Do I need a vapor barrier on the ground under a raised home?

If the space under the home will be closed in with skirting, usually yes. HUD's manufactured home installation standards require a ground vapor retarder under skirted homes except in arid regions with dry soil. It must be at least 6 mil polyethylene or equivalent, cover the whole area under the home except open porches, decks and recessed entries, and be lapped at least 12 inches at joints. For site-built crawl spaces, IRC-based local codes require a continuous Class I vapor retarder in unvented crawl spaces, with taped or sealed overlaps. The sheet keeps ground moisture from rising into the floor, insulation and steel frame, cutting mold and rust risk.

Check your local rule. This varies by state, county or city. Your permitting office has the final word; the official lookup pages below show you who that is.

Sources

• 24 CFR Part 3285, Model Manufactured Home Installation Standards (2020 edition)U.S. Government Publishing Office (HUD regulation)
• Residential Building GuideJersey County, IL