How to Read a Residential Duct Leakage Test Report
You already have a duct leakage report in hand. The useful question is not how the technician connected the fan; it is whether the document identifies the tested system, test boundary, pressure, units, normalization basis, and acceptance authority clearly enough to support a decision.
Read the report as a chain of defined fields. A CFM25 number without its boundary or denominator cannot be compared reliably with a code limit, another contractor’s test, or a post-sealing result.
Start with the tested system and construction stage
The report should identify the home or unit, date, tester, instrument, HVAC system, and portions of ductwork included. A house with two separate systems needs a clear statement showing whether each system was tested alone or whether a combined result was reported.
Construction stage matters. A rough-in test may exclude the air handler or use temporary closures that differ from a final test with equipment, registers, grilles, and cabinet connections installed. Existing-home testing should list inaccessible or intentionally excluded branches instead of implying the entire distribution system was included.
Find the test type before reading the number
Look for language such as total duct leakage or duct leakage to outside. Total leakage counts openings from the tested duct boundary into any connected space. Leakage to outside uses an additional building-pressure relationship to estimate the portion communicating beyond the building air barrier.
Those fields answer different questions. A report must not compare a total-leakage result with an outside-leakage limit, or describe the difference between the two as a measured supply-versus-return split. Side-specific operating effects need other diagnostics.
Confirm pressure, airflow units, and direction
Residential reports commonly state leakage airflow in cubic feet per minute at 25 pascals, written CFM25. The “25” is the test pressure difference, not the home’s normal duct pressure and not a target for the HVAC blower. A result at another pressure requires the applicable method and cannot be relabeled as CFM25.
The report should state whether the ducts were pressurized or depressurized and which location served as the reference baseline. Direction may be allowed by the governing procedure, but before-and-after comparisons should use compatible setups. Duct leakage airflow also must not be confused with the operating airflow concepts in HVAC static-pressure testing.
Read the raw value and the normalized value separately
Raw CFM25 describes leakage airflow under the stated test. A normalized result divides or scales that airflow by a defined quantity, commonly conditioned floor area under a program or code procedure. The report should show the denominator and the resulting unit, not only a percentage.
For example, a value expressed per 100 square feet cannot be checked without the floor area used by the tester. Conditioned floor area, total building area, and served floor area may differ. Use the definition required by the cited standard rather than selecting the denominator that produces the lowest ratio.
Do not substitute total leakage for leakage to outside
If both values appear, outside leakage would normally be a portion of the total under compatible test conditions, but the report still needs to identify how the building boundary was established. A vented attic, garage, or open crawlspace is usually different from a verified conditioned enclosure; an encapsulation label alone does not prove pressure-boundary continuity.
A large gap between total and outside leakage suggests that some measured openings communicate with spaces treated as inside the air barrier. It does not automatically make those openings harmless. Leakage inside can still affect room delivery, noise, cleanliness, or pressure relationships, and the space classification may need verification.
Locate the authority behind pass or fail
The tester should cite the code edition, energy program, contract specification, or other acceptance document used for the project. Limits can depend on construction stage, equipment inclusion, climate or jurisdiction, and whether the metric is total leakage or leakage to outside.
A contractor-created green check mark is not the authority. Confirm the reported unit and boundary match the cited requirement. Passing a tightness criterion also does not prove that branches follow Manual D duct design, that each room receives design airflow, or that system static pressure is acceptable.
Compare before and after results on the same basis
A sealing result is credible when the baseline and final reports use the same duct system boundary, pressure, equipment inclusion, register closures, normalization area, and compatible calibrated instruments. Note any ducts, cabinets, or building-boundary work added between tests because those changes can affect the comparison.
Calculate both the absolute CFM25 change and the normalized change when the report provides enough data. Do not convert the reduction directly into energy savings or room airflow. Delivery, runtime, duct location, operating pressure, insulation, and climate determine the practical effect.
Use notes and limitations to decide the next step
Useful field notes identify inaccessible sections, damaged or disconnected branches, temporary seals, test instability, weather limitations where relevant, and repairs completed between readings. Missing information should be requested before destructive access or a large duct replacement is authorized.
If the report passes but one room remains uncomfortable, the next measurement may be branch airflow, room pressure, load, or return-path performance. If leakage remains high after sealing, location diagnostics can define another repair scope. The broader decision about whether ductwork needs replacement also considers sizing, insulation, damage, access, and expected equipment airflow.
Example of a defensible report comparison
Assume a final report lists one system, final construction, air handler included, total leakage at 25 pascals, CFM25, conditioned floor area, and a cited program limit. The baseline uses the same fields and boundary. That pair can support a before-and-after tightness comparison and a compliance review.
If the second report instead lists leakage to outside, omits floor area, or excludes the air handler, the two headline numbers should not be presented as direct improvement. Ask the tester to reconcile the methods or repeat a compatible measurement. The problem is missing comparability, not automatically failed duct sealing.
Report-reading questions
Is CFM25 the same as normal duct airflow?
No. CFM25 is leakage airflow under a controlled test pressure with registers sealed. Normal system airflow is measured while the HVAC blower operates through open ducts and grilles.
Can a duct report show which connection leaks?
The total number does not locate individual defects. Field notes, inspection, pressure-pan screening, sectional isolation, or other location diagnostics are needed to assign a repair.
Does a passing leakage report prove the rooms are balanced?
No. Tightness and room airflow distribution are separate outcomes. A system can be tight but poorly balanced, or leaky while some rooms receive acceptable airflow.
Sources and verification
The technical statements in How to Read a Residential Duct Leakage Test Report were cross-checked against the primary references below. Final sizing, airflow, electrical, combustion, and refrigerant decisions must also follow the exact equipment instructions and applicable local code.