What is a 'Plenum' or 'Return-air plenum'?

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

• CMC § 0.94 High relevance — show source text

  1. In the main return air and exhaust air plenum of each air-conditioning system having a capacity greater than 2,000 cubic feet per minute (cfm) (0.94 m [3] /s). Such detectors shall be located in a serviceable area downstream of the last duct inlet.

  2. At each connection to a vertical duct or riser serving two or more stories from a return air duct or plenum of an airconditioning system. In Group R-1 and R-2 occupancies, a smoke detector is allowed to be used in each return air riser carrying not more than 5,000 cfm (2.4 m [3] /s) and serving not more than 10 air-inlet openings.

  2025 CALIFORNIA FIRE CODE 9-47

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

  FIRE PROTECTION AND LIFE SAFETY SYSTEMS

  907.2.13.2 Fire department communication system. Where a wired communication system is approved in lieu of an inbuilding, two-way emergency responder communication coverage system in accordance with Section 510, the wired fire department communication system shall be designed and installed in accordance with NFPA 72 and shall operate between a fire command center complying with Section 508, elevators, elevator lobbies, emergency and standby power rooms, fire pump rooms, areas of refuge and inside interior exit stairways. The fire department communication device shall be provided at each floor level within the interior exit stairway.

  907.2.13.3 Multiple-channel voice evacuation. In buildings with an occupied floor more than 120 feet (36 576 mm) above the lowest level of fire department vehicle access, voice evacuation systems for high-rise buildings shall be multiple-channel systems.

  907.2.14 Atriums connecting more than two stories. A fire alarm system shall be installed in occupancies with an atrium that connects more than two stories, with smoke detection in locations required by a rational analysis in Section 909.4 and in accordance with the system operation requirements in Section 909.17. The system shall be activated in accordance with Section 907.5. Such occupancies in Group A, E or M shall be provided with an emergency voice/alarm communication system complying with the requirements of Section 907.5.2.2.

  907.2.15 High-piled combustible storage areas. An automatic smoke detection system shall be installed throughout high-piled combustible storage areas where required by Section 3206.5.

  907.2.16 Aerosol storage uses. Aerosol product rooms and general-purpose warehouses containing aerosol products shall be provided with an approved manual fire alarm system where required by this code.

  907.2.17 Lumber, wood structural panel and veneer mills. Lumber, wood structural panel and veneer mills shall be provided with a manual fire alarm system.

  907.2.18 Underground buildings with smoke control systems. Where a smoke control system is installed in an underground building in accordance with the California Building Code, automatic smoke detectors shall be provided in accordance with Section 907.2.18.1.

  907.2.18.1 Smoke detectors. Not fewer than one smoke detector listed for the intended purpose shall be installed in all of the following areas:

  1. Mechanical equipment, electrical, transformer, telephone equipment, elevator machine or similar rooms.

  2. Elevator lobbies.

  3. The main return and exhaust air plenum of each air-conditioning system serving more than one story and located in a serviceable area downstream of the last duct inlet.

• CMC § 2.1 High relevance — show source text

  Exceptions:

  1. In Group I-3, Condition 2, 3 or 4, occupancies, the dead end in a corridor shall not exceed 50 feet (15 240 mm).

  2. In occupancies in Groups B, E, F, M, R-1, R-2, R-2.1, R-2.2, S and U, where the building is equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1, the length of the dead-end corridors shall not exceed 50 feet (15 240 mm).

  3. A dead-end corridor shall not be limited in length where the length of the dead-end corridor is less than 2.5 times the least width of the dead-end corridor.

  4. In Group I-2 occupancies, where the building is equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1, the length of dead-end corridors that do not serve patient rooms or patient treatment spaces shall not exceed 30 feet (9144 mm).

  2025 CALIFORNIA BUILDING CODE 10-43

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

  MEANS OF EGRESS

  1020.6 Air movement in corridors. Corridors shall not serve as supply, return, exhaust, relief or ventilation air ducts.

  Exceptions:

  1. Use of a corridor as a source of makeup air for exhaust systems in rooms that open directly onto such corridors, including toilet rooms, bathrooms, dressing rooms, smoking lounges and janitor closets, shall be permitted, provided that each such corridor is directly supplied with outdoor air at a rate greater than the rate of makeup air taken from the corridor.
  2. Where located within a dwelling unit, the use of corridors for conveying return air shall not be prohibited.
  3. Where located within tenant spaces of 1,000 square feet (93 m [2] ) or less in area, utilization of corridors for conveying return air is permitted.
  4. Transfer air movement required to maintain the pressurization difference within health care facilities and Group L occu- pancies in accordance with ASHRAE 170. 5. For health care facilities under the jurisdiction of the Office of Statewide Hospital Planning and Development (OSHPD), see the California Mechanical Code.

  1020.6.1 Corridor ceiling. Use of the space between the corridor ceiling and the floor or roof structure above as a return air plenum is permitted for one or more of the following conditions:

  1. The corridor is not required to be of fire-resistance-rated construction.
  2. The corridor is separated from the plenum by fire-resistance-rated construction.
  3. The air-handling system serving the corridor is shut down upon activation of the air-handling unit smoke detectors required by the California Mechanical Code .
  4. The air-handling system serving the corridor is shut down upon detection of sprinkler water flow where the building is equipped throughout with an automatic sprinkler system.
  5. The space between the corridor ceiling and the floor or roof structure above the corridor is used as a component of an approved engineered smoke control system.
• CMC § 1224.4.7.1 Medium relevance — show source text

  See California Building Code Section 1224.4.7.1 for Group I-2._|For SI: 1 inch = 25.4 mm.
  a. See California Building Code Section 1224.4.7.1 for Group I-2.|

  [BE] 1020.4 Obstruction. The minimum width or required capacity of corridors shall be unobstructed.

  Exception: Encroachments complying with Section 1005.7.

  [BE] 1020.5 Dead ends. Where more than one exit or exit access doorway is required, the exit access shall be arranged such that dead-end corridors do not exceed 20 feet (6096 mm) in length.

  Exceptions:

  1. In Group I-3, Condition 2, 3 or 4 occupancies, the dead end in a corridor shall not exceed 50 feet (15 240 mm).

  2. In occupancies in Groups B, E, F, I-1, M, R-1, R-2, R-2.1, R-2.2, S and U, where the building is equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1, the length of the dead-end corridors shall not exceed 50 feet (15 240 mm).

  3. A dead-end corridor shall not be limited in length where the length of the dead-end corridor is less than 2.5 times the least width of the dead-end corridor.

  4. In Group I-2 occupancies, where the building is equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1, the length of dead-end corridors that do not serve patient rooms or patient treatment spaces shall not exceed 30 feet (9144 mm).

  [BE] 1020.6 Air movement in corridors. Corridors shall not serve as supply, return, exhaust, relief or ventilation air ducts.

  Exceptions:

  1. Use of a corridor as a source of makeup air for exhaust systems in small rooms of 30 square feet or less that open directly onto such corridors, including toilet rooms, bathrooms, dressing rooms, smoking lounges and janitor closets, shall be permitted, provided that each such corridor is directly supplied with outdoor air at a rate greater than the rate of makeup air taken from the corridor.
  2. Where located within a dwelling unit, the use of corridors for conveying return air shall not be prohibited.
  3. Where located within tenant spaces of 1,000 square feet (93 m [2] ) or less in area, utilization of corridors for conveying return air is permitted.
  4. Transfer air movement required to maintain the pressurization difference within health care facilities and Group L occu- pancies, in accordance with ASHRAE 170. 5. For health care facilities under the jurisdiction of the Office of Statewide Health Planning and Development (OSHPD), see the California Mechanical Code.

  [BE] 1020.6.1 Corridor ceiling. Use of the space between the corridor ceiling and the floor or roof structure above as a return air plenum is permitted for one or more of the following conditions:

  1. The corridor is not required to be of fire-resistance-rated construction.
  2. The corridor is separated from the plenum by fire-resistance-rated construction.
  3. The air-handling system serving the corridor is shut down upon activation of the air-handling unit smoke detectors required by the California Mechanical Code .
• CMC § 1020.6 Medium relevance — show source text

  [BE] 1020.6 Air movement in corridors. Corridors shall not serve as supply, return, exhaust, relief or ventilation air ducts.

  Exceptions:

  1. Use of a corridor as a source of makeup air for exhaust systems in small rooms of 30 square feet or less that open directly onto such corridors, including toilet rooms, bathrooms, dressing rooms, smoking lounges and janitor closets, shall be permitted, provided that each such corridor is directly supplied with outdoor air at a rate greater than the rate of makeup air taken from the corridor.
  2. Where located within a dwelling unit, the use of corridors for conveying return air shall not be prohibited.
  3. Where located within tenant spaces of 1,000 square feet (93 m [2] ) or less in area, utilization of corridors for conveying return air is permitted.
  4. Transfer air movement required to maintain the pressurization difference within health care facilities and Group L occu- pancies, in accordance with ASHRAE 170. 5. For health care facilities under the jurisdiction of the Office of Statewide Health Planning and Development (OSHPD), see the California Mechanical Code.

  [BE] 1020.6.1 Corridor ceiling. Use of the space between the corridor ceiling and the floor or roof structure above as a return air plenum is permitted for one or more of the following conditions:

  1. The corridor is not required to be of fire-resistance-rated construction.
  2. The corridor is separated from the plenum by fire-resistance-rated construction.
  3. The air-handling system serving the corridor is shut down upon activation of the air-handling unit smoke detectors required by the California Mechanical Code .
  4. The air-handling system serving the corridor is shut down upon detection of sprinkler water flow where the building is equipped throughout with an automatic sprinkler system.
  5. The space between the corridor ceiling and the floor or roof structure above the corridor is used as a component of an approved engineered smoke control system.

  [BE] 1020.7 Corridor continuity. Fire-resistance-rated corridors shall be continuous from the point of entry to an exit, and shall not be interrupted by intervening rooms. Where the path of egress travel within a fire-resistance-rated corridor to the exit includes travel

  2025 CALIFORNIA FIRE CODE 10-45

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

  MEANS OF EGRESS

  along unenclosed exit access stairways or ramps, the fire-resistance-rating shall be continuous for the length of the stairway or ramp and for the length of the connecting corridor on the adjacent floor leading to the exit.

  Exceptions:

  1. Foyers, lobbies or reception rooms constructed as required for corridors shall not be construed as intervening rooms.
  2. Enclosed elevator lobbies as permitted by Item 1 of Section 1016.2 shall not be construed as intervening rooms. 3. [SFM] In fully sprinklered office buildings, corridors may lead through enclosed elevator lobbies if all areas of the building have access to at least one required exit without passing through the elevator lobby.

  SECTION 1021—EGRESS BALCONIES

  [BE] 1021.1 General. Balconies used for egress purposes shall conform to the same requirements as corridors for minimum width, required capacity, headroom, dead ends and projections.

• CMC § 1.9 Medium relevance — show source text

  9| |SUPPLY AND RETURN DUCTS FOR HEATING ONLY|SUPPLY AND RETURN DUCTS FOR HEATING ONLY|SUPPLY AND RETURN DUCTS FOR HEATING ONLY|SUPPLY AND RETURN DUCTS FOR HEATING ONLY| |0 to 1|none|none|none| |2 to 4|R-6|R-6|R-1.9| |5 to 8|R-12|R-6|R-1.9| |SUPPLY AND RETURN DUCTS FOR COOLING ONLY|SUPPLY AND RETURN DUCTS FOR COOLING ONLY|SUPPLY AND RETURN DUCTS FOR COOLING ONLY|SUPPLY AND RETURN DUCTS FOR COOLING ONLY| |0 to 6|R-8|R-6|R-1.9| |7 to 8|R-1.9|R-1.9|R-1.9|

  Notes: 1 Insulation R-values, measured in [°F•h•ft 2 /(Btu•in)] [(m•K)/W], are for the insulation as installed and do not include film resistance. The required minimum thicknesses do not consider water vapor transmission and possible surface condensation. Where portions of the building envelope are used as a plenum enclosure, building envelope insulation shall be as required by the most restrictive condition of Section E 503.4.7.1 or ASHRAE 90.1, depending on whether the plenum is located in the roof, wall, or floor. Insulation resistance measured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 75°F (24°C) at the installed thickness. 2 Includes attics above insulated ceilings, parking garages and crawl spaces. 3 Includes return air plenums, with or without exposed roofs above. 4 Return ducts in this duct location do not require insulation.

  TABLE E 503.7.3(1) MINIMUM PIPE INSULATION THICKNESS FOR HEATING AND HOT WATER SYSTEMS [1, 2, 3, 4, 5]

  (STEAM, STEAM CONDENSATE, HOT WATER HEATING, AND DOMESTIC WATER SYSTEMS)

  [ASHRAE 90.1: TABLE 6.8.3-1]

• CMC § 4.15 Medium relevance — show source text

  91|AHRI 1250|July 10, 2020| |Unit cooler, low temperature,
  net capacity (qnet) ≥ 15 500 Btu/h|UC.L,
  ≥15 500 Btu/h|4.15|AHRI 1250|July 10, 2020|

  For SI units: 1000 British thermal units per hour = 0.293 kW

  • qnet is net capacity (Btu/h) as determined in accordance with AHRI 1250.

  2025 CALIFORNIA MECHANICAL CODE 473

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  APPENDIX E

  Col1	TABLE E 503.7.2 MINIMUM DUCT INSULATION R-VALUE1 [ASHRAE 90.1: TABLE 6.8.2]	Col3	Col4
  CLIMATE ZONE	DUCT LOCATION	DUCT LOCATION	DUCT LOCATION
  CLIMATE ZONE	EXTERIOR2	UNCONDITIONED SPACE AND BURIED DUCTS	INDIRECTLY CONDITIONED SPACE3, 4
  SUPPLY AND RETURN DUCTS FOR HEATING AND COOLING	SUPPLY AND RETURN DUCTS FOR HEATING AND COOLING	SUPPLY AND RETURN DUCTS FOR HEATING AND COOLING	SUPPLY AND RETURN DUCTS FOR HEATING AND COOLING
  0 to 4	R-8	R-6	R-1.9
  5 to 8	R-12	R-6	R-1.9
  SUPPLY AND RETURN DUCTS FOR HEATING ONLY	SUPPLY AND RETURN DUCTS FOR HEATING ONLY	SUPPLY AND RETURN DUCTS FOR HEATING ONLY	SUPPLY AND RETURN DUCTS FOR HEATING ONLY
  0 to 1	none	none	none
  2 to 4	R-6	R-6	R-1.9
  5 to 8	R-12	R-6	R-1.9
  SUPPLY AND RETURN DUCTS FOR COOLING ONLY	SUPPLY AND RETURN DUCTS FOR COOLING ONLY	SUPPLY AND RETURN DUCTS FOR COOLING ONLY	SUPPLY AND RETURN DUCTS FOR COOLING ONLY
  0 to 6	R-8	R-6	R-1.9
  7 to 8	R-1.9	R-1.9	R-1.9

  Notes: 1 Insulation R-values, measured in [°F•h•ft 2 /(Btu•in)] [(m•K)/W], are for the insulation as installed and do not include film resistance. The required minimum thicknesses do not consider water vapor transmission and possible surface condensation. Where portions of the building envelope are used as a plenum enclosure, building envelope insulation shall be as required by the most restrictive condition of Section E 503.4.7.1 or ASHRAE 90.1, depending on whether the plenum is located in the roof, wall, or floor. Insulation resistance measured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 75°F (24°C) at the installed thickness. 2 Includes attics above insulated ceilings, parking garages and crawl spaces. 3 Includes return air plenums, with or without exposed roofs above.

• CMC § 603.13 Medium relevance — show source text

  603.13 Clearances. Duct clearances shall be in accordance with Section 603.13.1 through Section 603.13.6.

  603.13.1 General. Duct clearances to combustible construction shall be in accordance with the listing and the manufacturer’s installation instructions.

  603.13.2 Supply Air Ducts to Listed Furnaces. Supply air ducts connecting to listed central heating furnaces shall have the same minimum clearance to combustibles as required for the furnace supply plenum for a distance of not less than 3 feet (914 mm) from the supply plenum. Clearance shall not be required beyond the 3 feet (914 mm) distance. [NFPA 54:10.3.3.7] 603.13.3 Supply Air Ducts to Unlisted Furnaces. Supply air ducts connecting to unlisted central heating furnaces equipped with temperature limit controls with a maximum setting of 250°F (121°C) shall have a minimum clearance to combustibles of 6 inches (152 mm) for a distance of not less than 6 feet (1829 mm) from the furnace supply plenum. Clearance shall not be required beyond the 6 feet (1829 mm) distance. [NFPA 54:10.3.3.8]

  140 2025 CALIFORNIA MECHANICAL CODE

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  DUCT SYSTEMS

  603.13.4 Furnace Plenums and Air Ducts. Where an air conditioner is installed within an enclosure, the installation shall comply with Section 604.4. [NFPA 54:10.2.6]

  603.13.5 Duct Furnaces. Duct furnaces shall be

  installed in accordance with Section 905.0.

  603.13.6 Fuel-Burning Kilns. A hood and duct serving a fuel-burning kiln shall have a clearance from combustible construction of not less than 18 inches (457 mm). This clearance shall be permitted to be reduced in accordance with Table 303.10.1.

  604.0 Furnace Plenums and Air Ducts Used in Fuel- Gas Appliances. 604.1 Furnace Plenums and Air Ducts. Furnace plenums and air ducts shall be installed in accordance with this Chapter, and NFPA 90A or NFPA 90B. {NFPA 54:10.3.8.1} 604.2 Supplied as a Part of Furnace. A furnace plenum supplied as a part of a furnace shall be installed in accordance with the manufacturer’s instructions. [NFPA 54:10.3.8.2] 604.3 Not Supplied with the Furnace. Where a furnace plenum is not supplied with the furnace, any fabrication and installation instructions provided by the manufacturer shall be followed. The method of connecting supply and return ducts shall facilitate proper circulation of air. [NFPA 54:10.3.8.3]

  604.4 Return Air. Where a furnace is installed so supply ducts carry air circulated by the furnace to areas outside the space containing the furnace, the return air shall also be handled by a duct(s) sealed to the furnace casing and terminating outside the space containing the furnace. Return air shall not be taken from the mechanical room containing the furnace.

  [NFPA 54:10.3.8.4]

• CMC § 8.5 Medium relevance — show source text

  21_–0.27|75|0.75 inch|0.75 inch|1.0 inch|1.0 inch|1.0 inch| |Residential
  40–60|0.21–0.27|75|R-6|R-5|R-7|R-6|R-5| |Nonresidential
  40–60|0.21–0.27|75|0.5 inch|0.5 inch|1.0 inch|1.0 inch|1.0 inch| |Nonresidential
  40–60|0.21–0.27|75|R-3|R-3|R-7|R-6|R-5| |Below 40|0.20–0.26|50|1.0 inch|1.5 inches|1.5 inches|1.5 inches|1.5 inches| |Below 40|0.20–0.26|50|R 8.5|R 14|R 12|R 10|R 9|

  Footnote to Table 120.3-A-1 and Table 120.3-A-2: These thicknesses are based on energy efficiency considerations only. Issues such as water vapor permeability or surface condensation sometimes require vapor retarders or additional insulation.

  SECTION 120.4—REQUIREMENTS FOR AIR DISTRIBUTION SYSTEM DUCTS AND PLENUMS

  Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).

  Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.

  (a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.

  Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:

  1. Outdoors; or

  2. In a space between the roof and an insulated ceiling; or

  3. In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or

  4. In an unconditioned crawlspace; or

  5. In other unconditioned spaces.

  2025 CALIFORNIA ENERGY CODE 77

• CMC § 602.2.4 Medium relevance — show source text

  602.2.4 Discrete Products in Plenums. Discrete plumbing, mechanical, and electrical products that are located in a plenum and have exposed combustible material shall be listed and labeled in accordance with UL

  602.3 Tall Wood (Mass Timber) Buildings. Duct systems installed in tall wood (mass timber) buildings shall comply with the following:

  (1) Be designed by a registered design professional in accordance with this code and the building code.

  (2) Be designed to accommodate expansion, contraction, and differential movement between parts of a tall wood (mass timber) building in accordance with Section 316.2.

  2025 CALIFORNIA MECHANICAL CODE 137

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  DUCT SYSTEMS

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  602.4 Metallic. Ducts, plenums, or fittings of metal shall comply with SMACNA HVAC Duct Construction Standards – Metal and Flexible. Flexible metallic ducts shall comply with UL 181.

  602.5 Nonmetallic Ducts. Nonmetallic ducts shall comply with Section 602.5.1, Section 602.5.2, Section 602.5.3, Section 602.5.4, or Section 602.5.5.

  602.5.1 Phenolic. Phenolic duct, plenum, or fitting material shall comply with UL 181. Ducts, plenums, or fittings of phenolic shall be constructed in accordance with SMACNA Phenolic Duct Construction Standards or the conditions of its listing.

  602.5.2 Gypsum. Where gypsum products are exposed in ducts or plenums, the air temperature shall be restricted to a range from 50°F (10°C) to 125°F (52°C), and moisture content shall be controlled so that the material is not adversely affected. All gypsum products shall have a mold or mildew resistant surface. For the purpose of this section, gypsum products shall not be exposed in supply ducts.

  602.5.3 Air Dispersion Systems. Air dispersion systems shall be listed and labeled in accordance with UL

  602.5.4 Fibrous Glass Duct. Fibrous glass ducts, plenums, or fittings shall be constructed in accordance with SMACNA Fibrous Glass Duct Construction Stan dards or NAIMA Fibrous Glass Duct Construction Stan dards.

  602.5.5 Other Materials. Flexible and rigid ducts, plenums, or fittings for use in heating, ventilation, and air conditioning systems of other nonmetallic materials listed and labeled to UL 181 shall be permitted.

  Exception: Plastic ducts shall comply with Section 603.5.

  602.6 Vibration Isolators. Vibration isolation connectors installed between mechanical equipment and metal ducts (or casings) shall be made of an approved material and shall not exceed 10 inches (254 mm) in length.

  602.7 Corridors. Corridors shall not be used to convey air to or from rooms where the corridor is required to be of fire-resistive construction in accordance with the building code except where permitted by the building code. Corridors shall not serve as supply, return, exhaust, relief, or ventilation air ducts.

  603.0 Installation of Ducts.

• CMC § 0.0929 Medium relevance — show source text

  Exception: In Seismic Design Categories A, B and C, columns not more than 48 inches (1219 mm) in height on a pier or footing are exempt from the bottom end lateral displacement requirement within under-floor areas enclosed by a continuous foundation.

  SECTION R408—UNDER-FLOOR SPACE

  R408.1 Moisture control. The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement) shall comply with Section R408.2 or R408.3.

  R408.2 Openings for under-floor ventilation. Ventilation openings through foundation or exterior walls surrounding the underfloor space shall be provided in accordance with this section. The minimum net area of ventilation openings shall be not less than 1 square foot (0.0929 m [2] ) for each 150 square feet (14 m [2] ) of under-floor area. One ventilation opening shall be within 3 feet (915 mm) of each external corner of the under-floor space. Ventilation openings shall be covered for their height and width with any of the following materials provided that the least dimension of the covering shall not exceed [1] / 4 inch (6.4 mm), and operational louvers are permitted:

  1. Perforated sheet metal plates not less than 0.070 inch (1.8 mm) thick.
  2. Expanded sheet metal plates not less than 0.047 inch (1.2 mm) thick.
  3. Cast-iron grill or grating.
  4. Extruded load-bearing brick vents.
  5. Hardware cloth of 0.035 inch (0.89 mm) wire or heavier.
  6. Corrosion-resistant wire mesh, with the least dimension being [1] / 8 inch (3.2 mm) thick.

  Exceptions:

  1. The total area of ventilation openings shall be permitted to be reduced to [1] / 1,500 of the under-floor area where the ground surface is covered with an approved Class I vapor retarder material.
  2. Where the ground surface is covered with an approved Class 1 vapor retarder material, ventilation openings are not required to be within 3 feet (915 mm) of each external corner of the under-floor space provided that the openings are placed to provide cross ventilation of the space.

  R408.3 Unvented crawl space. For unvented under-floor spaces, the following items shall be provided:

  1. Exposed earth shall be covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall extend not less than 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall or insulation.
  2. One of the following shall be provided for the under-floor space: 2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m [2] ) of crawl space floor area, including an air pathway to the common area (such as a duct or transfer grille). 2.2. Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m [2] ) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille). 2.3. Plenum in existing structures complying with the California Mechanical Code, if under-floor space is used as a plenum.
• CMC § 120.4 Medium relevance — show source text

  Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).

  Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.

  (a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.

  Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:

  1. Outdoors; or

  2. In a space between the roof and an insulated ceiling; or

  3. In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or

  4. In an unconditioned crawlspace; or

  5. In other unconditioned spaces.

  2025 CALIFORNIA ENERGY CODE 77

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

  Portions of supply-air ducts that are not in one of these spaces, including ducts buried in concrete slab, shall be insulated to a minimum installed level of R-4.2 or be enclosed in directly conditioned space.

  (b) Duct and plenum materials.

  1. Factory-fabricated duct systems. A. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. B. All pressure-sensitive tapes, heat-activated tapes, and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. C. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. D. All ductwork and plenums with pressure class ratings shall be constructed to Seal Class A. Joints and seams of duct systems and their components shall not be sealed with cloth-back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands.

  Exception to Section 120.4(b)1D: Ductwork located in occupied space and exposed to view is not required to meet Seal Class A.

• CMC § 160.3 Medium relevance — show source text

  iii. Pipe insulation buried below grade must be installed in a waterproof and noncrushable casing or sleeve.

  D. Insulation thickness.

  i. For insulation with a conductivity in the range shown in Table 160.3-D for the applicable fluid temperature range, the insulation shall have the applicable minimum thickness or R- value shown in Table 160.3-D. ii. For insulation with a conductivity outside the range shown in Table 160.3-D for the applicable fluid temperature range, the insulation shall have a minimum R- value shown in Table 160.3-D or thickness as calculated with Equation 160.3-A:

  (Equation 160.3-A)

  T = PR  1 + ------ PRt 

  K

  [-] k [-] = PR  1 + ------ t  – 1

  where:

  T = Minimum insulation thickness for material with conductivity K, inches. PR = Pipe actual outside radius, inches.

  218 2025 CALIFORNIA ENERGY CODE

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  MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS

  t = Insulation thickness from 160.3-D, inches.

  K = Conductivity of alternate material at the mean rating temperature indicated in Table 160.3-D for the applicable fluid temperature range, in Btu-inch per hour per square foot per °F. k = The lower value of the conductivity range listed in Table 160.3-D for the applicable fluid temperature range, Btu-inch per hour per square foot per °F. Exception 1 to Section 160.3(c)1: Factory-installed piping within space-conditioning equipment certified under Section 110.1 or 110.2.

  Exception 2 to Section 160.3(c)1: Piping that conveys fluids with a design operating temperature range between 60°F and 105°F.

  Exception 3 to Section 160.3(c)1: Where the heat gain or heat loss to or from piping without insulation will not increase building source energy use. Exception 4 to Section 160.3(c)1: Piping that penetrates framing members shall not be required to have pipe insulation for the distance of the framing penetration. Metal piping that penetrates metal framing shall use grommets, plugs, wrapping or other insulating material to ensure that no contact is made with the metal framing. 2. Requirements for air distribution system, ducts and plenum. Multifamily common areas shall comply with the applicable requirements of Sections 160.3(c)2A through 160.3(c)2F. A. CMC compliance. All air distribution system ducts and plenums, including, but not limited to, building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums shall meet the requirements of CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B.

• CMC § 0.50 Medium relevance — show source text

  (8) Materials in a hazardous fabrication area including the areas above and below the fabrication area sharing a common air recirculation path with the fabrication area.

  (9) Plastic water distribution piping listed and labeled for use in plenums in accordance with UL 2846 as having a peak optical density not greater than 0.50, an average optical density not greater than 0.15, and a flame spread distance not greater than 5 feet (1524 mm), and installed in accordance with its listing, shall be permitted.

  602.2.1 Electrical. Electrical wiring in plenums shall comply with the California Electrical Code . Electrical wires and cables and optical fiber cables shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15, and a peak optical density not exceeding 0.5, where tested in accordance with NFPA 262.

  602.2.2 Fire Sprinkler Piping. Nonmetallic fire sprinkler piping in plenums shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15 and, a peak optical density not exceeding 0.5, where tested in accordance with UL 1887.

  602.2.3 Pneumatic Tubing. Nonmetallic pneumatic tubing in plenums shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15, and a peak optical density not exceeding 0.5, where tested in accordance with UL 1820.

  602.2.4 Discrete Products in Plenums. Discrete plumbing, mechanical, and electrical products that are located in a plenum and have exposed combustible material shall be listed and labeled in accordance with UL

  602.3 Tall Wood (Mass Timber) Buildings. Duct systems installed in tall wood (mass timber) buildings shall comply with the following:

  (1) Be designed by a registered design professional in accordance with this code and the building code.

  (2) Be designed to accommodate expansion, contraction, and differential movement between parts of a tall wood (mass timber) building in accordance with Section 316.2.

  2025 CALIFORNIA MECHANICAL CODE 137

  ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

  DUCT SYSTEMS

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  602.4 Metallic. Ducts, plenums, or fittings of metal shall comply with SMACNA HVAC Duct Construction Standards – Metal and Flexible. Flexible metallic ducts shall comply with UL 181.

  602.5 Nonmetallic Ducts. Nonmetallic ducts shall comply with Section 602.5.1, Section 602.5.2, Section 602.5.3, Section 602.5.4, or Section 602.5.5.

  602.5.1 Phenolic. Phenolic duct, plenum, or fitting material shall comply with UL 181. Ducts, plenums, or fittings of phenolic shall be constructed in accordance with SMACNA Phenolic Duct Construction Standards or the conditions of its listing.

• CMC § 907.2.13.1 Medium relevance — show source text

  [F] 907.2.13.1 Automatic smoke detection. Automatic smoke detection in high-rise buildings and Group I-2 occupancies having occupied floors located more than 75 feet above the lowest level of fire department vehicle access shall be in accordance with Sections 907.2.13.1.1 and 907.2.13.1.2.

  [F] 907.2.13.1.1 Area smoke detection. Area smoke detectors shall be provided in accordance with this section. Smoke detectors shall be connected to an automatic fire alarm system. The activation of any detector required by this section shall activate the emergency voice/alarm communication system in accordance with Section 907.5.2.2. In addition to smoke detectors required by Sections 907.2.1 through 907.2.9, smoke detectors shall be located as follows:

  1. In each mechanical equipment, electrical, transformer, telephone equipment or similar room that is not provided with sprinkler protection.
  2. In each elevator machine room, machinery space, control room and control space and in elevator lobbies.

  [F] 907.2.13.1.2 Duct smoke detection. Smoke detectors listed for use in air duct systems shall be provided in accordance with this section and the California Mechanical Code. The activation of any detector required by this section shall initiate a visible and

  2025 CALIFORNIA BUILDING CODE 9-33

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  FIRE PROTECTION AND LIFE SAFETY SYSTEMS

  audible supervisory signal at a constantly attended location. Duct smoke detectors complying with Section 907.3.1 shall be located as follows:

  1. In the main return air and exhaust air plenum of each air-conditioning system having a capacity greater than 2,000 cubic feet per minute (cfm) (0.94 m [3] /s). Such detectors shall be located in a serviceable area downstream of the last duct inlet.

  2. At each connection to a vertical duct or riser serving two or more stories from a return air duct or plenum of an airconditioning system. In Group R-1 and R-2 occupancies, a smoke detector is allowed to be used in each return air riser carrying not more than 5,000 cfm (2.4 m [3] /s) and serving not more than 10 air-inlet openings.

  [F] 907.2.13.2 Fire department communication system. Where a wired communication system is approved in lieu of an inbuilding two-way emergency responder communication coverage system in accordance with Section 510 of the California Fire Code, the wired fire department communication system shall be designed and installed in accordance with NFPA 72 and shall operate between a fire command center complying with Section 911, elevators, elevator lobbies, emergency and standby power rooms, fire pump rooms, areas of refuge and inside interior exit stairways. The fire department communication device shall be provided at each floor level within the interior exit stairway.

  [F] 907.2.13.3 Multiple-channel voice evacuation. In buildings with an occupied floor more than 120 feet (36 576 mm) above the lowest level of fire department vehicle access, voice evacuation systems for high-rise buildings shall be multiple-channel systems.

• CMC § 160.3 Medium relevance — show source text

  Exception 2 to Section 160.3(c)1: Piping that conveys fluids with a design operating temperature range between 60°F and 105°F.

  Exception 3 to Section 160.3(c)1: Where the heat gain or heat loss to or from piping without insulation will not increase building source energy use. Exception 4 to Section 160.3(c)1: Piping that penetrates framing members shall not be required to have pipe insulation for the distance of the framing penetration. Metal piping that penetrates metal framing shall use grommets, plugs, wrapping or other insulating material to ensure that no contact is made with the metal framing. 2. Requirements for air distribution system, ducts and plenum. Multifamily common areas shall comply with the applicable requirements of Sections 160.3(c)2A through 160.3(c)2F. A. CMC compliance. All air distribution system ducts and plenums, including, but not limited to, building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums shall meet the requirements of CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used. B. Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:

  i. Outdoors; or ii. In a space between the roof and an insulated ceiling; or iii. In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or iv. In an unconditioned crawl space; or v. In other unconditioned spaces. Portions of supply-air ducts that are not in one of these spaces, including ducts buried in concrete slab, shall be insulated to a minimum installed level of R-4.2 or be enclosed in directly conditioned space. C. Duct and plenum materials. i. Factory-fabricated duct systems. a. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. b. All pressure-sensitive tapes, heat-activated tapes and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. c. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. d. Ductwork and plenums with pressure class ratings shall be constructed to Seal Class A. Joints and seams of duct systems and their components shall not be sealed with cloth back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands.