Leaky ducts: the 20-30% of your cold air that goes into the attic

In a typical house, between 20% and 30% of the air moving through the ducts is lost to leaks, holes and poorly connected ducts, according to the DOE. In Florida that is worse than in any other state: ducts usually run through an attic above 120°F, so every leak cools the attic and the air that does reach your room arrives reheated. It is the cheapest repair with the biggest impact you will find, and almost nobody looks at it before selling panels.

What are duct leaks and why do they happen?

Ducts are the tubes (flexible or sheet metal) that carry cold air from the unit to the registers and bring it back. Leaks appear at joints, elbows, register boot connections and the plenum (the big box attached to the unit). The DOE describes it this way: leaks, holes and poorly connected ducts that let conditioned air escape into unconditioned spaces, adding hundreds of dollars a year to heating and cooling bills.

Typical causes in Florida:

• Original installation with tape that dried out and peeled in the attic heat.
• Flex ducts crushed, kinked or with a torn outer jacket.
• Rodents, attic work, previous technicians who disconnected and never reconnected.
• A return that pulls hot attic air through an open joint.

How does it affect the bill and comfort?

With air conditioning at about 50% of a Florida home’s bill (FPL), a 25% duct leak is a leak in the money you pay every month.

Heads up: a solar system sized on leaky ducts is a system that pays to cool the attic. If you seal after installing, the system ends up oversized and the annual surplus is settled at the generation rate, lower than what you pay. Nobody who quotes panels without asking about the ducts is designing for your house. You are right to distrust that.

How are they tested and sealed?

Test. A technician connects a calibrated fan (duct blaster) to the system, seals the registers and measures how much air escapes at a reference pressure. The result is expressed in CFM (cubic feet per minute) of leakage; Duke Energy Florida, for example, publishes the leakage-per-ton threshold above which its duct test-and-repair rebate applies, and the minimum share of leakage that must be sealed.

Sealing. The DOE is clear: duct mastic is the preferred material for sealing seams and joints because it is more durable than any tape and easier to apply. The gray tape everyone calls “duct tape” is not made for ducts and peels in the heat. Joints, elbows, register boots and, above all, the plenum and unit connections get sealed. Afterward, the DOE recommends insulating ducts that run through unconditioned spaces like the attic: usually very cost-effective.

Verification. Test after sealing to confirm the final CFM. Without an exit test, you do not know what you bought.

What does duct sealing change for a new A/C?

The load calculation. A Manual J run on sealed, insulated ducts comes out lower than one run on leaky ducts, and a lower load can mean a smaller, cheaper unit. That is why sealing goes before buying equipment, not after: the DOE’s 20-30% loss is part of what an oversized unit is bought to overcome.

When sealing does NOT make sense (yet)

• Flex ducts are crushed or torn over long runs: replace the run, do not seal it.
• You are replacing the unit with a different size and the duct design will change: seal after the new design.
• The attic is about to be foamed under the roof deck (encapsulated attic): the strategy changes; ask first.
• The technician proposes sealing without an entry and exit test: do not accept. Without measurement there is no way to know it worked.

What to check before you decide

• Ask for a leakage test with a number (CFM at reference pressure) before any work, and another when finished.
• Climb into the attic or ask for photos: peeling tape, kinked ducts, open joints, unsealed return.
• Confirm mastic and mesh are used, not gray tape, and that the plenum and unit connections are sealed.
• Ask your utility (Duke, FPL) whether it has a duct rebate and what prior inspection it requires.
• If you have a solar or new A/C quote, ask for it to be recalculated after sealing. Start with the 3 pillars and the energy audit.