Duct leakage is one of the most overlooked sources of energy waste in commercial buildings. Conditioned air that escapes before it reaches occupied zones means wasted fan and conditioning energy, higher utility bills, and comfort complaints that drain facility management resources.
Energy and mechanical codes address this with sealing, construction, and verification requirements. The International Energy Conservation Code (IECC), ASHRAE Standard 90.1, SMACNA's HVAC Air Duct Leakage Test Manual, and California's Title 24 Part 6 each impose specific requirements on commercial duct systems. Understanding how these frameworks interact -- and what your permit package must demonstrate -- is essential for any facility manager overseeing new construction, renovation, or equipment replacement projects.
This guide breaks down the major compliance layers, explains the testing approach inspectors rely on, and walks through the documentation your project team needs to close out a mechanical permit.
Why Duct Leakage Testing Is Mandated for Commercial Buildings
Conditioned air that escapes ductwork before reaching supply registers represents direct energy loss. Beyond efficiency, duct leakage can create pressure imbalances that draw unconditioned air and moisture into the building, degrade indoor air quality, and make it harder to deliver design airflow. In healthcare, laboratory, and cleanroom environments, duct leakage can also compromise pressurization relationships required for infection control or process integrity.
Commercial codes focus leakage testing on the ductwork where leakage matters most -- high-pressure systems -- and rely on sealing requirements for the rest. Some state and local codes go further, as noted below.
IECC Commercial Duct Leakage Requirements: Section C403
Scope and Applicability
The commercial provisions of the IECC are in Chapter 4 [CE]. Section C403 governs mechanical systems. In the 2021 IECC, duct construction requirements are in Section C403.12.2, with high-pressure duct systems in Section C403.12.2.3. In the 2024 IECC, the same provision is numbered C403.13.2.3.
The IECC classifies commercial ductwork by design static pressure (low- and medium-pressure thresholds shown are from the 2018 IECC as adopted in Colorado; the high-pressure provisions are the same in the 2021 and 2024 editions):
- Low-pressure systems (2 inches water gauge or less) -- joints, seams, and connections must be securely fastened and sealed.
- Medium-pressure systems (over 2 and under 3 inches w.g.) -- must be insulated and sealed.
- High-pressure systems (3 inches w.g. or greater) -- must be insulated and sealed and leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual, with a rate of air leakage (CL) of 4.0 or less.
Documentation must show that representative sections totaling at least 25 percent of the duct area have been tested and that all tested sections comply.
Local Amendments Can Expand Testing
Some jurisdictions amend the IECC to require more testing. For example, the Seattle Commercial Energy Code (2021) also requires leak testing of supply and return ductwork located outside the building thermal envelope that serves conditioned space, regardless of pressure class. Always check your adopted code and amendments.
The Leakage Coefficient Defined
The leakage class CL relates measured leakage to test pressure using the SMACNA relationship:
CL = F / P^0.65
Where F is the measured leakage rate in CFM per 100 square feet of duct surface area and P is the test pressure in inches water column. Because 1 raised to any power is 1, a CL of 4.0 corresponds to 4.0 CFM per 100 square feet of duct surface at a 1-inch w.c. test pressure; at higher test pressures, the allowable leakage rises with P raised to the 0.65 power (for example, about 8.2 CFM per 100 square feet at 3 inches w.c.).
Test Pressure Selection
Test pressure requirements vary by code. The Seattle amendment, for example, requires testing at a pressure equal to the average operating pressure or the design duct construction pressure class level, and New York City's energy code requires testing at the design duct pressure class rating. The contractor must know the system's design pressure class before scheduling the test, so it should appear on the mechanical drawings submitted for permit.
Representative Sampling vs. Full System Testing
The IECC allows testing of representative sections rather than 100 percent of the high-pressure duct system, with the 25 percent minimum noted above. The authority having jurisdiction (AHJ) may require more on specific projects; confirm sampling requirements with your local building department before work begins.
IECC Compliance Summary
| Duct Category | Trigger Condition | Requirement | Test Reference Standard |
|---|---|---|---|
| Low-pressure systems | Design static pressure 2 in. w.g. or less | Fasten and seal joints, seams, and connections | No IECC leakage test |
| Medium-pressure systems | Over 2 and under 3 in. w.g. | Insulate and seal | No IECC leakage test |
| High-pressure systems | 3 in. w.g. or greater | Insulate, seal, and leak-test at least 25% of duct area; CL 4.0 or less | SMACNA HVAC Air Duct Leakage Test Manual |
SMACNA HVAC Air Duct Leakage Test Manual: The Testing Framework
Standard Overview
The SMACNA HVAC Air Duct Leakage Test Manual (ANSI/SMACNA 016-2012, second edition) is the reference test method named by the IECC and ASHRAE 90.1 for commercial duct leakage testing. According to SMACNA, the second edition covers revised leakage criteria, expected leakage rates for ductwork built to the SMACNA duct construction standards, test procedures, recommendations on the use of leakage testing, test apparatus and setup, sample leakage analysis, and a new educational chapter for designers and specifiers.
SMACNA Seal Classes and Leakage Classes
SMACNA's leakage classes express leakage in CFM per 100 square feet of duct surface at 1 inch w.g. The table below reproduces the traditional SMACNA applicable-leakage-class table as summarized by McGill AirFlow (from the 1985 SMACNA test manual and the 2005 HVAC Duct Construction Standards). It shows expected leakage, not an allowable leakage specification, and codes such as the IECC set their own limits.
| Duct Pressure Class | Seal Class | Sealing Applicable | Leakage Class -- Rectangular Metal | Leakage Class -- Round Metal |
|---|---|---|---|---|
| ½-, 1-, or 2-in. w.g. | C | Transverse joints only | 24 | 12 |
| 3-in. w.g. | B | Transverse joints and seams | 12 | 6 |
| 4-, 6-, or 10-in. w.g. | A | Joints, seams, and all wall penetrations | 6 | 3 |
McGill AirFlow also notes that when a designer does not designate duct pressure classes on the contract drawings, SMACNA's basis of compliance is 2-inch w.g. for ducts between the supply fan and variable volume boxes and 1-inch w.g. for all other ducts. Current energy codes (ASHRAE 90.1 and Title 24) now require Seal Class A for ductwork with pressure class ratings, so the lower seal classes in this table are mainly of historical and design-estimating interest.
SMACNA Seal Classes Defined
- Seal Class A -- All transverse joints, longitudinal seams, and duct wall penetrations sealed. This is the most stringent class.
- Seal Class B -- All transverse joints and longitudinal seams sealed.
- Seal Class C -- Only transverse joints sealed.
Test Equipment and Setup
A duct leakage test generally requires:
- A calibrated test fan (flow-measuring apparatus) capable of supplying the test flow at the required pressure.
- A calibrated pressure gauge or manometer to measure duct static pressure during the test.
- Blanking materials (caps, plugs, bladders, or sealed covers) to isolate the duct section being tested by sealing open ends, registers, grilles, and diffusers.
The duct section is pressurized (or depressurized) to the specified test pressure. The airflow the fan must supply to maintain that pressure is the measured leakage. Results are converted to CFM per 100 square feet of duct surface area and compared to the applicable limit.
Test Section Requirements
Under the IECC and ASHRAE 90.1, representative sections totaling at least 25 percent of the high-pressure duct area must be tested. The contractor and the AHJ should agree on which sections make up the representative sample before testing. Failed sections must be repaired and retested, and all results -- pass and fail -- should be documented.
ASHRAE 90.1 Duct Sealing Requirements: Section 6.4.4.2
Standard Context
ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings, is recognized by the IECC as an alternative compliance path for commercial buildings, and some jurisdictions adopt it directly. ASHRAE's newest edition is 90.1-2025; 90.1-2019 and 90.1-2022 remain widely referenced because jurisdictions adopt editions on their own schedules.
Section 6.4.4.2.1: Duct Sealing
ASHRAE 90.1 Section 6.4.4.2.1 requires ductwork and all plenums with pressure class ratings to be constructed to Seal Class A. As reproduced in New York City's 2025 Energy Code (which is based on 90.1), the provision also requires sealing of all connections, specifies how pressure-sensitive tape may be used, exempts spiral lock seams, and requires openings for rotating shafts to be sealed with bushings or other devices.
Key Sealing Details Under ASHRAE 90.1
| Connection Type | ASHRAE 90.1 Requirement |
|---|---|
| Transverse joints | Must be sealed (Seal Class A) |
| Longitudinal seams | Must be sealed (Seal Class A) |
| Duct wall penetrations | Must be sealed (Seal Class A) |
| Spin-ins, taps, and other branch connections | All connections must be sealed |
| Access doors and panels | Must be sealed |
| Duct connections to equipment | Must be sealed |
| Rotating shaft openings | Sealed with bushings or other devices |
| Spiral lock seams | Exception -- need not be sealed |
Tape Restrictions
Under 90.1 Section 6.4.4.2.1, pressure-sensitive tape shall not be used as the primary sealant unless it is certified by an independent testing laboratory to comply with UL-181A or UL-181B and is used in accordance with that certification. California's Title 24 separately prohibits sealing joints and seams with cloth-back rubber adhesive duct tape unless it is used with mastic and drawbands. Mastic or properly listed tape is the practical choice for durable, code-compliant sealing.
Section 6.4.4.2.2: Duct Leakage Tests
Section 6.4.4.2.2 requires ductwork designed to operate at more than 3 inches w.c. to be leak-tested, with representative sections totaling at least 25 percent of the total installed duct area tested, and duct pressure ratings above 3 inches w.c. identified on the drawings. Maximum permitted leakage is Lmax = CL × P^0.65 in CFM per 100 square feet of duct surface area. New York City's 2025 version sets CL at 4 and the test pressure at the design duct pressure class rating, and also applies testing to outdoor ductwork; confirm the values in your adopted edition.
ASHRAE 90.1 vs. IECC: Which Standard Controls?
Where a jurisdiction adopts the IECC, the design team generally chooses either the IECC commercial provisions or ASHRAE 90.1 as the compliance path, and must then meet that path's requirements. Because ASHRAE 90.1 and Title 24 both require Seal Class A for pressure-rated ductwork, many owners simply specify Seal Class A for all new commercial ductwork unless the AHJ confirms otherwise.
California Title 24 Part 6: Duct Leakage Requirements
Overview of California's Commercial Energy Code
California's Building Energy Efficiency Standards (Title 24, Part 6), administered by the California Energy Commission (CEC), impose some of the most stringent duct requirements in the United States. The 2025 Energy Code took effect January 1, 2026, replacing the 2022 code. The provisions described below are from the 2022 code as summarized by Energy Code Ace; confirm them against the 2025 code for new permits.
Class A Sealing Mandate
Section 120.4 of the 2022 Energy Code requires air distribution ducts to meet California Mechanical Code requirements and the SMACNA duct construction standards, and requires ductwork with pressure class ratings to be constructed to Seal Class A. Sealing materials must comply with UL 181, UL 181A, or UL 181B, and cloth-back rubber adhesive duct tape may not be used on joints and seams unless combined with mastic and drawbands.
Duct Leakage Testing Requirements Under Title 24
Title 24 uses two testing routes for new duct systems:
| System Type | Requirement | Verification |
|---|---|---|
| Constant volume, single-zone systems serving less than 5,000 ft² with more than 25% of duct surface area outdoors or in unconditioned space (not serving healthcare facilities) | Leakage not to exceed 6 percent of nominal air handler airflow | HERS field verification and diagnostic testing |
| Other new duct systems | California Mechanical Code duct leakage testing | Per CMC (see below) |
For duct systems outside the HERS route, the California Mechanical Code requires leak testing of representative sections totaling at least 10 percent of the installed duct area, escalating to 40 percent and then 100 percent if tested sections fail, with maximum leakage calculated as Lmax = CL × P^0.65 using CL = 6. The section number varies by CMC edition, so check the edition adopted for your project.
HERS Rater Involvement
For the small single-zone systems described above, duct leakage must be confirmed through HERS (Home Energy Rating System) field verification and diagnostic testing. For other systems, the CMC requires tests to be performed by technicians certified through AABC, NEBB, TABB, or equivalent approved agencies. Verify requirements with your local jurisdiction before scheduling inspections.
Leakage Rate Pass/Fail Thresholds
California uses a percentage-of-airflow limit (6 percent of nominal air handler airflow) for the HERS-verified small systems and a surface-area-based limit (CL = 6) for CMC testing. Projects that fail must be repaired, re-sealed, and retested, and test documentation should be kept with the compliance package.
Understanding CFM25: The Test Metric for Commercial Duct Leakage
What CFM25 Means
CFM25 is the airflow, in cubic feet per minute, needed to create a 25 Pascal (about 0.10 inch water column) pressure difference in the ductwork. It is the common reporting metric for residential duct leakage testing.
Important distinction for facility managers: Commercial duct leakage testing under IECC Section C403 and ASHRAE 90.1 does not use CFM25 as its metric. It uses the SMACNA leakage-class (CL) method, with test pressures tied to the system's design pressure class, which can be several inches of water column for high-pressure systems. California's small single-zone systems use a percentage-of-airflow limit instead.
When CFM25 Testing Is Used in Commercial Contexts
CFM25-style testing may appear on commercial projects where a light-commercial system is being tested with residential-style equipment and procedures, or where a state or local amendment or project specification calls for it. Confirm the required metric and test pressure with your AHJ and the engineer of record before testing.
Commercial Duct Leakage Pass/Fail at a Glance
| Standard / Code | Metric | Pass Threshold | Trigger |
|---|---|---|---|
| IECC 2021 Section C403.12.2.3 (2024: C403.13.2.3) | Leakage class (CL) | CL 4.0 or less | Design static pressure 3 in. w.g. or greater |
| ASHRAE 90.1 Section 6.4.4.2.2 | Lmax = CL × P^0.65 | Per CL in adopted edition (CL 4 in NYC 2025 version) | Ductwork designed above 3 in. w.c. |
| ASHRAE 90.1 Section 6.4.4.2.1 | Seal Class A (construction standard) | Seal Class A required | Ductwork and plenums with pressure class ratings |
| California Title 24 (2022) -- small single-zone systems | % of nominal air handler airflow | 6 percent or less | Constant volume single-zone, under 5,000 ft², more than 25% of duct area outdoors/unconditioned |
| California Mechanical Code -- other systems | Lmax = CL × P^0.65 | CL = 6 | Other new duct systems (10% sample, escalating on failure) |
Mechanical Permit Documentation: What Inspectors Require
Drawing Requirements
Mechanical permit drawings for commercial projects should include the following duct-related information to support IECC, ASHRAE 90.1, and SMACNA-based review:
- Design duct static pressure class for each system or duct section (ASHRAE 90.1 requires ratings above 3 inches w.c. to be identified on the drawings)
- Seal class designation
- Leakage class (CL) or allowable leakage for sections requiring testing
- Test pressure for each duct section requiring leakage verification
- Identification of ductwork located outside the building thermal envelope
If the designer does not designate pressure classes on the drawings, SMACNA's default basis of compliance applies, which may not match what the designer intended.
Test Report Documentation
After testing is complete, the following documentation is typically included in the compliance package submitted to the AHJ:
| Document | Typical Content |
|---|---|
| Duct leakage test report | Test date, tested duct section(s), surface area, test pressure, measured leakage, pass/fail determination |
| Test equipment calibration records | Calibration certificate for test fan/flow meter and pressure gauge, calibration date |
| Representative sample identification | Floor plan or schematic identifying which sections were tested and their percentage of total duct surface area |
| Sealing material product data | Manufacturer data sheets for mastic, tape (with UL 181A/181B listing), or other sealants used |
| Repair and retest records | Documentation of any failed sections, repairs performed, and subsequent retest results |
| Tester or verifier credentials | Where required by the AHJ or state code (e.g., California HERS Rater, ATT, or AABC/NEBB/TABB certification) |
COMcheck and Compliance Software
DOE's COMcheck software is widely used to document commercial energy code compliance on permit applications. COMcheck documents design intent; it does not replace the physical leakage test report, which documents field verification.
Common Compliance Failures and How to Avoid Them
Failure 1: Using Cloth-Backed Duct Tape
A common sealing failure is using standard cloth-backed duct tape as the primary sealant. ASHRAE 90.1 does not allow pressure-sensitive tape as the primary sealant unless it is certified to UL-181A or UL-181B, and California's Title 24 prohibits cloth-back rubber adhesive duct tape on joints and seams unless combined with mastic and drawbands. Use listed tape or mastic for all primary sealing.
Failure 2: Missing Pressure Class Designations on Drawings
When mechanical drawings omit pressure class designations, SMACNA's default basis applies and ASHRAE 90.1's requirement to identify ratings above 3 inches w.c. is not met. Designers should label every duct section with its design pressure class and the corresponding seal and leakage requirements.
Failure 3: Testing at the Wrong Pressure
Testing at a pressure lower than required will produce artificially low measured leakage and a false pass. Confirm the required test pressure in your adopted code (for example, the design duct pressure class rating) with the mechanical engineer of record before conducting the test.
Failure 4: Inadequate Sample Size
Testing less than the required share of duct area -- 25 percent under the IECC and ASHRAE 90.1, 10 percent initially under the California Mechanical Code -- is a common cause of permit delays. Document total duct surface area by pressure class, calculate the required sample, and identify specific sections on the drawings before the test is scheduled.
Failure 5: Incomplete Records for Repairs
When a duct section fails, the repair and retest should be documented with the same rigor as the original test. A closeout package that shows a failed test with no repair record and passing retest invites rejection.
Practical Steps for Facility Managers
For facility managers overseeing new construction or major HVAC renovation projects, the following sequence will keep duct leakage compliance on track:
- Confirm the adopted code -- verify which IECC edition, local amendments, and whether ASHRAE 90.1 or California Title 24 applies in your jurisdiction before design begins.
- Require pressure class designations on drawings -- make this a standard checklist item during mechanical drawing review before permit submission.
- Specify Seal Class A -- it is required for pressure-rated ductwork under ASHRAE 90.1 and Title 24, and specifying it universally eliminates ambiguity.
- Pre-qualify the testing contractor -- confirm the testing firm has calibrated equipment, SMACNA-based procedures, any required certifications, and experience producing AHJ-acceptable test reports in your jurisdiction.
- Schedule tests before concealment -- duct leakage tests should be performed before ducts are enclosed in chases, ceilings, or insulation. Coordinate with the general contractor to build a testing window into the project schedule.
- Retain all test records -- both passing and failing test reports belong in the permanent building file and may be useful for future permit applications or energy audits.
Sources and References
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Michigan Commercial Energy Code 2021 (based on 2021 IECC) -- Section C403.12.2.3 High-Pressure Duct Systems -- ICC Digital Codes. https://codes.iccsafe.org/s/MIECC2021P1/chapter-4-ce-commercial-energy-efficiency/MIECC2021P1-Ch04-SecC403.12.2.3
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C403.13.2.3 High-Pressure Duct Systems (Colorado IECC 2024) -- UpCodes. https://up.codes/s/high-pressure-duct-systems
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Construction of HVAC System Elements (Colorado IECC 2018 duct pressure classes) -- UpCodes. https://up.codes/s/construction-of-hvac-system-elements-mandatory
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Duct Leak Testing (Seattle Commercial Energy Code 2021, Section C403.10.2.4) -- UpCodes. https://up.codes/s/duct-leak-testing
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6.4.4 HVAC System Construction and Insulation (NYC Energy Code 2025) -- UpCodes. https://up.codes/s/hvac-system-construction-and-insulation
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ANSI/ASHRAE/IES Standard 90.1-2019: HVAC (training presentation, PNNL-SA-153210) -- U.S. DOE Building Energy Codes Program, hosted by Oregon Building Codes Division. https://www.oregon.gov/bcd/codes-stand/Documents/90.1-2019-HVAC-training.pdf
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Understanding ASHRAE 90.1's Role in Energy Codes and Regulations -- Consulting-Specifying Engineer. https://www.csemag.com/understanding-ashrae-90-1s-role-in-energy-codes-and-regulations/
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ASHRAE Standard 90.1 -- ASHRAE Bookstore. https://www.ashrae.org/technical-resources/bookstore/standard-90-1
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SMACNA -- HVAC Air Duct Leakage Test Manual (ANSI/SMACNA 016-2012) -- SMACNA Store. https://store.smacna.org/hvac-air-duct-leakage-test-manual/
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What is an Allowable Duct System Leakage Specification? (Tech Bulletin CAS-TB3.1-2006) -- McGill AirFlow. https://www.mcgillairflow.com/pdf/cas/techBulletin3.1.pdf
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Section 120.4 -- Requirements for Air Distribution System Ducts and Plenums (2022) -- Energy Code Ace. https://energycodeace.com/content/section-1204-requirements-for-air-distribution-system-ducts
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Duct Sealing for New Duct Systems, Section 120.4(g)1 (2022) -- Energy Code Ace. https://www.energycodeace.com/content/1-new-duct-systems-that-meet-the-criteria-in-subsections-a-b
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Duct Sealing, Insulation and Leakage Testing (California Mechanical Code) -- GoCodeBook. https://gocodebook.com/us/california/california-mechanical-code/duct-systems/installation-supports-and-sealing/duct-sealing-insulation-and-leakage-testing
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Overview of 2025 Title 24, Part 6 Changes (effective date) -- Schnackel Engineers. https://schnackel.com/blogs/overview-of-2025-title-24part-6-changes
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Duct Leakage Testing (CFM25 definition) -- Wikipedia. https://en.wikipedia.org/wiki/Duct_leakage_testing
