Troubleshooting Ride-Along: Why Are My AC Ducts Sweating?
Location: Palm Coast, Florida
Complaint: Water forming on the ductwork and causing moisture problems around the AC ducts.
Today’s call:
“My AC ducts are sweating.”
That’s pretty common in Florida.
Cold ductwork.
Hot, humid attic.
Damaged insulation.
High humidity.
Air leakage.
There are several things we’re going to look for.
So up into the attic we go.
And this time we didn’t need gauges, psychrometers or a particularly complicated diagnostic process to identify our first problem.
Apparently, the homeowner had hired a subcontractor before we arrived.
A raccoon.
And he had absolutely destroyed the insulation around the metal ductwork.
Excellent work, sir.
Why Ducts Sweat in the First Place
Supply ducts carry cold air through some extremely unpleasant places.
Especially Florida attics.
The metal duct underneath the insulation can get very cold. Meanwhile, the surrounding attic air is hot and loaded with moisture.
Normally, insulation and its vapor barrier keep that humid attic air away from the cold metal surface.
But expose the metal?
Now we’ve created exactly what condensation needs:
Cold surface + warm humid air = water.
Same reason a cold drink sweats outside.
Except your Coke isn’t suspended above a drywall ceiling.
That’s the important difference.
Into the Palm Coast Attic
We climbed up and started inspecting the duct system.
The homeowner had metal ductwork with exterior insulation.
Or at least…
it used to.
Sections of the insulation had been ripped apart.
Other areas were pulled away from the metal.
And portions of the cold metal duct were now exposed directly to attic air.
This wasn’t insulation that had simply deteriorated gracefully over thirty years.
Something had gotten into it.
The culprit?
Raccoon.
Apparently he didn’t appreciate the original R-value.
Now Run the AC
With the system operating, cold supply air moves through that metal duct.
The metal gets cold.
Then hot, humid Palm Coast attic air hits the exposed surface.
And moisture begins forming directly on the duct.
There’s our sweat.
This wasn’t water leaking from inside the duct.
It wasn’t a condensate drain overflowing.
And it wasn’t necessarily a roof leak.
The water was coming directly from the air surrounding the ductwork.
Florida provided the water.
The raccoon provided access.
Teamwork.
Why We Still Inspect the Rest of the System
The destroyed insulation was pretty compelling evidence.
But we still don’t want to assume it’s the only issue.
Duct sweating can also be affected by:
- damaged or missing insulation
- failed vapor barriers
- duct leakage
- high attic humidity
- unusually cold supply-air temperatures
- airflow problems
- high indoor humidity
- poorly sealed duct connections
So we inspect the system rather than simply wrapping the first wet spot we find.
In this case, though, large areas of damaged insulation around cold metal ductwork gave us a very obvious reason for the condensation.
Why Can’t We Just Dry It and Leave It?
Because tomorrow the AC is going to get cold again.
And Palm Coast is going to remain Florida.
If we don’t restore the insulation and vapor barrier, we haven’t changed the conditions causing the condensation.
The metal gets cold.
Humid air reaches it.
Water forms.
Repeat.
Eventually that moisture can affect surrounding insulation, wood and drywall. Persistent duct condensation can cause water damage and other moisture problems if the underlying insulation or air-sealing issue isn’t corrected.
So this isn’t something we want dripping away in the attic indefinitely.
The Insulation Does More Than Keep the Cold Air Cold
This is something homeowners don’t always realize.
Duct insulation obviously reduces heat transfer.
We don’t want 55-degree supply air traveling through a blazing-hot attic inside bare sheet metal.
But the insulation system also helps keep the outside surface exposed to humid air from becoming cold enough to sweat.
And the outer vapor barrier helps prevent moisture-laden air from reaching colder surfaces underneath.
If that insulation system gets ripped apart, the metal underneath can become a condensation magnet. Damaged or missing insulation is a well-established cause of sweating ductwork in humid climates.
Which brings us back to our furry HVAC technician.
The Repair
The damaged sections of insulation needed to be restored so the metal ductwork was once again properly insulated and protected from the humid attic environment.
That means we’re not just throwing a little tape over a giant destroyed section and declaring victory.
We need to restore the thermal insulation and vapor barrier around the affected ductwork.
Then we can operate the system and make sure the sweating condition has been corrected.
And perhaps address the other fairly obvious building-maintenance issue:
Stop the raccoon from getting back into the attic.
Because our HVAC warranty does not include disagreements with wildlife.
The Palm Coast Verdict
Complaint: Sweating metal ductwork.
What we found: Large sections of duct insulation damaged and removed.
Cause of insulation damage: Raccoon.
Yes.
Actually.
Why the ducts were sweating: Cold metal duct surfaces were exposed to hot, humid attic air, allowing condensation to form.
Repair: Restore damaged duct insulation and vapor barrier.
Also recommended: Evict raccoon.
Preferably before he discovers the low-voltage wiring.
The Bigger Lesson
When AC ducts sweat, don’t automatically assume the air conditioner is producing too much moisture.
First figure out where the water is actually coming from.
Sometimes it’s a duct leak.
Sometimes it’s excessive humidity.
Sometimes it’s failed insulation.
Sometimes it’s a combination of several things.
And sometimes you climb into a Palm Coast attic and discover that a raccoon has removed enough insulation to create his own little condensation experiment.
The physics were actually pretty simple.
Cold metal.
Humid air.
Missing insulation.
Water.
The only unusual part was figuring out who removed the insulation.