Why is my attic ductwork wet?
Attic ductwork sweats when the outside surface of the duct falls below the dew point of the air around it. In a vented Florida attic that air is hot and extremely humid, and the duct is carrying air at roughly 55°F, so the two conditions meet easily. The water is condensing out of the attic air onto the duct, the same way a cold glass sweats on a porch in August. It is a sign that the insulation, the vapor barrier or the attic humidity has a problem — not that the duct is leaking water.
The short version
- Sweating ducts are a dew point problem. The duct surface is colder than the air touching it.
- A vented attic is the moisture source. As Building Science Corporation puts it, "the attic venting is responsible for the sweating ducts."
- Florida attics routinely run a dew point in the 60s and 70s°F while a supply duct carries air near 55°F. The margin is thin by design.
- The usual culprits are thin, compressed, torn or previously wet insulation, a failed vapor jacket, or duct leakage cooling the surrounding air.
- Cleaning a sweating duct does not fix it. Insulation that has been wet is replaced, not cleaned — that is EPA, NADCA and NAIMA guidance, not our opinion.
What is actually happening
Air holds a certain amount of water vapour. Cool that air down far enough and it cannot hold it any more, and the water comes out as liquid on the nearest cold surface. The temperature at which that happens is the dew point.
A vented attic in Northeast Florida pulls in outside air all summer. In July that air can carry a dew point in the mid-70s. Meanwhile the supply ducts running through that attic are carrying conditioned air at around 55°F. If the outer surface of the duct gets anywhere near the attic's dew point, water forms on it.
Building Science Corporation, which is about as authoritative as this field gets, states it plainly: "Cold air handlers and cold ducts in vented attics in hot humid climates sweat." And in a companion piece: "The attic venting is responsible for the sweating ducts. This outside air is the source for the condensation on your attic ductwork."
So the water is not coming from inside the duct. It is coming out of the attic air, onto the duct.
Why the margin is so thin here
A US Department of Energy field study in a hot-humid climate measured attic dew points ranging from 60°F to 85°F across a season, with attic temperatures peaking near 120°F. In the same study, correctly buried ducts held their surface temperature above the local dew point by only six to eight degrees.
Six degrees is the entire safety margin. Anything that erodes it — insulation squashed flat by someone's knee, a torn vapour jacket, a section that got wet during a roof leak and never properly dried — and that length of duct starts condensing.
What we actually find causing it
- Compressed or displaced insulation. Flex duct insulation only works at its rated thickness. Where it is pinched over a truss chord or crushed under stored boxes, the R-value at that spot is a fraction of what the label says, and that is exactly where the dripping starts.
- A failed vapour jacket. The outer plastic skin is not just a wrapper. It keeps humid attic air from reaching the cold inner surface. Once it is split, taped badly or UV-degraded, the insulation itself gets damp and stops insulating.
- Duct leakage. Supply air escaping into the attic chills the air immediately around the duct, dropping that pocket of air closer to its dew point. A leak and a sweat often sit in the same place.
- Return leakage in reverse. A leaky return pulls hot humid attic air straight into the system, raising the moisture load indoors and keeping the equipment running longer in a wet attic.
- Undersized or crushed runs. Low airflow means colder supply air temperatures, which means a colder duct surface for the same attic conditions.
Why this is worth taking seriously
Condensation is a water problem, and water in an attic finds things to damage. Wet insulation loses its R-value permanently. Water tracks down the duct and stains ceilings, usually several feet from where it actually formed. Repeatedly wet fibrous material is where microbial growth starts.
The nuance Building Science makes, which is worth repeating, is that occasional sweating followed by a proper dry-out is survivable. The damage comes when the system runs so continuously that nothing ever gets a chance to dry.
What fixes it, in order
- Find out where and when. Photographs of the wet runs, and which part of the system they are on. A pattern on the return side points somewhere different from a pattern at the far end of a supply trunk.
- Seal the leaks. If leakage is chilling the air around the duct or dragging attic air into the system, sealing comes before anything else. Aeroseal reaches runs buried in insulation that nobody can hand-seal.
- Replace what got wet. Insulation that has been wet is replaced. EPA, NADCA and NAIMA all say the same thing about wet or mouldy fibrous duct material, and so do we. See duct replacement and insulation.
- Get the indoor humidity down. EPA's guidance is below 60% relative humidity, ideally 30–50%. If the house is sitting above that, the equipment is not removing enough moisture and sweating ducts are one symptom among several.
What does not fix it is cleaning. A clean sweating duct is a sweating duct. If someone proposes duct cleaning as the answer to a water problem, that is worth a second opinion.
What the field research shows
Common questions
Is sweating ductwork dangerous?
Not immediately, but it is a water problem and water problems compound. Wet fibrous insulation loses its R-value permanently and is where microbial growth starts. The ceiling staining that usually brings people to call us has typically been developing for a while.
Can you just add more insulation over the wet spot?
Not over wet material. Insulation that has been wet is replaced — adding a layer on top traps the moisture against the duct and makes it worse. Once the cause is dealt with and the replacement is in, the new insulation stays dry.
Does this happen in every Florida house?
No. It happens where the margin between duct surface temperature and attic dew point gets eaten up. Houses with ductwork inside the conditioned envelope largely avoid it. Houses with long runs across a vented attic, older insulation, or significant duct leakage are where we see it.
Why does it seem worse some years than others?
Because it tracks the weather and the run time. A wetter, hotter stretch raises the attic dew point and lengthens run times at the same time, so the system spends more hours making cold surfaces in humid air and fewer hours drying out.
Will sealing the attic vents fix it?
Converting to an unvented, conditioned attic does remove the moisture source, and it is a legitimate solution — but it is a substantial building envelope project with code implications, not a duct service. We would rather tell you that than sell you a cleaning that does not address it.
Sources and standards
Where this page relies on published guidance, here is what it relies on.
- Building Science Corporation — Insight 109: Duct Sweating (Lstiburek, 2019) — Why air handlers and ducts in vented attics sweat in hot humid climates.
- Building Science Corporation — Insight 094: No Sweat — How attic ventilation supplies the moisture that condenses on attic ductwork.
- US DOE Building America — Compact Buried Ducts in a Hot-Humid Climate — Field measurements of attic dew point, attic temperature and duct surface temperature.
- US EPA — Mold Remediation in Schools and Commercial Buildings — Federal guidance on indoor relative humidity targets and drying wet materials.
