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How Do I Know If My Ducts Are Leaking? Duct Sealing in Las Vegas

The most common signs of leaking ducts are rooms that never get enough air, a house that’s dustier than it should be, and an AC that runs long while some rooms stay warm. Most leaks hide where you can’t see them: at joints in the attic, at the boxes behind supply vents, and where flex duct connects to metal fittings. On September 30, 2026, we re-routed duct runs and sealed air leaks at the supply vents in a large Las Vegas home in the 89117 ZIP that has several AC systems. The connections we worked on were sealed with duct mastic, and we measured airflow at the supply registers.

What We Found

The homeowner asked us to re-route some ductwork and seal air leaks at the supply vents. In the attic, one duct end had been closed off with overlapping patches of foil tape, with the insulation pulled back around it. At the supply vents, the connections where the ducts meet the boxes in the ceiling are where cooled air can slip out into the attic before it reaches the room.

How We Fixed It

Re-routed the duct runs the homeowner wanted moved.
Coated the taped duct end in mastic, a thick paste that cures into a flexible, airtight seal.
Sealed the duct joints and the connections at the supply vent boxes with mastic.
Measured airflow at the supply registers with an anemometer.

How it turned out

The re-routed runs and the supply connections were sealed with mastic, and airflow at the registers was measured and photographed.

Can I check my ducts for leaks myself?

You can spot some clues. In the attic, look for flex duct that’s crushed, kinked, or pulled loose from a fitting, and tape that’s peeling off joints. Inside, dark dust lines around the edges of ceiling vents, a whistle at a register, or one room that’s always weak are other signs. A duct leakage test finds the rest. Our ductwork troubleshooting page walks through it.

Why use mastic on duct joints?

Foil tape loses its grip in attic heat over the years, and cloth duct tape fails even faster. Mastic bonds to the duct, fills gaps, and holds up to Las Vegas attic temperatures. That’s why we coated over the old taped end here.

Do leaky ducts cost me money?

Yes. Cooled air that leaks into an attic that can top 130 degrees in summer is air you paid to cool and then lost. The system runs longer to make up for it, and the rooms at the end of the leaky runs stay warm. See weak AC airflow and rooms at different temperatures.

Expert insight

“Foil tape isn’t a lasting seal on ductwork. In Las Vegas attics it dries out and lets go, and the air leaks into the attic instead of your rooms. We seal duct joints and vent connections with mastic, which cures into a flexible, airtight seal, then measure airflow at the registers.”

J.C. PauliAir Duct Cleaning Manager

Safety note: HVAC repairs involve high voltage, refrigerant and, on gas furnaces, combustion gases. Leave them to a licensed technician.

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