Vermont / Grand Isle County
Score 46 · elevated · #2 of 14 in VT
Crawl space moisture in Grand Isle County, VT
54% of Grand Isle County's mapped soil is somewhat poorly to very poorly drained, which means ground that stays wet near the surface for significant periods. Where a seasonal water table is reported — 29% of acres — it averages 1 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1 ft
- 2Wet soils 54%
- 3Moisture score 46
- 4EPA radon zone 3
Reading the Grand Isle County soil survey
The score of 46 combines four parts, each from the USDA survey: wet drainage classes contribute 54.6 points of 100 on their own scale, group D soils 71.6, a shallow water table 24.6, and flooding 0.2; the weights are on the methodology page. The dominant hydrologic group is D (50% of acres): very slow infiltration, high runoff (clays, high water table).
A water table under two feet is the single most important number here. A crawl-space floor is usually 18 to 36 inches below the joists and often below outside grade, so in a wet season groundwater can stand at or above the crawl-space floor on the soils that report a water table — about 29% of Grand Isle County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Grand Isle County against the rest of Vermont
Grand Isle County ranks 2 of 14 Vermont counties, 11 points above the state median of 35. Only 1 other county in the state score higher. Statewide, 20% of mapped soil is in the wet drainage classes; here it is 54%, half again the state figure or more. After poorly drained, the next most common class is somewhat excessively drained at 24%.
Flooding. No mapped soil in Grand Isle County carries a flooding frequency above "none", so the score here is about water held in and under the soil rather than water arriving over it.
What it means for a crawl space here
In an elevated county the soil supplies more moisture than average, but a vented crawl space can work if the ground is covered: the code lets vent area drop from 1 sq ft per 150 to 1 per 1500 with a Class I vapor retarder on the ground, which is an admission of how much moisture comes from below. Grading, gutters and a ground cover usually decide whether a crawl space here stays dry before any decision about sealing it.
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 Grand Isle County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area VT013; drainage covers 42% of 124,400 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.