CMC · California Mechanical Code

What are the duct sealing, insulation and leakage testing requirements (Title 24 cross‑refs)?

Q: What if the 10% sample fails in the CMC sampling plan?

If the initial 10% tested area fails, test 40% ; if that fails, test 100% — all per § 603.9.2 . Sections to be tested are selected by the owner or their representative.

If your nonresidential duct system meets Title 24’s criteria you must demonstrate leakage ≤6% of the air‑handler airflow using the Title 24 acceptance test; if not, the California Mechanical Code requires certified testing and limits per § 603.9.2 (Lmax = CL·P^0.65, CL=6) with specified sampling, escalation and technician qualifications.

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Last reviewed: July 6, 2026

What the code requires — plain English (controlling §)

Duct systems must be sealed, insulated and, where required, leakage‑tested to meet either the Title 24 acceptance targets or the California Mechanical Code leakage limits and test procedures. The nonresidential acceptance target is a maximum leakage of 6% of nominal air‑handler airflow for certain new systems under § 120.4(g). If a system is not subject to that Title 24 acceptance path it must meet the CMC duct leakage testing requirements and limits in § 603.9.2 (with the specific cross‑reference guidance in § 603.9.2.1).

The single most important rule: meet the Title 24 leakage targets where they apply; otherwise test and limit leakage per CMC § 603.9.2 using the Lmax = CL·P^0.65 method.

Requirements in detail

Which rule applies (high level)

If the project meets the Title 24 acceptance criteria for new nonresidential systems (constant‑volume single‑zone, < 5,000 ft², not serving healthcare, and > 25% of duct surface area outdoors/unconditioned), use the § 120.4(g) leakage target (6% of nominal air‑handler airflow) and the referenced acceptance test procedure.
If the system is not subject to § 120.4(g)1 testing, the system must instead meet CMC § 603.9.2 leakage testing and limits; § 603.9.2.1 points builders/testers to Title 24 acceptance procedures where appropriate.

Key definitions and thresholds (first‑use bolded)

Air handler airflow — the nominal airflow rate of the system used as the denominator for Title 24 %‑leakage targets.
Test pressure (P) — the design duct pressure class rating (inches water column) used in the CMC leakage formula.
Duct leakage class (CL) — numeric class used in the CMC Lmax formula; CL = 6 per § 603.9.2.

CMC testing method and limit (formula)

CMC uses a surface‑area normalized leakage limit for ducts tested to a pressure class:
- Lmax = CL · P^0.65 (units: (ft³/min)/100 ft² duct surface area). CL = 6 and P = design duct pressure (in. wc). See § 603.9.2 for the formula, units and the exception for very low‑pressure transfer ducts.

Who may perform tests and sampling protocol

Duct leakage tests must be performed by a certified technician (examples listed: AABC, NEBB, TABB or equivalent). Representative sections totaling not less than 10% of total installed duct area shall be tested; if the 10% sample fails, test 40%, then 100% if the 40% fails. Sections are selected by the building owner or their representative. Positive pressure testing is allowed for negative pressure ductwork. See § 603.9.2.

Duct insulation requirements (Title 24 cross‑refs)

Title 24 requires duct insulation R‑values and sets how installed thickness and R are determined (insulation only, tested C‑values at 75°F, ASTM references). Insulation thickness rules and labeling requirements for flexible ducts appear in the energy code (see the cited Title 24 sections). For example, for many altered/new ducts Title 24 tables specify R‑6 or R‑8 depending on climate zone and location (see referenced tables in Title 24).

Decision‑relevant values — quick reference table

Decision dimension	Value / limit	When to use it	Code Reference
Title 24 nonresidential new system leakage target	6% of nominal air handler airflow	New constant‑volume single‑zone systems meeting § 120.4(g) criteria	§ 120.4(g)
Title 24 extension / other nonresidential alternative	15% (extensions meeting specific criteria referenced in T24)	Some extensions / combined systems per Title 24	Title 24 acceptance sections referenced by § 120.4(g)
CMC surface‑area normalized leakage limit	Lmax = CL·P^0.65 where CL = 6	When testing per CMC § 603.9.2 (not using the Title 24 % method)	§ 603.9.2
CMC sampling / test escalation	10% → 40% → 100% tested area if failures	Representative sampling protocol in § 603.9.2	§ 603.9.2
Technician qualifications	AABC, NEBB, TABB, or equivalent	Who may perform leakage testing	§ 603.9.2
Duct insulation R‑values (example table)	R‑6 or R‑8 depending on zone/location	Title 24 duct insulation tables for ducts in unconditioned spaces	Title 24 duct insulation tables (see Table references in Title 24)

(Where Title 24 gives multiple acceptance targets for residential and altered systems those specific values and procedures are found in the referenced Title 24 acceptance and residential appendices; § 603.9.2.1 in the CMC points to those Title 24 cross‑refs.)

Exceptions & special cases

Title 24 § 120.4(g) only applies when the system meets the listed criteria (constant‑volume single zone, < 5,000 ft², not healthcare, and >25% duct surface outdoors/unconditioned). If any criterion is not met the CMC formula or other T24 acceptance paths apply.
The CMC formula has an explicit exception for transfer air ducts operating at less than 1 in. wc. See § 603.9.2.
Title 24 contains alternative compliance targets for residential, multifamily and altered systems (e.g., 5%, 7%, 10%, 12%, 15% depending on the situation and the Reference Appendices used). Those are Title 24 acceptance thresholds — the CMC § 603.9.2.1 directs you to the proper T24 sections where those criteria apply.

Common mistakes

Treating the Title 24 percent leakage targets and the CMC Lmax formula as interchangeable — they are different metrics and apply in different situations. Use § 120.4(g) targets only when Title 24 criteria apply; otherwise use § 603.9.2 testing and limits.
Failing to test the required minimum sample area (start at 10% of duct area and escalate per § 603.9.2).
Using uncertified technicians or unapproved test procedures — the CMC requires tests by certified balance agencies (AABC, NEBB, TABB or equivalent).
Forgetting to apply the insulation thickness rules (e.g., assuming nominal thickness for wrap or flexible ducts without applying the Title 24 installed‑thickness conversions). Title 24 defines how to calculate installed thickness and R‑value.

Worked example — concrete numbers (two paths)

Scenario A — Title 24 path (nonresidential, meets criteria)

System: constant‑volume single‑zone, 4,000 ft², not healthcare, combined outdoor/unconditioned duct surface > 25%, nominal air‑handler airflow = 5,000 CFM.
Required leakage: ≤ 6% of nominal air‑handler airflow → 0.06 × 5,000 = 300 CFM total leakage allowed. Use Title 24 acceptance test procedure (Reference Nonresidential Appendix NA7.5.3) to confirm. § 120.4(g).

Scenario B — CMC path (does not meet Title 24 criteria)

Same building but suppose it does not meet the Title 24 criteria (e.g., system >5,000 ft² or variable volume). Duct surface area = 2,000 ft²; design duct pressure class = 1.0 in. wc; CL = 6 per CMC.
CMC per‑area limit: Lmax = CL · P^0.65 = 6 × (1.0^0.65) ≈ 6 (cfm) per 100 ft².
Total allowable leakage = 6 × (2,000 / 100) = 6 × 20 = 120 CFM (total). Test per § 603.9.2 and by certified technician; sample 10% of duct area, escalate if necessary.

Note how the two compliance paths produce different numerical allowances: 300 CFM allowed under the Title 24 % method vs 120 CFM under the CMC surface‑area formula in this example — which is why determining the correct compliance path up front is essential.

Related provisions (CMC)

§ 603.9.2 — Duct leakage testing procedure and Lmax formula (CMC).
§ 603.9.2.1 — Cross‑reference to Title 24 acceptance criteria for buildings that meet those duct sealing criteria.
§ 603.10 — Cross‑contamination requirements for exhaust ducts (relevant when pressure relationships and leakage affect contaminant control).
§ 603.11 — Underground duct installation (construction and sealing considerations).

Code references

Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:

CMC § 110.8. High relevance — show source text
(c) All duct insulation product R -values shall be based on insulation only (excluding air films, vapor retarders or other duct components) and tested C -values at 75°F mean temperature at the installed thickness, in accordance with ASTM C518 or ASTM C177, incorporated herein by reference, and certified pursuant to Section 110.8.

(d) The installed thickness of duct insulation used to determine its R -value shall be determined as follows:
1. For duct board, duct liner and factory-made rigid ducts not normally subjected to compression, the nominal insulation thickness shall be used.
2. For duct wrap, installed thickness shall be assumed to be 75 percent (25 percent compression) of nominal thickness.
3. For factory-made flexible air ducts, the installed thickness shall be determined by dividing the difference between the actual outside diameter and nominal inside diameter by two.
(e) Insulated flexible duct products installed to meet this requirement must include labels, in maximum intervals of 3 feet, showing the thermal performance R -value for the duct insulation itself (excluding air films, vapor retarder or other duct components), based on the tests in Section 120.4(c) and the installed thickness determined by Section 120.4(d)3.

(f) Protection of insulation. Insulation shall be protected from damage, including that due to sunlight, moisture, equipment maintenance and wind, but not limited to the following:

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NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS

Insulation exposed to weather shall be suitable for outdoor service, e.g., protected by aluminum, sheet metal, painted canvas or plastic cover. Cellular foam insulation shall be protected as above or painted with a coating that is water retardant and provides shielding from solar radiation that can cause degradation of the material.

(g) Duct sealing. Duct systems shall comply with Subsection 1 or 2 below:
1. New duct systems that meet the criteria in Subsections A, B, C and D below shall be sealed to a leakage rate not to exceed 6 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with Reference Nonresidential Appendix NA7.5.3; A. The duct system does not serve a healthcare facility; and B. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system; and C. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and D. The combined surface area of the ducts located outdoors or in unconditioned space is more than 25 percent of the total surface area of the entire duct system.
2. New duct systems that are not subject to testing under Section 120.4(g)1 shall instead meet the duct leakage testing requirements of CMC Section 603.9.2.
Note: Authority: Sections 25213, 25218, 25218.5, 25402 and 25402.1, Public Resources Code. Reference: Sections 25007, 25008, 25218.5, 25310, 25402, 25402.1, 25402.4, 25402.8, and 25943, Public Resources Code.

SECTION 120.5—REQUIRED NONRESIDENTIAL MECHANICAL SYSTEM ACCEPTANCE
CMC § 0.65 High relevance — show source text
Duct leakage tests shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:

Lmax = CLP [0.65 ] (Equation 603.9.2)

Where:

Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface

area.

CL = six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).

P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).

Exception: Transfer air duct operating at less than 1 inch of water column (0.25 kPa). 603.9.2.1 Duct Leakage Tests for Buildings that Meet Air Distribution System Duct Leak- age Sealing Criteria in Title 24, Part 6. For duct leakage testing, see California Energy Code Sec- tions 150.0(m)(11) for single family buildings, Sec- tion 160.3(b) for multifamily buildings, and Section 120.4(g) for nonresidential and Hotel Motel build- ings. 603.10 Cross Contamination. Exhaust ducts that convey Class 4 air shall be negatively pressurized relative to ducts, plenums, or occupiable spaces through which the ducts pass. Exhaust ducts under positive pressure that convey Class 2 or Class 3 air shall not extend into or pass through ducts, plenums, or occupiable spaces other than the space from which the exhaust air is drawn.

603.11 Underground Installation. Ducts installed underground shall be approved for the installation and shall have a slope of not less than [1] ⁄ 8 inch per foot (10.4 mm/m) back to the main riser. Ducts, plenums, and fittings shall be permitted to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab, provided the joints are sealed and duct is secured in accordance with SMACNA

HVAC Duct Construction Standards – Metal and Flexible.

Metal ducts where installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured in accordance with SMACNA HVAC Duct Con struction Standards – Metal and Flexible.
CMC § 180.2 High relevance — show source text
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MULTIFAMILY BUILDINGS—ADDITIONS, ALTERATIONS AND REPAIRS TO EXISTING MULTIFAMILY BUILDINGS

C. If it is not possible to meet the duct sealing requirements of either Section 180.2(b)2AiicI or II then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified ECC-Rater utilizing the methods specified in Reference Residential Appendix RA3.1.4.3.5.

Exception to Section 180.2(b)2AiiaII: duct sealing. Existing duct systems that are extended, which are constructed, insulated or sealed with asbestos. Exception 1 to 180.2(b)2Aii: The field verification and ECC-Provider data registry requirements of Reference Residential Appendix RA2 and RA3 are not required for multifamily dwelling units in buildings four stories and greater. The installer shall certify that diagnostic testing was performed in accordance with the applicable procedures. iii. Altered space-conditioning system—duct sealing. In all climate zones, when a space-conditioning system serving a multifamily dwelling is altered by the installation or replacement of space-conditioning system equipment, including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, or cooling or heating coil, the duct system that is connected to the altered space-conditioning system equipment shall be sealed, as confirmed through field verification and diagnostic testing in accordance with the applicable procedures for duct sealing of altered existing duct systems as specified in Reference Residential Appendix RA3.1 and the leakage compliance criteria specified in Subsection a, b or c below. a. The measured duct leakage shall be equal to or less than 15 percent of air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or b. The measured duct leakage to outside shall be equal to or less than 10 percent of air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4; or c. If it is not possible to meet the duct sealing requirements of either Section 180.2(b)2Aiiia or b, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified ECC-Rater utilizing the methods specified in Reference Residential Appendix RA3.1.4.3.5. Exception 1 to Section 180.2(b)2Aiii : duct sealing. Duct systems that are documented to have been previously sealed as confirmed through field verification and diagnostic testing in accordance with procedures in Reference Residential Appendix RA3.1. Exception 2 to Section 180.2(b)2Aiii: duct sealing. Duct systems with less than 40 linear feet as determined by visual inspection. Exception 3 to Section 180.2(b)2Aiii: duct sealing. Existing duct systems constructed, insulated or sealed with asbestos.

Exception 4 to Section 180.2(b)2Aiii: The field verification and ECC-Provider data registry requirements of Reference Residential Appendix RA2 and RA3 are not required for multifamily dwelling units in buildings four stories and greater. The installer shall certify that diagnostic testing was performed in accordance with the applicable procedures. iv. **Altered space-conditioning system mechanical cooling.
CMC § 120.4 High relevance — show source text
The system is not a single package air-cooled commercial unitary air conditioner or heat pump; and 2. The cooling capacity of the system is less than 54,000 Btu/h. D. Altered duct systems. New or replacement space-conditioning system ducts installed to serve an existing building shall meet the requirements of Section 120.4 (a) through (f) and meet i, ii or iii below: i. Entirely new or complete replacement duct systems installed as part of an alteration shall be leakage tested in accordance with Section 120.4(g). This applies to replacement duct systems installed as part of an alteration that are constructed of at least 75 percent new duct material. Up to 25 percent of that alteration may consist of reused parts from the building’s existing duct system (including registers, grilles, boots, air handlers, coils, plenums and ducts) if the reused parts are accessible and can be sealed to prevent leakage. ii. If the new ducts are an extension of an existing duct system and the combined new and existing duct system meets the criteria in Subsections 1, 2, 3 and 4 below, the duct system shall be sealed to a leakage rate not to exceed 15 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with the applicable procedures in Reference Nonresidential Appendix NA7.5.3:
1. The duct system does not serve a healthcare facility;
2. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system;
3. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and
4. The combined surface area of the ducts located outdoors or in unconditioned space is more than 25 percent of the total surface area of the entire duct system. Exception 1 to Section 141.0(b)2Dii: When it is not possible to achieve the duct leakage criteria in Section 141.0(b)2Dii, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test performed by a certified mechanical acceptance test technician utilizing the methods specified in Reference Nonresidential Appendix NA7.5.3. Exception 2 to Section 141.0(b)2Dii: Duct sealing. Existing duct systems that are extended, which are constructed insulated or sealed with asbestos are not required to comply with Subsection 141.0(b)2Dii. iii. If new ducts installed as part of an alteration are not required to comply with leakage testing specified by Section 141.0(b)2Di or 141.0(b)2Dii, then the new ducts shall meet the duct leakage testing requirements of CMC Section 603.9.2.
E. Altered space-conditioning systems. When a space-conditioning system is altered by the installation or replacement of space-conditioning system equipment (including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, or cooling or heating coil: i. For all altered units where the existing thermostat does not comply with the requirements for demand responsive controls specified in Section 110.12, the existing thermostat shall be replaced with a demand responsive thermostat that complies with Section 110.12. All newly installed space-conditioning systems requiring a thermostat shall be equipped with a demand responsive thermostat that complies with Section 110.12; and ii. The duct system that is connected to the new or replaced space-conditioning system equipment shall be sealed in accordance with Section 141.0(b)2Dii.
CMC § 150.2 High relevance — show source text
D. Altered duct systems—duct sealing. In all climate zones, when more than 25 feet of new or replacement spaceconditioning system ducts are installed, the ducts shall comply with the applicable requirements of Subsections i and ii below. Additionally, when altered ducts, air-handling units, cooling or heating coils, or plenums are located in garage spaces, the system shall comply with Subsection 150.2(b)1Diic regardless of the length of any new or replacement space-conditioning ducts installed in the garage space. i. New ducts located in unconditioned space shall meet the applicable requirements of Sections 150.0(m)1 through 150.0(m)10, and the duct insulation requirements of Table 150.2-D; and

TABLE 150.2-D—DUCT INSULATION R-VALUE Col2 Col3

Climate Zone 3, 5 through 7 1, 2, 4, 8 through 16

DuctR-Value R-6 R-8

ii. The altered duct system, regardless of location, shall be sealed as confirmed through field verification and diagnostic testing in accordance with all applicable procedures for duct sealing of altered existing duct systems as specified in Reference Residential Appendix Section RA3.1, utilizing the leakage compliance criteria specified in Subsection a or b below.

a. Entirely new or complete replacement duct system. If the new ducts form an entirely new or complete replacement duct system directly connected to the air handler, the duct system measured leakage shall be equal to or less than 5 percent of the system air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1. Entirely new or complete replacement duct systems installed as part of an alteration is constructed of at least 75 percent new duct material, and up to 25 percent may consist of reused parts from the dwelling unit’s existing duct system, including but not limited to registers, grilles, boots, air handler, coil, plenums, duct material; if the reused parts are accessible and can be sealed to prevent leakage. Entirely new or complete replacement duct systems shall also conform to the requirements of Sections 150.0(m)12 and 150.0(m)13. If the air handler and ducts are located within a vented attic, the requirements of Section 150.2(b)1J shall also be met.

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b. Extension of an existing duct system. If the new ducts are an extension of an existing duct system serving single-family dwellings, the combined new and existing duct system shall meet one of the following requirements: I. The measured duct leakage shall be equal to or less than 10 percent of system air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or II. The measured duct leakage to outside shall be equal to or less than 7 percent of system air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4; or III.
CMC § 141.0 High relevance — show source text
Exception 1 to Section 141.0(b)2Dii: When it is not possible to achieve the duct leakage criteria in Section 141.0(b)2Dii, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test performed by a certified mechanical acceptance test technician utilizing the methods specified in Reference Nonresidential Appendix NA7.5.3. Exception 2 to Section 141.0(b)2Dii: Duct sealing. Existing duct systems that are extended, which are constructed insulated or sealed with asbestos are not required to comply with Subsection 141.0(b)2Dii. iii. If new ducts installed as part of an alteration are not required to comply with leakage testing specified by Section 141.0(b)2Di or 141.0(b)2Dii, then the new ducts shall meet the duct leakage testing requirements of CMC Section 603.9.2.

E. Altered space-conditioning systems. When a space-conditioning system is altered by the installation or replacement of space-conditioning system equipment (including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, or cooling or heating coil: i. For all altered units where the existing thermostat does not comply with the requirements for demand responsive controls specified in Section 110.12, the existing thermostat shall be replaced with a demand responsive thermostat that complies with Section 110.12. All newly installed space-conditioning systems requiring a thermostat shall be equipped with a demand responsive thermostat that complies with Section 110.12; and ii. The duct system that is connected to the new or replaced space-conditioning system equipment shall be sealed in accordance with Section 141.0(b)2Dii. Exception 1 to Section 141.0(b)2Eii: Duct sealing. Buildings altered so that the duct system no longer meets the criteria of Section 141.0(b)2Dii. Exception 2 to Section 141.0(b)2Eii: Duct sealing. Duct systems that are documented to have been previously sealed as confirmed through field verification and diagnostic testing in accordance with procedures in Reference Nonresidential Appendix NA2. Exception 3 to Section 141.0(b)2Eii: Duct sealing. Existing duct systems constructed, insulated or sealed with asbestos are not required to comply with Subsection 141.0(b)2Eii. F. Spaces with lighting systems installed for the first time shall meet the requirements of Sections 110.9, 130.0, 130.1, 130.2, 130.4, 140.3(c), 140.6 and 140.7. G. When the requirements of Section 130.1(d) are triggered by the addition of skylights to an existing building and the lighting system is not recircuited, the daylighting control need not meet the multilevel requirements in Section 130.1(d). H. New internally and externally illuminated signs shall meet the requirements of Sections 110.9, 130.3 and 140.8. I. Altered indoor lighting systems. Alterations to indoor lighting systems that include 10% or more of the luminaires serving an enclosed space shall meet the requirements of i, ii, or iii below: i. The alteration shall comply with the indoor lighting power requirements specified in Section 140.6 and the lighting control requirements specified in Table 141.0-F; ii.
CMC § 150.2 High relevance — show source text
Additionally, when altered ducts, air-handling units, cooling or heating coils, or plenums are located in garage spaces, the system shall comply with Section 150.2(b)1Diic regardless of the length of any new or replacement space-conditioning ducts installed in the garage space. i. The measured duct leakage shall be equal to or less than 10 percent of system air handler airflow as determined utilizing the procedures in Reference Residential Appendix RA3.1.4.3.1; or ii. The measured duct leakage to outside shall be equal to or less than 7 percent of system air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4; or iii. If it is not possible to meet the duct sealing requirements of either Section 150.2(b)1Ei or 150.2(b)1Eii, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified ECCRater utilizing the methods specified in Reference Residential Appendix Section RA3.1.4.3.5. Exception 1 to Section 150.2(b)1E: Duct sealing. Duct systems that are documented to have been previously sealed as confirmed through field verification and diagnostic testing in accordance with procedures in the Reference Residential Appendix RA3.1. Exception 2 to Section 150.2(b)1E: Duct sealing. Duct systems with less than 40 linear feet as determined by visual inspection. Exception 3 to Section 150.2(b)1E: Duct sealing. Existing duct systems constructed, insulated or sealed with asbestos.

F. Altered space-conditioning system—mechanical cooling. When a space-conditioning system is an air conditioner or heat pump that is altered by the installation or replacement of refrigerant-containing system components such as the compressor, condensing coil, evaporator coil, refrigerant metering device or refrigerant piping, the altered system shall comply with the following requirements: i. All thermostats associated with the system shall be replaced with setback thermostats meeting the requirements of Section 110.2(c).

ii. Air-cooled air conditioners in Climate Zones 2 and 8 through 15 and air-source heat pumps in all climate zones, including but not limited to ducted split systems, ducted package systems, small duct high-velocity air systems, and minisplit systems shall comply with Subsections a and b, unless the system is of a type that cannot be verified using the specified procedures. Systems that cannot comply with the requirements of Section 150.2(b)1Fii shall comply with Section 150.2(b)1Fiii. Exception to Section 150.2(b)1Fii: Entirely new or complete replacement packaged systems for which the manufacturer has verified correct system refrigerant charge prior to shipment from the factory are not required to have refrigerant charge confirmed through field verification and diagnostic testing. The installer of these packaged systems shall certify on the Certificate of Installation that the packaged

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system was pre-charged at the factory and has not been altered in a way that would affect the charge.
CMC § 150.0 High relevance — show source text
Entirely new or complete replacement duct systems installed as part of an alteration is constructed of at least 75 percent new duct material, and up to 25 percent may consist of reused parts from the dwelling unit’s existing duct system, including but not limited to registers, grilles, boots, air handler, coil, plenums, duct material; if the reused parts are accessible and can be sealed to prevent leakage. Entirely new or complete replacement duct systems shall also conform to the requirements of Sections 150.0(m)12 and 150.0(m)13. If the air handler and ducts are located within a vented attic, the requirements of Section 150.2(b)1J shall also be met.

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b. Extension of an existing duct system. If the new ducts are an extension of an existing duct system serving single-family dwellings, the combined new and existing duct system shall meet one of the following requirements: I. The measured duct leakage shall be equal to or less than 10 percent of system air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or II. The measured duct leakage to outside shall be equal to or less than 7 percent of system air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4; or III. If it is not possible to meet the duct sealing requirements of either Section 150.2(b)1DiibI or 150.2(b)1DiibII, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified ECC-Rater utilizing the methods specified in Reference Residential Appendix RA3.1.4.3.5. Exception to Section 150.2(b)1Diib: Duct sealing. Existing duct systems that are extended, which are constructed, insulated or sealed with asbestos.

c. Altered ducts and duct system components in garage spaces. When new or replacement space-conditioning ducts, air-handling units, cooling or heating coils, or plenums are located in a garage space, compliance with either I or II below is required. I. The measured system duct leakage shall be less than or equal to 6 percent of system air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or II. All accessible leaks located in the garage space shall be sealed and verified through a visual inspection and a smoke test by a certified ECC-Rater utilizing the methods specified in Reference Residential Appendix RA3.1.4.3.5. E. Altered space-conditioning system—duct sealing. In all climate zones, when a space-conditioning system serving a single-family dwelling is altered by the installation or replacement of space-conditioning system equipment, including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, or cooling or heating coil, the duct system that is connected to the altered space-conditioning system equipment shall be sealed, as confirmed through field verification and diagnostic testing in accordance with the applicable procedures for duct sealing of altered existing duct systems as specified in Reference Residential Appendix RA3.1, and the leakage compliance criteria specified in Subsection i, ii, or iii below.
CMC § 150.2 High relevance — show source text
Exception to Section 150.2(b)1Diib: Duct sealing. Existing duct systems that are extended, which are constructed, insulated or sealed with asbestos.

c. Altered ducts and duct system components in garage spaces. When new or replacement space-conditioning ducts, air-handling units, cooling or heating coils, or plenums are located in a garage space, compliance with either I or II below is required. I. The measured system duct leakage shall be less than or equal to 6 percent of system air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or II. All accessible leaks located in the garage space shall be sealed and verified through a visual inspection and a smoke test by a certified ECC-Rater utilizing the methods specified in Reference Residential Appendix RA3.1.4.3.5. E. Altered space-conditioning system—duct sealing. In all climate zones, when a space-conditioning system serving a single-family dwelling is altered by the installation or replacement of space-conditioning system equipment, including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, or cooling or heating coil, the duct system that is connected to the altered space-conditioning system equipment shall be sealed, as confirmed through field verification and diagnostic testing in accordance with the applicable procedures for duct sealing of altered existing duct systems as specified in Reference Residential Appendix RA3.1, and the leakage compliance criteria specified in Subsection i, ii, or iii below. Additionally, when altered ducts, air-handling units, cooling or heating coils, or plenums are located in garage spaces, the system shall comply with Section 150.2(b)1Diic regardless of the length of any new or replacement space-conditioning ducts installed in the garage space. i. The measured duct leakage shall be equal to or less than 10 percent of system air handler airflow as determined utilizing the procedures in Reference Residential Appendix RA3.1.4.3.1; or ii. The measured duct leakage to outside shall be equal to or less than 7 percent of system air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4; or iii. If it is not possible to meet the duct sealing requirements of either Section 150.2(b)1Ei or 150.2(b)1Eii, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified ECCRater utilizing the methods specified in Reference Residential Appendix Section RA3.1.4.3.5. Exception 1 to Section 150.2(b)1E: Duct sealing. Duct systems that are documented to have been previously sealed as confirmed through field verification and diagnostic testing in accordance with procedures in the Reference Residential Appendix RA3.1. Exception 2 to Section 150.2(b)1E: Duct sealing. Duct systems with less than 40 linear feet as determined by visual inspection. Exception 3 to Section 150.2(b)1E: Duct sealing. Existing duct systems constructed, insulated or sealed with asbestos.
CMC § 204.1 High relevance — show source text
1. Lighting for industrial sites, including but not limited to, rail yards, maritime shipyards and docks, piers and marinas, chemical and petroleum processing plants, and aviation facilities.
2. Lighting of tunnels, bridges, stairs, wheelchair elevator lifts for American with Disabilities Act (ADA) compliance, and ramps that are not parking garage ramps.
3. In theme parks: outdoor lighting only for themes and special effects.
4. Lighting for outdoor theatrical and other outdoor live performances, provided that these lighting systems are additions to area lighting systems and are controlled by a multi-scene or theatrical cross-fade control station accessible only to authorized operators.
5. Outdoor lighting systems for qualified historical buildings, as defined in the California Historical Building Code (Title 24, Part 8), if they consist solely of historical lighting components or replicas of historical lighting components. If lighting systems for qualified historical buildings contain some historical lighting components or replicas of historical components, combined with other lighting components, only those historical or historical replica components are exempt. All other outdoor lighting systems for qualified historical buildings shall not be exempted.
APPENDIX A4-10 2025 CALIFORNIA GREEN BUILDING STANDARDS CODE

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

APPENDIX A4 — RESIDENTIAL VOLUNTARY MEASURES

SECTION A4.204 — REQUIREMENTS FOR ALTERATIONS TO EXISTING BUILDINGS

A4.204.1 Energy efficiency. Alterations to existing residential buildings shall comply with Sections A4.204.1.1 and A4.204.1.2.

A4.204.1.1 Altered space-conditioning system serving existing single-family dwelling units – mechanical cooling. When a space-conditioning system serving an existing single-family dwelling unit is altered in Climate Zones 1 through 14 and 16 by installation or replacement of an air conditioner, the altered system shall comply with either a or b below in addition to the requirements for installation specified by Title 24, Part 6, Sections 150.2(b)1E and 150.2(b)1F: a. A heat pump shall be the primary heating source and sized according to the system selection requirements specified by Title 24, Part 6 of Section 150.0(h)5. Supplemental heating may be provided by an existing gas furnace or existing electric resistance heating; or b. An air conditioner shall meet the following requirements: i. R-8 duct insulation for ducts located in unconditioned space; and ii. The duct system measured air leakage shall be equal to or less than 5 percent of the system air handler airflow as confirmed through field verification and diagnostic testing, per the requirements in Title 24, Part 6, Reference Residential Appendix Section RA3.1.4.3.1; and iii. Demonstrate, in every control mode, airflow greater than or equal to 400 CFM per ton of nominal cooling capacity through the return grilles, and an air-handling unit fan efficacy less than or equal to 0.35 W/CFM. The airflow rate and fan efficacy requirements in this section shall be confirmed through field verification and diagnostic testing, following the procedures outlined in Title 24, Part 6, Reference Residential Appendix RA3.3; and iv.
CMC § 180.2 High relevance — show source text

TABLE 180.2-C—DUCT INSULATION R-VALUE Col2 Col3

Climate Zone 3, 5 through 7 1, 2, 4, 8 through 16

Duct_R-_Value R-6 R-8

I. Entirely new or complete replacement duct system. If the new ducts form an entirely new or complete replacement duct system directly connected to the air handler, the duct system shall meet one of the following requirements: A. The total leakage of the duct system shall not exceed 12 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1,

or

B. The duct system leakage to outside shall not exceed 6 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4. Entirely new or complete replacement duct systems installed as part of an alteration are constructed of at least 75 percent new duct material, and up to 25 percent may consist of reused parts from the dwelling unit's existing duct system, including but not limited to registers, grilles, boots, air handler, coil, plenums and duct material, if the reused parts are accessible and can be sealed to prevent leakage. Entirely new or complete replacement duct systems shall also conform to the requirements of Sections 160.2(a)1 and 160.3(b)5L. If the air handler and ducts are located within a vented attic, the requirements of Section 180.2(b)1Bi shall also be met. II. Extension of an existing duct system. If the new ducts are an extension of an existing duct system serving multifamily dwellings, the combined new and existing duct system shall meet one of the following requirements: A. The measured duct leakage shall be equal to or less than 15 percent of air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or B. The measured duct leakage to outside shall be equal to or less than 10 percent of air handler airflow as confirmed by field verification and diagnostic testing utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4; or

276 2025 CALIFORNIA ENERGY CODE

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

MULTIFAMILY BUILDINGS—ADDITIONS, ALTERATIONS AND REPAIRS TO EXISTING MULTIFAMILY BUILDINGS

C. If it is not possible to meet the duct sealing requirements of either Section 180.2(b)2AiicI or II then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified ECC-Rater utilizing the methods specified in Reference Residential Appendix RA3.1.4.3.5.

Exception to Section 180.2(b)2AiiaII: duct sealing. Existing duct systems that are extended, which are constructed, insulated or sealed with asbestos. Exception 1 to 180.2(b)2Aii: The field verification and ECC-Provider data registry requirements of Reference Residential Appendix RA2 and RA3 are not required for multifamily dwelling units in buildings four stories and greater. The installer shall certify that diagnostic testing was performed in accordance with the applicable procedures. iii. **Altered space-conditioning system—duct sealing.
CMC § 110.8. High relevance — show source text
I. Drawbands shall be either stainless-steel worm-drive hose clamps or UV-resistant nylon duct ties. II. Drawbands shall have a minimum tensile strength rating of 150 pounds. III. Drawbands shall be tightened as recommended by the manufacturer with an adjustable tensioning tool.

f. Aerosol-sealant closures.

I. Aerosol sealants shall meet the requirements of UL 723 and be applied according to manufacturer specifications. II. Tapes or mastics used in combination with aerosol sealing shall meet the requirements of this section.

D. All duct insulation product R- values shall be based on insulation only (excluding air films, vapor retarders or other duct components) and tested C-values at 75°F mean temperature at the installed thickness, in accordance with ASTM C518 or ASTM C177, incorporated herein by reference, and certified pursuant to Section 110.8.

E. The installed thickness of duct insulation used to determine its R- value shall be determined as follows:

i. For duct board, duct liner and factory-made rigid ducts not normally subjected to compression, the nominal insulation thickness shall be used.

ii. For duct wrap, installed thickness shall be assumed to be 75 percent (25 percent compression) of nominal thickness.

iii. For factory-made flexible air ducts, the installed thickness shall be determined by dividing the difference between the actual outside diameter and nominal inside diameter by two. F. Insulated flexible duct products installed to meet this requirement must include labels, in maximum intervals of 3 feet, showing the thermal performance R- value for the duct insulation itself (excluding air films, vapor retarder or other duct components), based on the tests in Section 160.3(c)2D and the installed thickness determined by Section 160.3(c)2Eiii. G. Insulation shall be protected from damage, including that due to sunlight, moisture, equipment maintenance and wind but not limited to the following: Insulation exposed to weather shall be suitable for outdoor service; e.g., protected by aluminum, sheet metal, painted canvas or plastic cover. Cellular foam insulation shall be protected as above or painted with a coating that is water retardant and provides shielding from solar radiation that can cause degradation of the material. H. Duct systems shall be tested in accordance with i or ii below: i. New duct systems that meet the criteria in Subsections a, b and c below or ductwork that is part of a system that meets the criteria of Section 180.2(b)2B shall be sealed to a leakage rate not to exceed 6 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with Reference Nonresidential Appendix NA7.5.3. a. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system; and b. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and c. The combined surface area of the ducts located in the following spaces is more than 25 percent of the total surface area of the entire duct system:

I. Outdoors; or

II. In a space directly under a roof that has a U- factor greater than the U- factor of the ceiling, or if the roof does not meet the requirements of Section 170.2(a)1; or III. In a space directly under a roof that has fixed vents or openings to the outside or unconditioned

TABLE 150.2-D—DUCT INSULATION R-VALUE	Col2	Col3
Climate Zone	3, 5 through 7	1, 2, 4, 8 through 16
DuctR-Value	R-6	R-8

TABLE 180.2-C—DUCT INSULATION R-VALUE	Col2	Col3
Climate Zone	3, 5 through 7	1, 2, 4, 8 through 16
Duct_R-_Value	R-6	R-8

Frequently asked questions

Who decides whether Title 24 § 120.4(g) applies or whether CMC § 603.9.2 applies?

The project designer/contractor in coordination with the building owner or their representative should confirm system type and the Title 24 criteria (constant‑volume single‑zone, <5,000 ft², not healthcare, and >25% outdoor/unconditioned duct surface). If the criteria are met use § 120.4(g); otherwise plan to test per § 603.9.2.

What test standard or appendix is used for Title 24 acceptance testing?

Title 24 acceptance testing for ducts points to the Reference Nonresidential Appendix NA7.5.3 for the acceptance procedure for nonresidential systems referenced in § 120.4(g).

What if the 10% sample fails in the CMC sampling plan?

If the initial 10% tested area fails, test 40%; if that fails, test 100% — all per § 603.9.2. Sections to be tested are selected by the owner or their representative.

Are there special rules for flexible duct insulation labeling?

Yes — Title 24 requires insulated flexible ducts installed to meet duct insulation requirements to include labels (maximum intervals of 3 ft) showing the thermal performance R‑value for the insulation itself. See the Title 24 insulation labeling and thickness rules.

Who can perform duct leakage testing?

Tests must be performed by a certified technician (examples listed in the CMC are AABC, NEBB, TABB, or other equivalent approved agencies) as required in § 603.9.2.

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