The leakage problem
DOE / LBNL research consistently shows residential ductwork leaks 25–40% of conditioned air between supply registers and return grilles. Air leaving the supply boot at 110°F arrives at the room at 88°F because half of it went to the attic on the way. The opposite happens in summer: 55°F supply air mixes with attic air and arrives at 72°F instead of 60°F.
The result is a homeowner running the system longer to hit setpoint, paying for energy that’s actively conditioning unoccupied attics and crawl spaces. ENERGY STAR Home Improvement programs target ≤8% leakage post-sealing, measured with a calibrated duct blaster.
Materials — flex, sheet metal, fiber board
- Insulated flex duct. Inner liner + helical wire + R-6 or R-8 fiberglass insulation + outer vapor barrier. Cheapest material ($1.50–$3/linear ft), fastest install, dominant in 2026 residential installs. Trade-off: pressure drop is higher than rigid metal, 15–20 year service life, susceptible to crushing.
- Sheet metal (galvanized steel). Rigid rectangular or round duct, externally insulated (R-6 or R-8 wrap) in unconditioned spaces. Most expensive ($4–$8/linear ft installed), 40+ year life, lowest pressure drop. Right call for main trunks and any duct you can’t access easily later.
- Fiberglass duct board. Rigid panels of compressed fiberglass with foil facing. Integrated insulation (no separate wrap needed). Cheaper than sheet metal, good acoustic dampening. Trade-off: prone to mold if it gets wet, must not be used in high-humidity uninsulated spaces.
Best-practice install: sheet metal trunk, insulated flex branches to individual registers. Avoid all-flex on long runs over 25 ft.
Sealing — mastic + mesh is the standard
The IECC and most state energy codes require duct joints to be sealed with mastic + fiberglass mesh tape, not duct tape (despite the name). Mastic is a paste-like sealant applied to every joint and seam, then reinforced with mesh. Properly applied, mastic creates a permanent flexible seal that survives temperature cycling.
Avoid duct tape. Standard foil-faced duct tape (UL-181 listed or not) typically fails within 5–10 years from thermal cycling and adhesive breakdown. Acceptable for temporary use only.
Aeroseal — sealing from inside
Aeroseal is a proprietary process that pressurizes the duct system with a polymer aerosol; the polymer drifts to leaks (where air is escaping), accumulates, and bridges holes up to ~5/8 inch. Independent studies show 60–90% leakage reduction.
Best fit: finished homes where opening drywall to access joints is destructive. Worst fit: open ductwork in unfinished basements where manual mastic is faster and cheaper. Cost: $1,500–$3,200 typical, plus a pre/post Duct Blaster test included. Some HOMES rebate programs accept Aeroseal pre/post as evidence of measured savings.
Heat pump airflow problem
Gas furnaces produce 130–140°F supply air. Heat pumps produce 90–105°F. To deliver the same heating BTU/hr, the heat pump moves 30–50% more CFM. Existing return ducts and trunk lines sized for a gas furnace are usually undersized for the heat pump replacement.
Symptoms of inadequate return: heat pump short-cycles, hot/cold spots, blower motor whines, ECM blower runs at 100% constantly. Symptoms of inadequate trunk: high static pressure (over 0.8 inWC), blower overheats, registers don’t deliver design airflow.
Manual D analysis is required to verify. Common upgrades when retrofitting heat pump onto existing ducts: larger return ducts (often adding a second return), trunk-line upsizing or addition of a second trunk, balancing dampers, additional supply registers. Budget $1,500–$5,000 for these upgrades on top of the heat pump itself.
Reading a duct line item
- Per-sqft pricing is a sanity check, not a quote. $1.30/sqft for seal + insulate is reasonable; $5+/sqft for the same scope is overpriced.
- Manual D design referenced in the proposal.
- Pre/post Duct Blaster numbers (CFM25 leakage rate) committed in writing.
- R-8 insulation on ducts in unconditioned space; R-6 if mostly conditioned.
- Mastic + mesh on every joint, not foil tape.
- Pressure-tested at supply boots after install.