Ductwork Sweating and Dripping in Summer

Quick Answer

Sweating ductwork happens when warm, humid air in your attic, basement, or crawlspace contacts the cold surface of air conditioning ducts. The moisture in that air condenses on the duct -- exactly like a cold glass of water beading up on a hot day. The fix is insulating the ducts to prevent warm air from reaching the cold metal surface, and reducing humidity in the space around the ducts.

Why Your Ducts Are Dripping

If you have ever set a cold drink down on a table in July and watched a puddle form underneath it, you already understand the physics behind sweating ductwork. When air conditioning pushes 55-degree air through metal ducts that run through a 90-degree attic or a humid basement, the surface temperature of the duct drops well below the dew point of the surrounding air. Moisture condenses on contact.

A little condensation is normal on particularly humid days. But when ducts are dripping steadily enough to stain ceiling tiles, wet insulation, or create puddles, you have a problem that will only get worse. Water and building materials are not friends, and it does not take long for persistent moisture to invite mold into the conversation.

I have seen this most often in homes where the original ductwork was installed with minimal insulation, or where the insulation has deteriorated over the years. Fiberglass duct wrap that was installed 20 years ago can sag, compress, or develop gaps where sections were joined -- and every gap is a spot where condensation will form.

The Two Things That Must Be True

Duct sweating requires two conditions to exist at the same time:

The duct surface is cold. This is unavoidable when the AC is running. Supply ducts carrying conditioned air will always be cold.
The surrounding air is warm and humid. This is the variable you can control.

If you eliminate either condition, the sweating stops. Since you cannot make the ducts warmer without defeating the purpose of your air conditioning, the strategy is to either keep humid air away from the duct surface (insulation) or reduce the humidity in the space around the ducts (ventilation and moisture control).

How to Fix Sweating Ductwork

What About Flex Duct?

Flexible duct (the round, corrugated hose-like material common in many homes) usually comes pre-insulated with an outer vapor barrier. If your flex duct is sweating, the vapor barrier may be torn or the insulation may have bunched up where the duct sags between supports.

Check for sections where the duct hangs too low between hangers, creating a U-shape where condensation can pool inside the outer jacket. Re-support these sections so the duct runs straight or with a gentle slope toward a register. If the outer vapor barrier is torn, patch it with foil tape or replace the section entirely.

The Crawlspace and Basement Factor

If your ducts run through a crawlspace with a dirt floor and no vapor barrier, that dirt is likely the biggest humidity source in your home. Ground moisture constantly evaporates upward, creating a humid environment that will overwhelm even well-insulated ductwork.

A 6-mil polyethylene vapor barrier on the crawlspace floor, with seams overlapped by 12 inches and taped, can reduce crawlspace humidity dramatically. Some homeowners go further and fully encapsulate the crawlspace with a sealed vapor barrier on walls and floor, plus a dehumidifier. This is a more significant project but essentially eliminates the humidity problem.

If your dehumidifier is running but humidity is not dropping, solve that issue before assuming the ductwork insulation is inadequate -- you may be fighting a losing battle against a moisture source.

When Sweating Ducts Signal a Bigger Problem

Duct sweating that appears suddenly in a home where it was never an issue before can indicate a change in the system:

A new roof or re-roofing may have altered attic ventilation, trapping more humid air.
A blocked soffit vent (from insulation pushed against it during an attic insulation upgrade) reduces airflow and raises attic humidity.
An oversized AC system produces colder supply air than necessary, making duct surfaces colder and more prone to condensation. If your house feels humid even with the AC running, an oversized unit is a prime suspect.
Duct leaks that have developed over time let humid attic or crawlspace air get sucked into the return side, increasing moisture load on the system and contributing to condensation.

The Cost of Ignoring It

A few drips on a summer afternoon might not seem urgent, but persistent duct condensation leads to:

Water-stained ceilings. The drips find their way to drywall, leaving yellow-brown stains that look like a roof leak.
Mold growth. Wet insulation and damp building materials in enclosed spaces are ideal for mold. By the time you see it, it has usually been growing for months.
Damaged insulation. Wet fiberglass insulation compresses and loses its R-value, making the sweating worse in a feedback loop.
Structural concerns. In extreme cases, persistent moisture can rot wood framing, decking, or subfloor near duct runs.

Fixing duct insulation is a weekend project for accessible ductwork. Materials for an average home run $150 to $400. Professional insulation of ductwork typically costs $800 to $2,000 depending on the linear footage and accessibility. Either way, it is far cheaper than mold remediation or ceiling repair.