Why is there moisture under my manufactured home, and why does it smell musty?

Almost always the bottom board — the belly wrap — has failed, and the underfloor ductwork is making it worse. This is the system with no site-built equivalent, and it is where most manufactured-home moisture damage starts. The bottom board is a membrane stretched across the underside of the floor framing. It is the primary air and vapor control layer for the floor system and it is also the rodent barrier. In homes more than roughly fifteen to twenty years old, punctures, sagging and missing sections are common, whether from rodents, from service work done under the home, or simply from age. Once it is open, ground moisture reaches the floor cavity, saturates the fiberglass batts, and drives up the moisture content of the floor decking. Above roughly 19 percent moisture content, wood rot and mold become active risks. The musty odor you notice at floor registers or around the base of the home is wet insulation held against the decking. The ductwork compounds it, because in a manufactured home the supply ducts run in the belly, below the floor, inside that same cavity. Ducts sag, separate at joints, collect water and disconnect outright — so one end of the home will not heat and the system pressurizes the crawlspace instead of the house. Cold ducts sitting in a humid belly cavity sweat and grow mold. On a double-wide the crossover duct between the two sections is a particular offender: lying on the ground, crushed by something set on it, or disconnected entirely is extremely common. Related findings we check: whether skirting is intact and properly vented or, alternatively, sealed with a continuous ground vapor retarder and a service access opening; whether ducts and plumbing are strapped up off the ground; rodent entry, which destroys belly insulation quickly; and grading and downspout discharge around the perimeter. One note: whether to vent or seal a crawlspace in a cold climate is a genuine technical debate, so we report what is there against what the manufacturer's installation instructions call for rather than asserting one universal answer.