Leaking Attic Supply Plenum in Fort Worth: Why Tape Won't Hold It

A Fort Worth summer afternoon, the thermostat set where it always is, and the AC running without a break. The back bedroom is still warm, and the electric bill is higher than last year. It is easy to assume the air conditioner is failing. Sometimes the equipment is fine and the cooled air is simply going somewhere else: into the attic, through a supply plenum that has come apart.

This post walks through a recent Lantern job where that happened, the signs to watch for, and why a roll of tape is not the fix.

What a supply plenum does

In most DFW homes with an attic system, the air handler pushes cooled air into a box attached to its outlet. That box is the supply plenum. From there, the air splits into the duct runs that feed each room.

Because it sits right where the air leaves the blower, the plenum carries the highest pressure on the supply side. It also sits in one of the hottest places in the house. A North Texas attic in summer runs far hotter than the outdoor air, while the plenum carries the coldest air the system makes. Years of that heat and cold, plus vibration from the blower and the weight of the ducts pulling on it, wear the joints loose.

Before: the seam at the air handler collar had split open, and the old tape and mastic had torn apart.

What we found in this attic

On a recent job in Fort Worth, the plenum was built from fiberglass duct board. The seam at the air handler collar had separated completely. The lower seam had torn as well, and it sat tight against the wall framing and the attic decking. Cooled air was blowing under the decking and into the attic.

Before: the lower seam tore right against the framing, where no patch could reach.

Why supply plenums come apart

When we look at failed plenum joints, the same three causes keep showing up.

• Tape and mastic that have aged out. Adhesive tapes and thin coats of mastic dry out and crack after years of attic heat on the outside and cold air on the inside.
• Duct board that has lost its strength. Fiberglass duct board is inexpensive and widely used. It is far less stiff than sheet metal, and over time the weight of the duct take-offs pulls its edges away from the metal collars until they tear.
• No screws and no support. A joint held only by tape or mastic has nothing mechanical keeping it together. Without sheet metal screws at the flange and straps holding the plenum up, gravity and blower pressure slowly pull it apart.

5 signs your supply plenum may be leaking

Because the air handler is out of sight in the attic, a failed plenum can go unnoticed for months. Watch for these:

• Weak airflow in the far rooms. Bedrooms at the ends of the house get less air, because some of it escapes before it reaches their ducts.
• Long runtimes on hot afternoons. The system runs and runs without reaching the setpoint.
• A higher electric bill. You are paying to cool the attic. ENERGY STAR says that in a typical house, about 20 to 30 percent of the air moving through the ducts is lost to leaks, holes and poorly connected ducts.
• Dust and drafts indoors. When supply air blows out into the attic, the house can pull attic air back in through gaps around ceiling vents, can lights and other openings. Dust streaks around those spots are a clue.
• A whooshing sound near the attic access. A large tear on the supply side can be loud enough to hear from the hallway or the attic stairs.

If any of these sound familiar, have someone look at the plenum and the first few feet of each duct, where the air pressure is highest.

Why tape over the split will not hold

The natural instinct is to put foil tape or spray foam over the split. Tape and mastic seal a joint. They do not hold one together. ENERGY STAR recommends mastic or metal tape for sealing duct leaks and says never to use plain duct tape, because it does not last. But when a joint has already pulled apart, or the duct board around it is tearing, sealing the surface does nothing about the force pulling it open.

Repair	What holds the joint together	How it holds up in a DFW attic
Tape alone over the split	Nothing but the adhesive	No tape holds a joint that has already pulled apart
A smear of mastic over the gap	Nothing behind the mastic	Cracks once the duct sags or flexes again
New metal plenum, screwed, strapped and sealed	Screws at the flange and straps to the framing	Built to stay put, with mastic sealing every seam

During: the old duct board coming out, brittle and torn away from the take-offs.

How we rebuilt the supply plenum

On this job, a proper repair meant moving the air handler. It sat flush against the wall framing and floor decking, so there was no room to reach the back, bottom and side flanges with sheet metal tools.

• Disconnect and shift the air handler. We disconnected the power and control wiring, the overflow float switch and the condensate line. Then we moved the unit on its platform to open up the whole collar.
• Build in a new metal plenum. The duct board came out. In its place went an insulated 26-gauge sheet metal supply plenum with new start collars.
• Screw it on. The plenum was fastened to the air handler’s discharge flange with self-tapping screws every 4 inches, so the joint cannot pull apart under blower load.
• Hang it on its own. The plenum and its take-offs were strapped and supported separately, so their weight no longer pulls on the air handler.
• Seal every joint. Each seam, collar and flange was wrapped in fiberglass mesh tape and coated with a continuous layer of UL 181A-M listed duct mastic.

After: the new metal plenum, screwed to the air handler, strapped, and sealed in mastic.

What changed after the repair

After the system was reconnected, static pressure was checked and read where it should for this system. With the plenum sealed, the cooled air goes to the rooms instead of the attic. That means shorter cooling cycles on hot days, less wear on the equipment, and far less attic air pulled down into the house through the ceiling.

What plenum repair looks like with Lantern

Plenum work depends on what is in your attic: how the plenum was built, what has failed, and how much room there is around the air handler. Lantern quotes it in writing after a look in the attic, before any work starts. If the problem turns out to be a part in the system instead, our AC repair page lists starting prices for the common ones.

If your AC is running but the house is not keeping up, our post on why an AC runs but does not cool covers the other usual causes. The HVAC services page shows everything we work on, and our Fort Worth HVAC page covers the area.

Tell us what you are noticing in the form below, and a Lantern team member will reach out within 24 hours.