What causes water to drip from an air-conditioning duct under the house?

The duct transfers the cold air produced by an air conditioning unit throughout the home. It is most commonly installed either in the attic of a house or in the mezzanine or basement below the house. Dripping water from a duct is common, but excessive dripping can trigger mold or mildew to bloom or damage soft materials such as drywall.


The condensation on the outside of the cold duct is the most common cause of water dripping. Heat, moist air surrounds the thin metal conduit used under houses and causes any moisture in the air to condense. The same process causes condensation to gather on the outside of a glass of water in a warm room. Keeping air conditioning at a specific temperature during the warm months of the year reduces condensation. This works to maintain the mezzanine or basement where the duct is cold enough to avoid condensation.

Poor insulation

Inadequate insulation in the ducts under your house can cause the water to collect and lead to decay of the wood or mold. A well-known home improvement expert, explains that poorly insulated ducts also raise the cost of cooling a home because the air heats up quickly as it travels through ducts without heat insulation. Flexible, pre-insulated plastic conduits can be installed to prevent water drips or traditional fiberglass insulation can be wrapped around the ducts.

Collected water

Although ducts in a house rarely go through both the attic and the basement or mezzanine of a house, if they do, dripping water would be coming from a leak in the ceiling. Inspection experts say that gaps or openings in the ducts located in the roof or attic of a building can leave water in the conduits, which will then seep into the lowest point in the system. The entrance of water into the ducts in the attic would also indicate the presence of a leaky roof.