Duct leakage is easy to underestimate because the largest losses often occur out of sight—at takeoffs, plenums, boots, filter racks, and disconnected runs in attics or crawlspaces. The result is not only wasted heating or cooling. A leaky supply system can leave rooms underserved, while a leaky return can pull dust, humidity, garage contaminants, or attic air into the air handler.

ENERGY STAR reports that a typical house can lose 20% to 30% of the air moving through its duct system because of leaks, holes, and poor connections. That national estimate is a reason to investigate, not a savings promise for a specific property. The right way to turn a suspicion into a repair plan is to measure the system, locate the important defects, correct them without creating airflow problems, and verify the result.

What a duct-leakage test actually measures

A calibrated duct-testing fan temporarily connects to the air distribution system while the supply and return registers are sealed. The technician pressurizes or depressurizes the ducts and records the airflow required to maintain the test pressure. Results are commonly reported as CFM25: cubic feet per minute of airflow at 25 pascals of pressure.

That number needs context. Two tests answer different questions:

  • Total duct leakage measures leakage from the entire duct system, including leakage that may remain inside the building enclosure.
  • Duct leakage to the outside estimates how much leakage communicates with outdoors or unconditioned space. This procedure generally combines duct testing with a blower-door setup.

The U.S. Department of Energy’s duct-leakage training module distinguishes these two methods and provides code and standards resources. Total leakage is useful for judging construction quality and repair progress. Leakage to outside is often more directly connected to energy waste because it identifies conditioned air escaping the enclosure or outdoor air being pulled into the system.

Do not compare raw CFM25 values across buildings without normalizing them. A 150-CFM25 result means something different in an 800-square-foot dwelling than in a 3,000-square-foot home. Program and code forms may normalize leakage by conditioned floor area, system airflow, or another defined denominator. Record the method, test boundary, system served, floor area, and whether the air handler was included.

Start with the whole system, not the fan

A test number alone does not tell a contractor what to fix. Begin with a visual and operational inspection.

Document the configuration

Map the supply trunks, branches, returns, air handler, filter location, zoning dampers, and every section that passes through an attic, crawlspace, garage, or other unconditioned area. Note building cavities used as returns, disconnected boots, crushed flex duct, unsupported runs, damaged insulation, and inaccessible sections.

The ANSI/BPI-1200 standard listed by BPI calls for duct-leakage evaluation and visual inspection for airflow restrictions. It also calls for a recommendation to measure total external static pressure after duct sealing is recommended. That last step matters: a tighter system is not automatically a better-performing system if repairs leave the blower working against excessive resistance.

Establish safe test conditions

Testing and sealing can change pressure relationships around combustion appliances. Identify fuel-fired equipment, exhaust fans, fireplaces, attached garages, and other potential pollutant sources before work starts. Follow the governing test standard and equipment instructions. Where natural-draft appliances are present, include the applicable combustion-safety procedure and post-work checks rather than treating duct sealing as an isolated task.

Turn measurements into a repair scope

The most useful scopes rank defects by consequence, accessibility, and cost.

1. Repair gross failures first

Reconnect separated ducts, replace collapsed or badly torn flex duct, secure loose takeoffs, and correct open plenums or missing end caps before chasing small seams. These failures can dominate the test and may also create major room-to-room comfort problems.

2. Prioritize leakage outside the enclosure

A supply leak into a vented attic throws away conditioned air. A return leak in a dusty crawlspace can add moisture and contaminants while increasing the load on the equipment. When access and budgets are limited, these locations usually deserve attention before small leaks that discharge into conditioned space.

3. Seal durable joints and transitions

ENERGY STAR recommends mastic, appropriate metal tape, or an aerosol-based sealant, and specifically warns that ordinary cloth-backed “duct tape” is not a durable repair. Seal boots to the surrounding air barrier, plenums to equipment cabinets where permitted, takeoffs, elbows, seams, and filter-rack gaps. Use products compatible with the duct material and their listings; DOE’s Building Science Education guidance points to UL 181, UL 181A, and UL 181B requirements for applicable mastics and tapes.

4. Restore insulation and geometry

Air sealing and duct insulation solve different problems. After joints are sealed and cured, restore continuous insulation on ducts outside conditioned space, with the vapor retarder correctly oriented and closed. Straighten compressed flex duct, support runs without sharp sagging, and eliminate kinks. A sealed but highly restrictive branch can still fail to deliver design airflow.

Verify more than the leakage number

A strong closeout package includes both the pre- and post-repair result, using the same test method and boundary. It should also show that the HVAC system still operates correctly.

At minimum, document:

  • Pre- and post-work total leakage and, when required, leakage to outside
  • Test pressure, equipment used, system boundary, and registers excluded
  • Conditioned floor area and the normalization method
  • Repairs completed and inaccessible areas left unresolved
  • Total external static pressure before and after work
  • Filter condition, blower setting, and measured or verified airflow where applicable
  • Room-delivery or pressure-balance issues that remain
  • Combustion-safety checks when relevant

If a room is still uncomfortable after leakage falls, investigate duct sizing, balancing, equipment airflow, envelope loads, or control problems. The next step may be a load calculation or distribution redesign—not another layer of mastic. For replacement projects, pair this process with BPT’s heat-pump specification and commissioning checklist.

Codes and incentives in 2026

Duct-leakage limits are not universal. They vary by code edition, project type, test stage, duct location, system configuration, and local amendments. DOE’s analysis of the 2024 IECC residential provisions notes new testing requirements for certain existing duct systems connected to new HVAC equipment in additions and alterations. But DOE’s model-code determination is not the same as local adoption. Confirm the enforced code and approved test protocol with the authority having jurisdiction before writing a pass/fail criterion into a contract.

Incentives also require a live eligibility check. As of October 28, 2026, DOE says Home Energy Rebates are available in select states, with individual states, territories, and Tribes setting availability and project rules. Duct sealing may be included within eligible whole-home work, but contractors and owners should verify program enrollment, approved scopes, contractor requirements, and pre-approval before construction begins.

Do not assume the former federal Energy Efficient Home Improvement Credit is available for 2026 work. Current IRS Form 5695 instructions state that the credit cannot be claimed for property placed in service after December 31, 2025. State and utility programs may still help, but their rules can change.

A practical next step

For an existing building, begin with utility-bill and comfort history, then connect duct testing to the broader diagnostic picture. BPT’s guide to whole-home audit baselines and its blower-door testing overview explain how distribution losses and enclosure leakage fit together.

A credible duct project ends with evidence: a clear baseline, a prioritized repair log, a repeatable post-test, and confirmation that airflow and safety were preserved. Building teams that need a consistent way to organize field observations, test results, and corrective actions can use the BPMS workflow from Building Performance Technologies to keep that evidence connected from audit through closeout.