States / Montana
41 counties · median score 13
Crawl space moisture in Montana
Most of Montana drains well: only 0 of 41 counties score high or elevated, and the median is 13. Sweet Grass County is the exception at 21; crawl-space moisture problems here usually come from grading, gutters and plumbing rather than the soil itself.
- 1Wettest county Sweet Grass County
- 2High or elevated 0 of 41
- 3Median score 13
- 4Driest county Missoula County
Montana's soil, taken as a whole
Across the 58,234,612 acres mapped in these 41 counties, 2% of soil with a drainage class is somewhat poorly, poorly or very poorly drained, and 3% of acres report a seasonal water table within about two metres. That is a dry figure; wet soils here are confined to river bottoms and wetlands. 39 of the 41 counties have mapped soils subject to occasional or frequent flooding.
| Drainage class | Share of mapped acres |
|---|---|
| Excessively drained | 1% |
| Somewhat excessively drained | 2.4% |
| Well drained | 93.6% |
| Moderately well drained | 1% |
| Somewhat poorly drained | 1% |
| Poorly drained | 0.8% |
| Very poorly drained | 0.2% |
Radon. By EPA zone, 34 Montana counties here are zone 1 (highest potential), 7 zone 2 and 0 zone 3. With most of the state in zone 1, any crawl-space sealing project here should start with a radon test and be designed to take a radon fan.
Every Montana county
Sorted wettest first. High and elevated counties link to their own page with the full soil breakdown.
What drives the Montana numbers
The wettest county, Sweet Grass County, is 94% well drained by dominant condition, with a seasonal water table averaging 1.9 ft below grade across the 9% of acres that report one, and sits in EPA radon zone 2. The driest, Missoula County, is 47% somewhat excessively drained. No Montana county reaches the high tier.
The score is a guide to the soil, not to a house: a well-graded lot with working gutters in a high county can have a dry crawl space, and a bad downspout can flood one in a low county. The R408 calculator sizes venting or encapsulation for any footprint.
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). Checked 2026-10-23.