Massachusetts / Plymouth County
Score 34 · moderate · #2 of 12 in MA
Crawl space moisture in Plymouth County, MA
Plymouth County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 50% of acres — it averages 0.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.9 ft
- 2Wet soils 29%
- 3Moisture score 34
- 4EPA radon zone 2
Reading the Plymouth County soil survey
The score of 34 combines four parts, each from the USDA survey: wet drainage classes contribute 36 points of 100 on their own scale, group D soils 31.2, a shallow water table 42.8, and flooding 8.7; the weights are on the methodology page. The dominant hydrologic group is A (42% of acres): high infiltration, low runoff (sands, gravels).
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 50% of Plymouth County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Plymouth County against the rest of Massachusetts
Plymouth County ranks 2 of 12 Massachusetts counties, 8 points above the state median of 26. Only 1 other county in the state score higher. Statewide, 19% of mapped soil is in the wet drainage classes; here it is 29%, half again the state figure or more. After excessively drained, the next most common class is moderately well drained at 21%.
Flooding. 4.6% of acres with a flooding class are rated frequent; 0.1% of acres with a flooding class are rated occasional; 9.5% of acres with a flooding class are rated rare. That is a large flood-prone share; a crawl space in the floodplain needs flood vents under most local floodplain ordinances, which is a separate rule from R408 and can rule out a fully sealed design. Ponding — water standing at the surface with nowhere to go — affects 1.1% of acres.
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. EPA radon zone 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Plymouth County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MA023; drainage covers 88% of 458,562 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.