Efficiency ratings live on the equipment, but the air has to get to the rooms. ENERGY STAR is blunt about what happens in between: "about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts," and the result is higher utility bills and rooms that won't stay comfortable "no matter how the thermostat is set" ([ENERGY STAR](https://www.energystar.gov/saveathome/heating-cooling/duct-sealing)). A high-SEER2 condenser pushing into a leaky duct system still pays for air you never feel.
Where leaks hurt most
Not all leaks are equal. A leak inside the conditioned space at least dumps its air into the house; a leak into an attic, crawlspace, or garage is gone for good — and ENERGY STAR specifically flags ducts in those unconditioned spaces as a top problem ([ENERGY STAR](https://www.energystar.gov/saveathome/heating-cooling/duct-sealing)). DOE-backed field work backs the scale of the payoff: heating and cooling energy savings from duct sealing ran about 10% where ducts were in a basement, and about 15–20% where they ran through an attic or crawlspace, with peak electricity demand savings of 25% or more ([DOE / LBNL](https://www.energy.gov/sites/default/files/2015/08/f25/LBNL_Duct_Sealings.pdf)). Leakage to outside — across the building's pressure boundary — is the kind to fix first.
That pressure-boundary point is also an indoor-air-quality point. When ducts leak in unconditioned spaces, the pressure imbalances they create can pull dust, insulation fibers, garage or crawlspace air, or combustion-appliance-zone air into the system and deliver it to the living area. EPA's building-codes guidance treats duct sealing as an IAQ measure, noting that ducts outside the pressure boundary are sealed to leakage limits (the code example cites no more than 6% leakage), that attached garages are sealed off with no supply or return air to or from them, and that combustion appliances are managed as part of the same pressure picture ([EPA](https://www.epa.gov/sites/default/files/2014-08/documents/building_codes_and_iaq.pdf)). In other words, leaky ducts aren't only a comfort and energy problem — they're an air-quality pathway.
How to seal them
ENERGY STAR's homeowner guidance is to "seal air leaks using mastic sealant or metal tape" on every duct you can reach, and — the line every tech repeats — "Never use duct tape, as it is not long-lasting" ([ENERGY STAR](https://www.energystar.gov/saveathome/heating-cooling/duct-sealing)). For runs you can't reach, aerosolized internal duct sealing is the option; DOE/LBNL field work describes an aerosol sealant that remains rubbery, seals holes up to about a half-inch across, and holds up for 10+ years ([DOE / LBNL](https://www.energy.gov/sites/default/files/2015/08/f25/LBNL_Duct_Sealings.pdf)).
| Method | What it does | Best use / limitation |
|---|---|---|
| Mastic sealant | Brush-on sealant that hardens over joints and gaps | Accessible joints, plenums, takeoffs; durable; needs dry, clean surfaces |
| UL-181 foil tape | Metal tape rated for duct use | Flex connections, seams; only UL-181-rated, not cloth "duct tape" |
| Aerosolized internal sealing | Sealant injected into pressurized ducts, depositing at leaks | Hard-to-reach runs; won't fix disconnected, crushed, undersized, or poorly designed ductwork |
| Ordinary cloth "duct tape" | — | Not a durable duct-sealing method; ENERGY STAR: "not long-lasting" |
Seal the worst leaks first, then test before and after.Measure duct leakage (or airflow / static pressure) before you seal and again after, so you know the work paid off — we cover those tests in our commissioning article. And sealing isn't sizing: tightening leaks changes airflow, so recheck static pressure afterward — you don't want to choke a system that was barely moving air to begin with (see the oversized-AC point in our humidity article).
A practical checklist
- Seal joints, plenums, boots, and takeoffs with mastic or UL-181 foil tape — not cloth duct tape.
- Prioritize ducts outside the conditioned space (attic, crawlspace, garage) first; that's leakage to outside.
- Test before and after with a duct-leakage or airflow/static-pressure check.
- Recheck airflow and static pressure after sealing so you haven't choked the system.
- Maintain balanced return paths — a supply fix without a return path just shifts the imbalance.
- Insulate ducts in unconditioned space once sealed, to cut conductive loss too.
What sealing can't fix
Duct sealing fixes leaks. It doesn't fix a badly designed or laid-out system — ducts that are undersized, crushed, sharply kinked, or routed through the wrong space. Aerosol sealing in particular won't rescue a disconnected or collapsed run. If the distribution system itself is the problem, sealing is a bandage, not the cure, and a redesign or reroute is the real fix. Access is the other limit: ducts buried in walls, slabs, or finished ceilings can't be reached without opening them up, which is why aerosolized internal sealing exists for the runs you can't get to by hand.
The bottom line
The ducts are part of the HVAC system. ENERGY STAR's 20–30% leakage figure is the number to remember: if you're upgrading equipment without checking the distribution, you're spending on the box and ignoring the leaks. Seal the joints and boots with mastic or UL-181 tape, prioritize the runs through unconditioned space, test before and after, and recheck airflow — because a tight duct system is what lets the equipment you paid for actually deliver.
HVAC Zone Inc is a multi-brand dealer; this article is brand-neutral and names no product as "best." This content is educational and not a substitute for a site-specific assessment by a licensed professional. Want your duct system leak-tested and sealed the right way — with before-and-after numbers and an airflow recheck? Request a consultation and we'll start by finding out how much air you're actually losing.