Ductwork Sweating: Why Ducts Drip in Summer

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

Ductwork Sweating: Why Ducts Drip in Summer

Short answer

Ducts sweat when cold air inside them chills the metal below the dew point of the damp air around them, in a basement, crawlspace or attic. It is condensation, not a leak. The fixes are keeping the damp air off the metal with insulation that has an unbroken vapor barrier, sealing the joints, drying the space around the ducts, and making sure enough air moves through them.

In summer, air conditioning ducts can bead with water, drip onto the floor or the ceiling, and stain the wood around them. It looks like a leak, but it is condensation: the same thing that happens to a cold glass on a humid day. The cold air inside the duct chills the metal, and the damp air around it gives up its water on the cold surface.

This page is about the duct runs themselves, in a basement, crawlspace or attic. If the water is on a ceiling grille instead, that has its own page: water dripping from a ceiling vent.

Before you start

What to carry: a torch, a dry cloth and a roll of paper towel. Twenty minutes.

Gas or electric? Neither matters here. It happens with any central air conditioner or heat pump in cooling.

Make sure it is condensation. Dry the duct, run the cooling and watch. Water that forms evenly on the metal while it runs is condensation. Water that runs along from one spot, or appears with the cooling off, is a leak from somewhere else.

Why it matters. Wet wood and mold. Water dripping for a whole summer soaks joists, subfloor and drywall, and damp dark spaces grow mold. Deal with it in the season it starts.

A wet attic ceiling. Drips from attic ducts land on the ceiling below and show up as stains. Check the duct above any new stain in summer.

Why ducts sweat

Cold air inside, damp air outside. Where the damp air touches metal that is colder than its dew point, water forms.

Check 1Free, fifteen minutes

Find where it sweats

Supply ducts more than returns. The supply side carries the coldest air, straight off the coil, so it sweats first and most.

Bare metal. An uninsulated duct in a damp basement or crawlspace sweats on its whole length.

Torn or wet insulation. Insulation only works if its outer vapor barrier is unbroken. A tear lets damp air in behind it, where it soaks the insulation and drips.

Around the joints. Leaky joints let cold air spill onto nearby surfaces, and they sweat too.

The space itself. A crawlspace with bare earth, a basement with a damp floor, or a hot attic all hold far more moisture than the house.

The fixes you can do

Four fixes: an unbroken vapor barrier on the insulation, sealed joints, a drier space around the ducts, and enough air through them.

Check 2

Keep damp air off the metal

Insulate the supply ducts. Duct wrap with a foil or plastic facing on the outside, with every seam taped, keeps the damp air off the cold metal.

Mend the vapor barrier. Tape every tear in the facing of flexible or wrapped duct. Replace insulation that has got soaked.

Seal the joints. Use duct mastic or foil tape made for ducts on the joints and boots, not cloth duct tape, which dries out and lets go.

Dry the space. A crawlspace with a ground cover and a basement kept dry sweat far less: why a house gets too humid.

When the air is too cold

Check 3

Check the airflow

A clogged filter. Less air across the coil makes it, and the air leaving it, colder than it should be: how often a filter needs changing.

Closed registers. Closing many registers restricts the airflow through the whole system, and the air in the ducts runs colder: why a vent blows little or nothing.

Ice on the coil. If the coil or the copper pipes are frosting, the system has an airflow or refrigerant problem, and the ducts after it run too cold: why an AC freezes up.

When it needs a pro

Check 4A quote

Know when to call

Ducts you cannot reach. Runs buried in a tight crawlspace or behind finished walls need a duct contractor to wrap and seal them.

A whole system of leaky ducts. Sealing a full duct system is bigger work than one joint.

A crawlspace that stays wet. Standing water or a constantly damp crawlspace needs its drainage and ventilation put right first.

Questions people ask

Why is my ductwork sweating?

Cold air inside the duct chills the metal below the dew point of the damp air around it, so water forms on it, like a cold glass in summer. Bare or torn insulation, leaky joints and a damp space make it worse.

Is sweating ductwork a leak?

No. It is condensation on the outside of the duct. Dry it and watch: water that forms evenly while the cooling runs is condensation.

How do I stop ducts sweating in the basement?

Wrap the supply ducts with faced insulation and tape every seam, seal the joints with mastic, and keep the basement drier.

Can a dirty filter make ducts sweat?

Yes. Less air across the coil makes the air in the ducts colder, and colder ducts sweat more. It is the cheapest thing to check.

Should I use duct tape on the joints?

Use duct mastic or foil tape made for ducts. Ordinary cloth duct tape dries out and lets go.

Why do only some ducts sweat?

The supply ducts carry the coldest air, and the ones in the dampest spot, or with torn insulation, sweat first.

Can flexible duct sweat?

Yes. Its outer plastic jacket is the vapor barrier. If it is torn, damp air reaches the cold inner core and water collects in the insulation. Tape the tear, or replace a soaked section.

Will a dehumidifier stop ducts sweating?

In a basement it helps, because drier air has a lower dew point and wets cold metal less. It does not fix bare metal or a torn vapor barrier, so do those too.

Sources: DOE Building America Solution Center "Sealed and Insulated Metal Ducts" and "Sealed and Insulated Flex Ducts" (duct sweating, vapor barrier, mastic, cloth duct tape), ENERGY STAR "Duct Sealing", and Lawrence Berkeley National Laboratory LBNL-54005 "Register Closing Effects on Forced Air Heating System Performance"