Excerpted from the May 2026 Palm Beach Air Conditioning Contractors Association (PBACCA) Plenum Newsletter, submitted by Sean Caplan, Service Manager, EDS Air Conditioning & Plumbing
If you’ve been in the field for 3 to 5 years, you’ve likely graduated from basic component swaps to the more “haunting” service calls. These are the ones where the client claims their ceiling is leaking, but there isn’t a plumbing pipe in sight.
In South Florida, Dew Point isn’t just a textbook term; it’s a daily adversary. As homes get tighter and high-efficiency equipment becomes the standard, the physics of sweating ducts is becoming a complex trigger for callbacks in our unique, high-humidity environment.
The “New” Physics of High-Efficiency in the Heat
In the past, we ran high-static, high-velocity blowers that moved air fast enough that the duct jacket often stayed just above the dew point. Today, we’re installing high-SEER2, variable-speed systems designed for long run cycles at lower CFMs to maximize dehumidification.
While this is essential for Florida comfort, it means ductwork in unconditioned attics, where temperatures often exceed 110°F to 120°F with humidity consistently above 70%, is staying cold for much longer. If that duct insulation has a minor “pinch” or a loose mastic seal, that cold surface will hit the dew point of the ambient attic air almost instantly. In Broward or Palm Beach, summer dew points are often between 70°F and 75°F, meaning any surface cooler than that will sweat.
Regional Triggers:
The Vapor Barrier Myth: You’ll see R-6 or R-8 flex duct that looks fine from five feet away. But in our tropical climate, if the outer silver jacket is nicked or the tape has failed at the plenum, moisture-laden air migrates under the insulation. Once that fiberglass gets damp, its R-value drops to near zero, and the “sweating” becomes a flood.
Building Pressure Imbalance: With tighter home envelopes, we see “infiltration-driven sweating.” If a house is under negative pressure due to leaky return ducts in the attic, it pulls humid outdoor air through can lights and top plates. That high Florida humidity hits your 55°F supply registers, and suddenly you have “sweating grills.”
Oversized AC / Undersized Duct Combo: A common “old school” mistake in South Florida is slapping a 3-ton unit on a 2-ton duct system. The restricted airflow makes the coil—and the ductwork—drastically colder than intended, pushing the surface temperature well below the local dew point.
Using the Psychrometric Chart to Explain the “Invisible”
When a customer in West Palm or Boca sees water dripping from their vents, they think the AC is broken. You know better, but proving it is easier with a Psychrometric Chart.
Plot the Room Air: Use your probe to find the indoor Dry Bulb (e.g., 75°F) and Relative Humidity (e.g., 50%).
Find the Dew Point: Follow the horizontal line to the left until you hit the 100% saturation curve. In this example, the dew point is about 55°F.
The “Smoking Gun”: Show them that if the supply air is 53°F, that register is colder than the air’s dew point. It’s not a mechanical failure; it’s physics. This visual helps them understand why they need to stop leaving the slider door open or why that attic duct needs a better seal.
For a quick refresher on reading the psychrometric chart to pinpoint dew point issues in the field, check out this tutorial.
The Pro Fix: Beyond the Tape
Don’t just wrap more insulation over a wet spot. Check your Static Pressure and your Target Evaporator Temperature. If you’re running a 35°F-40°F coil because of low airflow, you’re asking for a moisture disaster in this humidity.
Modern tech requires us to be “Airflow Surgeons.” Ensure your plenums are sealed with mastic (not just tape), verify your blower settings match the calculated load, and always check the attic’s ventilation. In South Florida, if the attic can’t breathe, your ducts can’t stay dry.
Stay dry out there.
