The duct under the house is raining. Here is why, and what it costs to stop.
Pier-and-beam houses in the old neighborhoods and manufactured homes across the county share one thing: ductwork hanging in damp air under the floor. In summer it sweats, and the house pays.
Why a duct sweats
Cold duct in warm, humid air does what a glass of iced tea does on a porch in July. The supply duct carries air at around 55 degrees. The crawlspace air under a Putnam County house in summer sits near 80 degrees with a dew point in the mid-70s. Any duct surface colder than that dew point condenses water out of the air, and an uninsulated metal trunk or a flex duct with torn or compressed insulation is colder than that for every minute the system runs. The water beads, runs, drips, and soaks whatever is below and above.
Three things make it worse: open crawlspace vents that keep feeding humid air in, bare ground that wicks moisture up from the water table, and duct leaks that blow cold air into the crawlspace and chill everything around them.
What the water costs
Wet duct insulation stops insulating, so the air arrives at the registers warmer and the system runs longer: a power bill that creeps up every summer. Dripping condensation rots the subfloor above and the joists beside it, which in the old frame houses of the historic districts and Welaka’s grid means a floor repair that dwarfs the duct repair. Mold grows on the wet insulation and the damp wood, and the return side of the system pulls that air into the house, which is the “it smells when the AC comes on” call. In a manufactured home the belly wrap sags with the weight of the water, the insulation falls away, and the trunk eventually rusts through.
Taken together, a sweating duct left alone for a few summers costs more than the whole fix below.
The three checks
- Where is the water coming from? Condensation on the duct is one source. A plumbing leak, a dripping condensate drain that terminates under the house, or ground water from a high river are others. We trace the drips before we blame the duct.
- Is the duct leaking? A pressure test or a simple hand along the seams with the system running. A leak at a joint chills the surrounding air and the duct next to it, and it is the first thing to seal.
- How wet is the crawlspace itself? A moisture reading on the joists, a look at the ground, and a check of whether vents are open and the ground is bare. This decides whether sealing the duct is enough or whether the crawlspace needs work too.
The fixes, in the order they should happen
Seal the duct. Mastic on every joint, seam and boot. This stops the leaks that blow cold air into the crawlspace and the warm air that leaks in. Cheapest step, biggest effect per dollar.
Repair or replace the insulation. Wet or crushed insulation comes off. New insulation goes on with a continuous vapor barrier on the outside, taped at the seams. On flex duct that is past saving, the run is replaced with new insulated duct hung high and straight.
Cover the ground. A heavy vapor barrier across the crawlspace floor stops the water table from feeding the air. On a manufactured home this is paired with repairing the belly wrap.
Close the vents. With the ground covered and the duct sealed, the vents are closed and the crawlspace stops breathing humid outdoor air.
Add a dehumidifier if needed. On a site-built house with a large, deep crawlspace, a small crawlspace dehumidifier draining to the outside holds the space dry year-round. On most manufactured homes the steps above are enough.
Encapsulate, for the long haul. A fully sealed crawlspace with barrier up the walls and conditioned air is the top of the ladder, and it is justified on a house the owner plans to keep for decades. It is overkill for most.
Two houses, worked through
A 1920s frame house in Welaka on brick piers. Metal trunk from a 1970s retrofit, bare, dripping onto sand six inches below. The hallway floor is soft. Fix: seal and insulate the trunk with a vapor-barrier wrap, lay a ground barrier, close the vents. A day and a half under the house, priced like a substantial repair, followed by the carpenter.
A 1990s double-wide near Hollister. Belly wrap torn, insulation hanging, crossover duct full of water. The ground is sand and drains well, but the belly is soaked. Fix: replace the crossover, re-seal the trunk, re-hang the insulation and repair the wrap, lay a barrier over the sand under the duct. A single day, priced in the range of a mid-level repair, and the floor over the hall dries out within weeks.
Where it happens most
The pier-and-beam houses of Palatka’s historic districts, the Heights and Newtown; the frame houses in Welaka, Crescent City and Melrose; and the manufactured homes everywhere from East Palatka to Interlachen to Huntington. Near the river and the lakes the ground is wetter and the problem is worse. On the sand ridge it is milder but the belly wrap tears just the same. If your floor registers blow cool but the house never feels dry, look under the floor first.
Crawlspace and duct questions from under the house
The floor in the hallway feels soft. Is that from the duct?
Often, yes. A trunk line that sweats under a hallway drips in the same spot for years, and the subfloor above it absorbs the water. Fixing the duct stops the source; the floor repair is a separate job for a carpenter once the crawlspace is dry.
Can I just wrap the duct in more insulation?
Only if the duct is sealed first and the insulation has a vapor barrier on the outside. Insulation alone over a leaking duct or without a barrier soaks up condensation and makes things worse. The order of operations is the whole fix.
Should the crawlspace vents be open or closed?
In this climate, open vents let humid outdoor air in all summer and the crawlspace never dries. Closing them and controlling the moisture with a vapor barrier and, where needed, a dehumidifier is the modern approach. It is a change from what older builders did, and it works.
Is a sealed crawlspace worth it on a manufactured home?
A full encapsulation is rarely justified on a manufactured home; a ground vapor barrier, repaired belly wrap and sealed, insulated duct achieve most of the benefit at a fraction of the cost. Encapsulation makes more sense on a site-built pier-and-beam house that the owner plans to keep.
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