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Leaky ducts waste 20–30% of every HVAC dollar — $405/yr at typical spend. Sealing pays back in 2.6 years; DIY mastic in under one.
The least glamorous efficiency upgrade is often the best one: DOE estimates typical duct systems lose 20–30% of conditioned air to attics, crawlspaces, and garages — you are paying to heat the outdoors. At $1,800/yr of heating and cooling with 25% leakage, that is $405/yr recoverable by sealing. A pro aerosol seal runs $1,200–2,000 (pays back in ~2.6 years); DIY mastic and foil tape on accessible joints costs $200–400 of materials and can pay back inside a year. Sealing also comes BEFORE equipment: a new high-efficiency heat pump on leaky ducts delivers old efficiency, and tighter ducts can drop the tonnage you need to buy. This calculator prices your leakage and both routes.
Homeowners use it before buying new HVAC equipment: ducts leak 20–30% of conditioned air — $405/yr at typical spend — and sealing pays back in 2.6 years while new equipment on leaky ducts delivers old efficiency.
Three signals before any test: rooms far from the air handler that never reach setpoint, dusty air right after the system kicks on (return leaks pull attic dust in), and an attic or crawlspace that feels oddly comfortable in extreme weather — that is your conditioned air. The cheap check: with the system running, hold a tissue at visible joints — movement means leakage; look for tape that has dried and peeled, disconnected runs, and flex duct kinks. The exact answer is a duct blaster test, $100–200 standalone or often free to cheap inside a utility-subsidized home energy audit — it pressurizes the duct system and gives leakage in CFM25, the number contractors quote against. If your ducts run entirely inside conditioned space (some newer homes), leakage matters little; if they run through an unconditioned attic or crawlspace — most American homes — assume 20–30% until tested.
It depends on access. Pro aerosol sealing (AeroSeal and similar) pressurizes the ducts and blows sealant particles that deposit exactly at the leaks — it seals joints you cannot physically reach, works the whole system, and typically finishes with a measured before-and-after leakage number, often guaranteed. Cost: $1,200–2,000 for a typical home. DIY mastic — brushed-on paste plus foil (never cloth) tape — is the right call when the ducts are accessible in an open attic or basement: $200–400 of materials and a weekend reaches most leaks; set the sealed slider to 60–70% for honesty. The mastic mistakes to avoid: cloth duct tape fails in years (it is literally not for ducts), and sealing only the visible joints while ignoring the air-handler cabinet connections misses the biggest leakers. Many homes do both: DIY the accessible runs now, aerosol later if bills stay high.
Because equipment efficiency ratings assume the air arrives. A 18-SEER2 heat pump pushing through 25% leakage delivers roughly 13.5-SEER2 results at the register — you paid for efficiency that leaks into the attic. The compounding win: after sealing, the home needs less capacity, so a Manual J load calculation on the tightened house often drops the required system size by half a ton or more — smaller equipment costs less upfront and runs more efficiently with longer cycles. Contractors quoting equipment replacement rarely volunteer this because duct work is a different, dirtier job. The right sequence: test ducts, seal, then run the load calculation and size equipment to the sealed house. If a quote for new equipment never mentions your ducts, that tells you which job the contractor is selling. Run the sealing numbers here first — at a 2–3 year payback, it usually outranks the equipment upgrade entirely.