Duct Leakage: Why 30% of Your Heating May Be Going Nowhere

When I commission HVAC systems as part of an energy audit, one of the first things I want to know is the duct leakage rate. Not the SEER rating on the equipment. Not the insulation levels in the attic. The duct leakage, because in my experience, it is one of the most impactful and consistently underaddressed energy problems in homes with forced-air heating and cooling.

The reason it gets missed so often is simple: the ducts are hidden. They're in the attic, in crawl spaces, inside walls. Nobody sees a leaking duct seam the way they see a drafty window. The air goes somewhere it wasn't supposed to go, the house doesn't get as comfortable as it should, the system runs longer to compensate, and the energy bill quietly comes in higher than it needs to be — month after month, year after year.

How Duct Leakage Works

A forced-air HVAC system has two sets of ducts: supply ducts that carry conditioned air from the air handler to the registers in each room, and return ducts that carry room air back to the air handler to be heated or cooled again. Both sides can leak.

Supply duct leakage sends conditioned air into unconditioned spaces — the attic, the crawl space, between wall cavities. In summer, you're paying to cool air that ends up heating your attic. In winter, you're paying to heat air that goes directly to the cold crawl space below your floors. Either way, the room you wanted to condition gets less airflow, the system runs longer, and the efficiency of even high-rated equipment drops significantly.

Return duct leakage is often worse from a comfort standpoint. Instead of drawing conditioned room air back to the system, a leaky return duct draws in whatever air is available at the location of the leak — which in an attic installation typically means very hot air in summer and very cold air in winter. The system then has to work harder to condition that air before delivering it, compounding the efficiency loss.

What a Duct Blaster Test Measures

A duct blaster is a calibrated fan that attaches to the air handler or to a register and pressurizes or depressurizes the duct system while all the supply registers are sealed with foam plugs or tape. The fan measures how much airflow is required to maintain a standard pressure difference — typically 25 Pascals — across the duct walls. That airflow measurement, in CFM25, represents how much air is escaping through unsealed joints and gaps.

The result is usually expressed as a percentage of total system airflow or as CFM25 per 100 square feet of conditioned floor area. Tight duct systems aim for 4 CFM25 per 100 square feet or less. Many existing homes measure 2 to 5 times that.

Some auditors also perform a "duct leakage to outside" test, which measures only the leakage that goes to unconditioned spaces — the most energy-impactful portion. This requires a simultaneous blower door test and a duct blaster test run together, which not all contractors offer but which gives you the most useful number.

Where Leaks Usually Are

In my field experience, the most common locations for duct leakage are:

• Connections between sections of ductwork. Flex duct connections to rigid trunk ducts, connections at equipment cabinets, and anywhere two duct sections meet are high-risk points that are often inadequately sealed at installation.
• Register boots. The sheet metal box where a duct connects to the floor or ceiling register is frequently poorly sealed where it meets the drywall or subfloor. This can also be a significant building envelope air leakage source in addition to duct leakage.
• Unsealed duct ends and abandoned connections. Older duct systems often have capped-off branches from previous room configurations that are simply folded over or stuffed with insulation rather than properly sealed.
• Air handler cabinet leakage. The equipment cabinet itself often has gaps around penetrations, filter access doors, and panel seams that add to the measured leakage even when they don't technically involve the duct runs.

How to Fix It

For accessible ductwork — runs in an open attic or crawl space where you can physically reach the connections — mastic sealant is the professional standard. Mastic is a thick paste applied by brush or gloved hand to all joints and seams. It cures to a flexible, durable coating that doesn't dry out or crack over time the way standard duct tape does. (Standard silver duct tape, confusingly, is one of the worst things you can use on ducts — it loses adhesion in heat and cold and fails within a few years.)

For ductwork inside walls, chases, or other inaccessible locations, Aeroseal is a pressurized sealant injection technology that coats leaky seam interiors from the inside. It's more expensive than manual mastic work but is highly effective for systems where the leaks can't be reached by hand.

After any duct sealing work, a post-work duct blaster test confirms what was achieved. Any contractor doing duct sealing work who doesn't offer pre- and post-testing either doesn't have the equipment or has no interest in demonstrating results. Both of those are red flags.

The Right Order of Operations

Duct sealing should come before equipment replacement — and often before significant building envelope work in homes where duct leakage is the dominant energy issue. A new high-efficiency furnace installed in a system with 30 percent duct leakage will not perform as rated. Fix the duct system first, then evaluate whether equipment replacement makes sense with the actual reduced load.

It's one of the most consistent patterns I see in home energy work: the expensive fix gets done first because it's visible and marketable, and the less glamorous fix that would have made the expensive fix more effective gets skipped. Duct sealing isn't exciting, but in homes with significant duct leakage, it frequently delivers a better return on investment than any other single improvement.