How the score works

One number per county, from four USDA soil-survey fields, with the weights published here so anyone can reproduce it. It describes the soil, not a house.

Source data

USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted. For every mapunit in every soil survey area we read the dominant-condition drainage class (drclassdcd), hydrologic group (hydgrpdcd), flooding frequency (flodfreqdcd), ponding presence (pondfreqprs) and the annual minimum water-table depth (wtdepannmin), and weight each by the mapunit's acreage. Radon zones are joined by county name from the EPA Map of Radon Zones, county table (June 2024 JSON).

From survey areas to counties

NRCS publishes soil surveys by survey area, which is usually a county but sometimes part of one ("Eastern Part"), an "Area", or several counties together. A county record merges every survey area whose name resolves to that county. Survey areas covering several counties, national forests and reservations cannot be assigned to one county and are left out — 605 of them — which is why a few states are missing or thin. 2,700 counties remain.

The score (0–100)

• Wet drainage — weight 0.4. Each drainage class carries a wetness value of 0 (well drained and drier), 1 (moderately well), 2 (somewhat poorly) or 3 (poorly, very poorly, subaqueous); the county figure is the acreage-weighted mean divided by 3.
• Group D soils — weight 0.25. Share of acres in group D (counted fully), dual x/D groups (0.6) and group C (0.3).
• Shallow water table — weight 0.25. 1 − (mean depth ÷ 200 cm), clamped to 0–1, then multiplied by the share of acres that report a water table at all. NRCS reports wtdepannmin only where a seasonal water table exists within roughly two metres, so without that multiplier a county with one wet valley would look as wet as a swamp.
• Flooding — weight 0.1. Frequent 1, occasional 0.6, rare 0.2, none 0.

Tiers: high ≥ 60, elevated ≥ 40, moderate ≥ 20, low ≥ 0. High, elevated and moderate counties get their own page; all counties appear on their state page and in the lookup.

What the score is not

It is not a measurement of any house. Soil survey data is mapped at county scale and describes typical conditions; a lot can be graded, drained, filled or perched differently from its mapunit. Gutters, downspouts, plumbing leaks and HVAC ducts cause many crawl-space moisture problems regardless of soil. The score says how hard the ground is working against you, nothing more.

Building code figures

• Vented crawl spaces: 1 sq ft of net vent area per 150 sq ft of under-floor area, reducible to 1 per 1500 with a Class I vapor retarder on the ground; one opening within 3 ft of each corner — 2012 IRC R408 (text as published by the Georgia Department of Agriculture).
• Unvented crawl spaces: continuous Class A (ASTM E1745) vapor retarder, seams lapped 6 in and sealed, edges 6 in up the stem wall; plus continuous exhaust or conditioned supply at 1 cfm per 50 sq ft, a plenum, or a dehumidifier sized per the manufacturer — IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII).
• Access: 18 × 24 in through the floor, 16 × 24 in through a wall.
• States amend R408. Washington, for example, requires 1 per 300 and a 6-mil ground cover — WAC 51-51-0408 (Washington amendments to 2021 IRC R408) (LII). The local adopted code governs.

No prices

There is no national, citable source for encapsulation or waterproofing prices, so this site publishes none. The calculator gives quantities — vent area, airflow, square feet of liner — that you can price locally.

Refresh

SSURGO is updated annually (each October); the EPA radon table rarely changes. Both are re-pulled yearly.

Data version 2026-10-23.