Answer first, context after
Why is my ductwork sweating in the attic?
Cold duct surfaces meeting hot humid air, which in a Bakersfield attic in August is a lot of hot air. Sweating ducts almost always mean the insulation or its vapor barrier has failed, or a leak is pulling attic air against cold metal. Left alone it soaks insulation, stains ceilings, and grows mold, and it is also a sign you are paying to cool an attic.
Condensation on ductwork is the same physics as a cold glass of tea on a hot porch. Air can only hold so much moisture, and when it touches something cold enough, that moisture comes out as water. Our attics hit 130° on a summer afternoon while the air inside the ducts is near 55°, which is a bigger difference than most parts of the country ever see.
Why it starts. A duct that is properly insulated and sealed does not sweat, because the warm air never reaches the cold surface. So sweating means something has broken down:
- Insulation is torn, compressed, or missing, often where someone crawled through the attic to run cable or set a box down.
- The vapor barrier has split. The outer jacket is not just insulation, it is a moisture barrier, and once it tears, humid air gets underneath it and condenses on the metal where you cannot see it.
- The duct is leaking. A leak lets cold air escape and chills the surrounding surfaces and nearby framing.
- Airflow is poor, so air sits in the duct getting colder than it should. A filter choked with valley dust does this.
- Humid air is getting into the attic from a bathroom fan venting into it rather than outside, which is more common than it should be.
Why it is worth fixing beyond the water. Wet insulation stops insulating, and once it packs down it never recovers. You get ceiling stains, sometimes mold, and occasionally drywall damage that costs more than the duct work would have. And the same failure that lets ducts sweat is leaking the air you paid to cool into a 130° attic, which shows up on the bill every month.
What we do about it. Our duct inspection and leak test is $189, and it is credited toward any duct work we do. We measure the leakage rather than eyeball it, find where the insulation or vapor barrier failed, and quote the repair flat: targeted repairs run $340 to $980, with a full hand-seal and re-strap quoted after the test. Details on the duct repair and sealing page.
One honest note. If your ducts are sweating and your rooms are also uneven, the two are usually the same story, and sealing the ducts often fixes both. See do my ducts need sealing and why is one room always hot.
Sweating ducts soak insulation.
It looks like a roof leak and behaves like one. The cause is usually missing or crushed duct insulation, which is a repair, not a replacement.