones, what they do and do not tell you, and why encapsulation and radon venting meet.

Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted; EPA Map of Radon Zones, county table (June 2024 JSON); 2012 IRC R408 (text as published by the Georgia Department of Agriculture); IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII). Checked 2026-10-23.