Why Ducts Sweat
Every parcel of air has a dew point, the temperature at which its moisture turns to liquid. Supply ducts carrying air-conditioned air are cold. When warm, humid air around them cools below its dew point on contact, water forms on the duct. Three things make it worse: missing or thin insulation, leaks that let cold air chill the surrounding space, and high humidity around the duct.
Where Ducts Sweat Most
| Location | Why it sweats | First fix |
|---|---|---|
| Attic | Hot, humid air around cold supply ducts | Insulate and seal the runs |
| Crawl space | Damp ground raises humidity under the house | Control moisture, then insulate |
| Unfinished basement | Summer humidity meets bare metal | Dehumidify and insulate |
| Ceiling register | Cold register in a humid room | Seal the boot, check humidity |
The attic ductwork page and the crawl space ductwork page cover each location in more detail.
Fixes That Last
Seal the Leaks
ENERGY STAR recommends mastic sealant or metal tape on duct joints, never cloth duct tape. Typical ducts lose 20 to 30 percent of their air to leaks.
Insulate to Code
The residential code requires at least R-8 on attic supply and return ducts 3 inches and larger, and R-6 on those ducts in other unconditioned parts of the house.
Bring Humidity Down
The EPA recommends 30 to 50 percent indoors and staying under 60 percent. A dehumidifier helps in basements and crawl spaces.
Check the Airflow
A dirty filter or coil slows air across the coil and can make supply air colder than it should be.
The duct sealing page covers materials and which joints to seal first.
When the Insulation Is Already Wet
The EPA says duct insulation that gets wet or moldy can't be cleaned effectively and should be replaced. Wet insulation also stops insulating, which makes the sweating worse. A musty smell at the registers is often the first sign; the musty smell guide covers what to check.