Duct Leakage Inspection and Report

Purpose

This standing instruction produces a duct leakage number that carries its own basis, and then converts that number into a repair scope ranked by measured contribution.

A leakage figure with no basis attached is worse than no figure, because it survives being repeated. Total leakage and leakage to outside can differ by a factor of two on the same house. A result normalized against a floor area somebody read off a county listing can be off by more than a tenth. And a number with no per-register data behind it tells a crew that the ducts leak without telling them where, which is how a shop sells a day of mastic and moves the number by almost nothing.

Safety actions that gate the work

• Before any depressurization test, shut off every atmospherically vented combustion appliance and confirm it stays off for the duration. A blower door pulling a house negative will draw flue gases back down a water heater vent, and the person in the crawlspace does not see it happen. Restore and verify normal draft on every appliance before you leave.
• Where a test finds ductwork connected to an attached garage, treat it as a live finding on the spot, tell the occupant not to run a vehicle in that garage until it is sealed, and put it at the top of the scope regardless of what it costs to reach.
• Attic work in summer and crawlspace work in any season are the actual injury risks on this job. Walk only on structure, carry a light and a second one, work in shorter cycles in heat, and do not enter a crawlspace alone without someone who knows you are down there.
• Fiberglass duct board and old duct liner release fibers when cut or abraded. Where you must open one to inspect, cut it wet or with a sharp blade rather than tearing it, wear eye protection and a fitted respirator selected under a program that meets 29 CFR 1910.134, and bag the offcuts rather than leaving them in the plenum.

Scope

Covers diagnostic duct leakage testing and reporting on existing residential and small light-commercial forced-air systems, including total leakage, leakage to outside, per-register pressure pan screening, and the repair scope that results.

Does not cover code-compliance duct tightness testing where a jurisdiction requires a registered rater and a specific form, which the California Title 24 duct tightness test SOP covers as an example of that shape. Does not cover duct design or replacement sizing. Does not cover airflow verification at the equipment, owned by the commissioning SOP.

What the number is measuring

Two different tests are both correctly called duct leakage, and quoting one against the other's limit is the most common error in this work.

     TOTAL LEAKAGE            LEAKAGE TO OUTSIDE
   +---------------+         +---------------+
   |    house      |         |    house      |
   |    0 Pa       |         |    25 Pa      | <- blower
   |   +-------+   |         |   +-------+   |    door
   |   | ducts |25 |         |   | ducts |25 |
   |   +-------+   |         |   +-------+   |
   +---------------+         +---------------+
     duct fan sees             no pressure across
     every leak, to            the duct-to-house
     house and to              boundary, so only
     outside alike             leaks to outside show

Total leakage is what you measure with the house left at outdoor pressure. Leakage to outside is measured with a blower door holding the house at the same 25 Pa as the ducts, so leaks into conditioned space register nothing and only the leaks that cost energy and drag in attic, crawlspace or garage air are counted. Both are reported in CFM at 25 Pa and both are normalized per 100 square feet of conditioned floor area.

Roles and handoffs

Procedure

1. Walk the system and record what you find before any fan comes off the truck. Attic and crawlspace access, duct material, obvious disconnections, panned-joist or platform returns, and any duct that passes through or touches an attached garage. Acceptance: a written inventory with photographs. Wrong looks like testing first, which produces a number you cannot explain because you have never seen the system it came from.

2. Measure the conditioned floor area rather than accepting a stated figure. Pace or tape the conditioned footprint and exclude the garage and any unconditioned space. Acceptance: an area you produced, with the method noted. Stop rule: if the measured area disagrees with the customer's or the listing's figure, use yours and say so in the report, because this number is the denominator of every result and an error in it moves every conclusion.

3. Set up to the test you are actually running, and write down which one it is. Seal every supply and return grille, connect the duct fan at a central return or at the air handler cabinet, and for leakage to outside set the blower door as well. Acceptance: the test named, the reference pressure named at 25 Pa, and the normalization basis named. Wrong looks like a report that says "duct leakage 268" with no test type, no pressure and no area.

4. Run the test and record the flow at 25 Pa, plus the ring you ran it on. Acceptance: a stable 25 Pa duct pressure and a flow reading within the fan's calibrated range on that ring. Stop rule: if you cannot reach 25 Pa, do not extrapolate, change to a larger ring or find and temporarily block the gross disconnection first, and note in the report that you did. Wrong looks like a reading taken off the bottom of a ring's range, where the accuracy basis of the fan is at its worst.

5. Compute the normalized result and state the threshold you are judging it against, with its edition. Divide flow at 25 Pa by the conditioned area in hundreds of square feet. Acceptance: a value in CFM25 per 100 square feet, next to the limit from the energy code edition your jurisdiction has adopted, which binds new work through the permit and is a benchmark rather than a requirement on an existing system. Wrong looks like quoting a national limit as though it applies to a thirty-year-old house.

6. Locate the leakage before you scope any sealing. Run a pressure pan at every register with the blower door holding the house negative, the air handler off and all other registers open, and record a value at each. Acceptance: a pan reading at every register plus visual confirmation at the plenums, the return platform and the boot-to-ceiling joints. As a field screen, readings under about half a Pascal indicate a register with little connection to outside and readings above roughly one Pascal indicate a significant one, which is a rule of thumb for ranking, not a pass criterion.

7. Rank the repair scope by measured contribution, then adjust only for safety. Acceptance: a scope list where each item cites the pan reading or the visual finding that put it there, with any garage-connected or combustion-zone duct pulled to the top regardless of its number. Wrong looks like a scope ordered by what is easy to reach, which is how a crew seals eight accessible boots and moves the total by a few percent.

8. Re-test after sealing and report the result you got, including a miss. Acceptance: the same test, the same fan, the same ring where possible, the same measured area, and a stated before and after. Stop rule: if the target was not met, the report says so and names what is left, rather than reporting the improvement and going quiet. A shop that only publishes the wins loses the ability to quote the next job honestly.

The record this produces

A duct report filed to the property:

• Measured conditioned floor area and the method used to get it
• Test type, reference pressure, fan ring, and the raw flow at 25 Pa for each test run
• Normalized CFM25 per 100 square feet for total and for to-outside, and the percentage of total that crosses to outside
• The threshold cited, with the code edition and the note that it binds new work
• A pressure pan value for every register, by room
• The ranked scope with the reading behind each item, and any safety finding flagged
• Post-repair results, stated against the same basis, including any remaining gap

The install crew works from the ranked scope. The customer keeps the before and after with the basis attached, so a future contractor cannot misread the number. The manager compares the before and after across jobs to learn what the shop's sealing work actually buys, which is the only way to quote the next one from evidence instead of hope.

Worked pass: single-story house, ducts in a vented attic

Step 2 FAILS. The county listing shows 1,850 square feet; the measured conditioned footprint is 2,140 square feet, because a permitted addition is not in the listing. Stop rule taken: the measured figure is used and the discrepancy is written into the report. It matters directly. The same measured flow normalized against 1,850 would read about 16 percent worse than against 2,140, and using a too-small area always overstates leakage.

Step 4: total leakage 268 CFM25 on the appropriate ring at a stable 25 Pa. Leakage to outside, with the blower door holding the house at 25 Pa, 174 CFM25.

Step 5: total is 268 / 21.40 = 12.5 CFM25 per 100 square feet, against a post-construction total leakage limit of 4.0 CFM25 per 100 square feet in the energy code edition this jurisdiction has adopted for new work, so more than three times that benchmark. To outside is 174 / 21.40 = 8.1 per 100 square feet, and 174 / 268 means about 65 percent of the total leakage crosses to outside rather than into the house.

Step 6 pressure pan sweep: return platform 4.1 Pa, the two supply registers in the room over the garage 3.2 and 2.9 Pa, four bedroom supplies between 0.3 and 0.6 Pa, the rest under 0.5 Pa. The return platform and the two garage-side supplies are where the leakage lives.

Step 7 scope, ranked: the two garage-side supply runs first on safety grounds even though the return platform reads higher, then the return platform, then the supply plenum seams found visually. Occupant told not to run a vehicle in the garage until those runs are sealed.

Step 8 re-test: total leakage 96 CFM25, which is 96 / 21.40 = 4.5 CFM25 per 100 square feet, a reduction of about 64 percent from 268. That is still above the 4.0 benchmark, and the report says so: a buried trunk section under the slab-adjacent soffit was not accessible, and closing the remaining gap means opening that soffit. It is written as an open item with a scope, not as a pass.

Checking the pass against the sections above: every leakage figure here is flow at 25 Pa normalized against the same measured 2,140 square feet, and the total and to-outside values are labeled as the two different tests the diagram distinguishes rather than compared as one quantity. The 4.0 figure is stated as a new-work code benchmark, exactly as step 5 requires, not as a requirement this existing house is failing. Step 7's ordering announces safety as the only override on measured contribution, and the pass shows that override being used once, on the garage runs, with the higher-reading return platform ranked immediately after.

References

• International Energy Conservation Code, in the edition your authority having jurisdiction has adopted, Section R403 duct testing provisions, which bind new and replacement work through the permit
• ANSI/RESNET/ICC 380, the standard test method for airtightness and airflow measurement, for duct test setup and reference pressure
• Duct testing fan manufacturer's documentation for ring ranges and the accuracy basis of each
• 29 CFR 1910.134 for the respirator program governing any respiratory protection used when duct board or liner is disturbed
• See related: the California Title 24 duct tightness test SOP, and the commissioning a replacement condensing unit SOP