CRC · California Residential Code

What are the CRC ventilation requirements for enclosed attics and rafter spaces?

Homes must provide cross ventilation for enclosed attics and rafter spaces sized by net free area: normally 1 ft² of vent per 150 ft² of attic (1/150), with a limited 1/300 reduction allowed only in Climate Zones 6–8 when a vapor retarder is used and 40–50% of vents are located near the ridge. Openings must be screened to specific mesh limits, and insulation must leave at least a 1‑inch airspace at eaves. See **§ R806.1** and **§ R806.2** for the controlling CRC requirements .

Ask a question free See coverage

Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

Enclosed attics and enclosed rafter spaces must have continuous cross ventilation to the outdoors using ventilating openings protected against rain, snow and pests. The minimum net free ventilating area is 1/150 of the vented space (reducible to 1/300 when the two-part exception is met). These requirements are set out in § R806.1 and § R806.2 of the California Residential Code (CRC) .

The single most important rule: provide cross‑venting to the outside sized at a minimum of 1 square foot of net free area for every 150 square feet of attic/rafter space (or 1/300 when the exception applies).

Requirements in detail

Which spaces are covered

  • Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters are covered by the rule. See § R806.1 .

Vent opening size and protection

  • Ventilation openings must have a least dimension of not less than 1/16 inch and not more than 1/4 inch. Openings larger than 1/4 inch must be fitted with corrosion‑resistant screening or equivalent with openings between 1/16 inch and 1/4 inch .
  • Required ventilators must open directly to outdoors and be protected against birds, rodents, snakes and similar creatures per § R806.1 .

Minimum net free ventilating area (NFA)

  • Standard requirement: NFA ≥ 1/150 of the area of the vented space (that is, attic area ÷ 150) § R806.2 .
  • Exception (reduced NFA to 1/300) allowed only when BOTH conditions are met:
    1. In IECC Climate Zones 6, 7 or 8, a Class I or II vapor retarder is installed on the warm‑in‑winter side of the ceiling.
    2. 40%–50% of required ventilation area is provided by upper ventilators located not more than 3 feet vertically below the ridge (the remainder in the bottom one‑third of the attic). See § R806.2 (Exception) .

Airflow clearance at eaves / insulation clearance

  • Where eave or cornice vents are installed, blocking/bridging and insulation must not impede airflow; provide at least a 1‑inch airspace between insulation and roof sheathing at vent locations per § R806.3 .

Installation and weather protection

  • Ventilators must be installed per manufacturer instructions and applicable CRC sections (roof vents per § R903, wall vents per § R703.1) — see § R806.4 for installation direction .

Table — decision‑relevant dimensions / values

Requirement / Dimension Value(s) (bolded key value) Where to apply / notes Code Reference
Standard minimum NFA 1/150 of vented area All enclosed attics & enclosed rafter spaces § R806.2
Reduced NFA (exception) 1/300 of vented area Only if both exception conditions are met § R806.2 (Exception)
Upper vent percentage (for exception) 40%–50% of required NFA Upper vents ≤ 3 ft below ridge; remainder in bottom 1/3 § R806.2 (Exception)
Vent opening least dimension 1/16 in. minimum Opening meshes/louvers must be at least this small § R806.1
Vent opening maximum free dimension 1/4 in. maximum Larger openings require screening meeting 1/16–1/4 in. § R806.1
Insulation‑to‑sheathing clearance 1 in. min. airspace At eave/cornice vents to allow free airflow § R806.3

Exceptions & special cases

  • Reduced ventilation area to 1/300 is allowed only in Climate Zones 6, 7 or 8 and only if a Class I or II vapor retarder is installed and the upper/lower vent split (40–50% upper, remainder bottom 1/3) is met. See § R806.2 .
  • Where the location of framing conflicts with locating upper vents exactly 3 ft below the ridge, install upper vents deeper than 3 ft is permitted (the code allows some flexibility where framing interferes) — see § R806.2 (Exception) .
  • Vent openings in the Wildland‑Urban Interface (WUI) must comply with the California WUI Code (Part 7); WUI vents have additional ember/flame resistance and mesh rules — see § R806.1.1 and the WUI Code guidance .
  • Unvented attic assemblies are allowed under a separate set of criteria (insulation location, vapor retarder prohibition, and other conditions) per § R806.5 — do not assume unvented unless those conditions are met .

Common mistakes

  • Counting gross vent area instead of net free area (NFA). Louvers, screens and grilles reduce free area — always use the NFA rating or calculate the free area for the product before sizing vents (see NFA requirements in § R806.2) .
  • Using vents with mesh/openings larger than 1/4 inch without installing compliant screening (must be 1/16–1/4 in.) — violates § R806.1 .
  • Blocking soffit/eave ventilation with insulation or mis‑placed baffles — maintain the 1‑inch airspace and keep airflow paths clear per § R806.3 .
  • Applying the 1/300 reduction outside Climate Zones 6–8, or without the vapor retarder / upper vent split — the exception is strict and limited to the conditions in § R806.2 .
  • Forgetting WUI requirements in high‑risk areas — WUI vent performance/mesh rules can be more restrictive than standard CRC vent rules (see § R806.1.1 and Part 7, WUI Code) .

Worked example — concrete scenario

Scenario: a simple gable house with a single enclosed attic measuring 1,200 sq ft (plan area).

  1. Standard NFA (1/150):

    • Required NFA = 1,200 ÷ 150 = 8.0 sq ft of net free ventilating area. (Convert to square inches: 8.0 ft² × 144 in²/ft² = 1,152 in².)
    • Provide vents (soffit + ridge or gable + roof vents) whose combined rated NFA ≥ 8.0 ft², ensuring product NFA values (not gross area) are used — § R806.2 .

  2. If the designer wants to use the reduced NFA (1/300):

    • Confirm the project is in Climate Zone 6, 7 or 8 and that a Class I or II vapor retarder will be installed on the warm‑in‑winter side of the ceiling — § R806.2 (Exception) .
    • Reduced required NFA = 1,200 ÷ 300 = 4.0 sq ft (576 in²).
    • Upper ventilation requirement: 40%–50% of the 4.0 ft² must be upper vents located ≤ 3 ft below ridge. So upper NFA = 0.4–0.5 × 4.0 = 1.6 to 2.0 ft². The remaining 2.4–2.0 ft² must be low/bottom vents (e.g., soffit vents) in the bottom third of the attic vertical profile — § R806.2 (Exception) .

  3. Practical note: choose vent products with rated NFA data. For example, if a soffit vent product has rated NFA 12 in² each, you would need 1,152 ÷ 12 = 96 such vents to meet the full 8.0 ft² requirement (or fewer higher‑NFA products). Always sum NFA ratings, not gross opening sizes — § R806.2 .

Related provisions (CRC sections)

  • § R806.2 — Minimum vent area (NFA ratios and exception)
  • § R806.3 — Vent and insulation clearance (1‑inch airspace at eaves)
  • § R806.4 — Installation and weather protection; follow manufacturer instructions and other CRC sections (roof/wall vent installation)
  • § R806.5 — Requirements allowing unvented attic and unvented enclosed rafter assemblies (separate criteria)
  • § R806.1.1 — Vents in the Wildland‑Urban Interface; compliance with WUI Code requirements (Part 7) for ember/flame resistance
  • § R802.7 — (Referenced for openings in roof framing members) — see the roof framing/opening requirements referenced in § R806.1

Code references

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

  • CRC § 3.8 High relevance — show source text

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

    ROOF-CEILING CONSTRUCTION

    R804.3.8 Roof tie-down. Roof assemblies shall be connected to walls below in accordance with Table R804.3. A continuous load path shall be provided to transfer uplift loads to the foundation.

    SECTION R805—CEILING FINISHES

    R805.1 Ceiling installation. Ceilings shall be installed in accordance with the requirements for interior wall finishes as provided in Sections R702.1 through R702.6.

    SECTION R806—ROOF VENTILATION

    Note: The IECC climate zones used by this section differ from those used by the California Energy Code to determine applicability of energy efficiency measures. Comparison of IECC and California Energy Code climate zones is shown in Table R702.7.3.

    R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of [1] / 16 inch (1.6 mm) minimum and [1] / 4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than [1] / 4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar material with openings having a least dimension of [1] / 16 inch (1.6 mm) minimum and [1] / 4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air and shall be protected to prevent the entry of birds, rodents, snakes and other similar creatures.

    R806.1.1 Vents in the Wildland Urban Interface (WUI). Where provided, ventilation openings for enclosed attics, gable ends, ridge ends, under eaves and cornices, enclosed eave soffit spaces, enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters, underfloor ventilation, foundations and crawl spaces, or any other opening intended to permit ventilation, either in a horizontal or vertical plane, shall be in accordance with Part 7, California Wildland-Urban Interface Code to resist building ignition from the intrusion of burning embers and flame through the ventilation openings. R806.2 Minimum vent area. The minimum net free ventilating area shall be [1] / 150 of the area of the vented space. Exception: The minimum net free ventilation area shall be [1] / 300 of the vented space provided both of the following conditions are met:

    1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
    2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space.
  • CRC § 3.7.1 High relevance — show source text

    FIGURE R804.3.7.1(2)—CEILING DIAPHRAGM TO SIDEWALL DETAIL

    ROOF-CEILING CONSTRUCTION

    ROOF RAFTER (OR TRUSS TOP CHORD)

    CEILING JOIST (OR TRUSS BOTTOM CHORD)

    WOOD STRUCTURAL

    PANEL OR GYPSUM

    BOARD DIAPHRAGM

    350T125-33 TRACK

    ROOF SHEATHING

    ROOF BLOCKING

    NO. 8 SCREWS AT 6 IN. O.C.

    STRUCTURAL WALL

    For SI: 1 inch = 25.4 mm.

    R804.3.7.2 Roof diaphragm. A roof diaphragm shall be provided by attaching not less than [3] / 8 -inch (9.5 mm) wood structural panel that complies with Section R803 to roof rafters or truss top chords in accordance with Table R804.3. Buildings with 3:1 or larger plan aspect ratio and with roof rafter slope (pitch) of 9:12 or larger shall have the roof rafters and ceiling joists blocked in accordance with Figure R804.3.7.2.

    FIGURE R804.3.7.2—ROOF BLOCKING DETAIL

    BE ATTACHED TO OUTSIDE FACE OF SHEATHING OR PREFERABLY DIRECTLY TO TRACK FLANGE PRIOR TO SHEATHING PLACEMENT AS SHOWN

    For SI: 1 mil = 0.0254 mm, 1 inch = 25.4 mm, 1 foot = 304.8 mm.

    2025 CALIFORNIA RESIDENTIAL CODE 8-43

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

    ROOF-CEILING CONSTRUCTION

    R804.3.8 Roof tie-down. Roof assemblies shall be connected to walls below in accordance with Table R804.3. A continuous load path shall be provided to transfer uplift loads to the foundation.

    SECTION R805—CEILING FINISHES

    R805.1 Ceiling installation. Ceilings shall be installed in accordance with the requirements for interior wall finishes as provided in Sections R702.1 through R702.6.

    SECTION R806—ROOF VENTILATION

    Note: The IECC climate zones used by this section differ from those used by the California Energy Code to determine applicability of energy efficiency measures. Comparison of IECC and California Energy Code climate zones is shown in Table R702.7.3.

    R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of [1] / 16 inch (1.6 mm) minimum and [1] / 4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than [1] / 4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar material with openings having a least dimension of [1] / 16 inch (1.6 mm) minimum and [1] / 4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7.

  • CRC § 1202.1 High relevance — show source text

    SECTION 1202—VENTILATION

    1202.1 General. Buildings shall be provided with natural ventilation in accordance with Section 1202.5, or mechanical ventilation in accordance with the California Mechanical Code .

    Dwelling units complying with the air leakage requirements of the California Energy Code or ASHRAE 90.1 shall be ventilated by mechanical means in accordance with Section 403 of the California Mechanical Code . Ambulatory care facilities and Group I-2 occupancies shall be ventilated by mechanical means in accordance with Section 407 of the California Mechanical Code .

    1202.2 Roof ventilation. Roof assemblies shall be ventilated in accordance with this section or shall comply with Section 1202.3.

    1202.2.1 Ventilated attics and rafter spaces. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing members shall have cross ventilation for each separate space by ventilation openings protected against the entrance of rain and snow. Blocking and bridging shall be arranged so as not to interfere with the movement of air. An airspace of not less than 1 inch (25 mm) shall be provided between the insulation and the roof sheathing. The net free ventilating area shall be not less than [1] / 150 of the area of the space ventilated. Ventilators shall be installed in accordance with manufacturer’s installation instructions.

    Exception: The net free cross-ventilation area shall be permitted to be reduced to [1] / 300 provided both of the following conditions are met:

    1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
    2. At least 40 percent and not more than 50 percent of the required venting area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically, with the balance of the ventilation provided by eave or cornice vents. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

    1202.2.2 Openings into attic. Exterior openings into the attic space of any building intended for human occupancy shall be protected to prevent the entry of birds, squirrels, rodents, snakes and other similar creatures. Openings for ventilation having a least dimension of not less than [1] / 16 inch (1.6 mm) and not more than [1] / 4 inch (6.4 mm) shall be permitted. Openings for ventilation having a least dimension larger than [1] / 4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar material with openings having a least dimension of not less than [1] / 16 inch (1.6 mm) and not more than [1] / 4 inch (6.4 mm). Where combustion air is obtained from an attic area, it shall be in accordance with Chapter 7 of the California Mechanical Code .

    1202.3 Unvented attic and unvented enclosed rafter assemblies. Unvented attics and unvented enclosed roof framing assemblies created by ceilings applied directly to the underside of the roof framing members/rafters and the structural roof sheathing at the top of the roof framing members shall be permitted where all of the following conditions are met: 1.

  • CRC § 1.1 High relevance — show source text

    Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air and shall be protected to prevent the entry of birds, rodents, snakes and other similar creatures.

    R806.1.1 Vents in the Wildland Urban Interface (WUI). Where provided, ventilation openings for enclosed attics, gable ends, ridge ends, under eaves and cornices, enclosed eave soffit spaces, enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters, underfloor ventilation, foundations and crawl spaces, or any other opening intended to permit ventilation, either in a horizontal or vertical plane, shall be in accordance with Part 7, California Wildland-Urban Interface Code to resist building ignition from the intrusion of burning embers and flame through the ventilation openings. R806.2 Minimum vent area. The minimum net free ventilating area shall be [1] / 150 of the area of the vented space. Exception: The minimum net free ventilation area shall be [1] / 300 of the vented space provided both of the following conditions are met:

    1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
    2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

    R806.3 Vent and insulation clearance. Where eave or cornice vents are installed, blocking, bridging and insulation shall not block the free flow of air. Not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.

    R806.4 Installation and weather protection. Ventilators shall be installed in accordance with manufacturer’s instructions. Installation of ventilators in roof systems shall be in accordance with the requirements of Section R903. Installation of ventilators in wall systems shall be in accordance with the requirements of Section R703.1.

    R806.5 Unvented attic and unvented enclosed rafter assemblies. Unvented attics and unvented enclosed roof framing assemblies created by ceilings that are applied directly to the underside of the roof framing members and structural roof sheathing applied directly to the top of the roof framing members/rafters, shall be permitted where all the following conditions are met:

    1. The unvented attic space is completely within the building thermal envelope.
    2. Interior Class I vapor retarders are not installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof framing assembly.
    3. Where wood shingles or shakes are used, a minimum [1] / 4 -inch (6.4 mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.
  • CRC § 1.11. High relevance — show source text

    The state agency does not adopt sections identified by the following symbol:

    The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.

    12-2 2025 CALIFORNIA BUILDING CODE

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

    12 INTERIOR ENVIRONMENT

    User notes:

    About this chapter: Chapter 12 provides minimum provisions for the interior of buildings—the occupied environment. Ventilation, lighting, and space heating are directly regulated in this chapter and in conjunction with the California Mechanical Code and the California Energy Code . Minimum room size, maximum room-to-room sound transmission and classroom acoustics are set for educational occupancies.

    ICC code development note: Code change proposals to sections preceded by the designation [P] will be considered by a code development committee meting during the 2024 (Group A) Code Development Cycle. All other code change proposals will be considered by a code development committee meeting during the 2025 (Group B) Code Development Cycle.

    SECTION 1201—GENERAL

    1201.1 Scope. The provisions of this chapter shall govern ventilation, temperature control, lighting, yards and courts, sound transmission, enhanced classroom acoustics, interior space dimensions, access to unoccupied spaces, toilet and bathroom requirements and ultraviolet (UV) germicidal irradiation systems associated with the interior spaces of buildings.

    SECTION 1202—VENTILATION

    1202.1 General. Buildings shall be provided with natural ventilation in accordance with Section 1202.5, or mechanical ventilation in accordance with the California Mechanical Code .

    Dwelling units complying with the air leakage requirements of the California Energy Code or ASHRAE 90.1 shall be ventilated by mechanical means in accordance with Section 403 of the California Mechanical Code . Ambulatory care facilities and Group I-2 occupancies shall be ventilated by mechanical means in accordance with Section 407 of the California Mechanical Code .

    1202.2 Roof ventilation. Roof assemblies shall be ventilated in accordance with this section or shall comply with Section 1202.3.

    1202.2.1 Ventilated attics and rafter spaces. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing members shall have cross ventilation for each separate space by ventilation openings protected against the entrance of rain and snow. Blocking and bridging shall be arranged so as not to interfere with the movement of air. An airspace of not less than 1 inch (25 mm) shall be provided between the insulation and the roof sheathing. The net free ventilating area shall be not less than [1] / 150 of the area of the space ventilated. Ventilators shall be installed in accordance with manufacturer’s installation instructions.

    Exception: The net free cross-ventilation area shall be permitted to be reduced to [1] / 300 provided both of the following conditions are met:

  • CRC § 504.9.3 High relevance — show source text

    Gaps between doors and door openings_ shall be controlled by one or more of the following methods: 1. Weather-stripping products constructed of materials that which comply with both of the following: 1.1. The tensile strength of the material shall be tested in accordance with ASTM D638 before and after exposure to ASTM G155 for a period of 2,000 hours, and the maximum allowable difference in tensile strength values between exposed and nonexposed samples shall not exceed 10 percent. 1.2. When tested to UL 94, the materials shall have a flammability rating of V-2 or better. 2. Door overlaps onto jambs and headers. 3. Garage door jambs and headers covered with metal flashing.

    504.9.3 Conditions of acceptance for ASTM E2707. The ASTM E2707 test shall be conducted on a minimum of three test specimens and meet the conditions of acceptance in Items 1 and 2. If any one of the three tests do not meet the conditions of acceptance, three additional tests shall be performed. All three additional tests must meet the conditions of acceptance. 1. Absence of flame penetration through the wall assembly at any time. 2. Absence of evidence of glowing combustion on the interior surface of the assembly at the end of the 70-minute test.

    504.10 Vents. Where provided, ventilation openings for enclosed attics, gable ends, ridge ends, under eaves and cornices, enclosed eave soffit spaces, enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters, underfloor venti

    5-8 2025 CALIFORNIA WILDLAND-URBAN INTERFACE CODE

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

    SPECIAL BUILDING CONSTRUCTION REGULATIONS

    lation, foundations and crawl spaces, or any other opening intended to permit ventilation, either in a horizontal or vertical surface, shall be in accordance with Section 504.10.1 or Section 504.10.2 to resist building ignition from the intrusion of burning embers and flame through the ventilation openings.

    504.10.1 Performance requirements. Ventilation openings shall be fully covered with Wildfire Flame and Ember Resistant vents approved and listed by the California State Fire Marshal, or WUI vents, tested in accordance with ASTM E2886, to demonstrate compliance with all the following requirements:

    1. There shall be no flaming ignition of the cotton material during the Ember Intrusion Test.
    2. There shall be no flaming ignition during the Integrity Test portion of the Flame Intrusion Test.
    3. The maximum temperature of the unexposed side of the vent shall not exceed 662°F (350°C).

    504.10.2 Off ridge and ridge vents. Vents that are installed on a sloped roof, such as dormer vents, shall comply with all of the following: 1. Vents shall be covered with a mesh where the dimensions of the mesh therein shall be a minimum of [1] / 16 inch (1.6 mm) and shall not exceed [1] / 8 inch (3.2 mm) in diameter.

    2. The vent and mesh material shall be noncombustible.

    3. The vent and mesh material shall be corrosion resistant.

  • CRC § 0.025. High relevance — show source text

    i. Roof and ceiling insulation in a ventilated attic shall meet one of the following requirements: a. In Climate Zones 1, 2, 4, and 8 through 16, achieve an overall assembly U -factor not exceeding 0.025. In wood framed assemblies, compliance with U -factors may be demonstrated by installing insulation with an R -value of R-38 or greater.

    2025 CALIFORNIA ENERGY CODE 271

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

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

    b. In Climate Zones 3 and 5 through 7, achieve an overall assembly U- factor not exceeding 0.031. In wood framed assemblies, compliance with U- factors may be demonstrated by installing insulation with an R- value of R-30 or greater. ii. Radiant barrier. For buildings three habitable stories or less, radiant barriers shall be installed in attics with exposed attic deck undersides in Climate Zones 2–15. iii. Extensions of existing wood-framed walls may retain the dimensions of the existing walls and shall install cavity insulation of R-15 in a 2x4 framing and R-21 in a 2x6 framing; and iv. Fenestration products must meet the U- factor, RSGHC and VT requirements of Table 180.2-B. v. Quality insulation installation (QII) requirements of Section 170.2(a)6 do not apply. Exception to Section 180.1(a)1B: Insulation in an enclosed rafter ceiling shall meet the requirements of Section 160.1(a). Exception to Section 180.1(a)1: Additions that increase the area of the roof by 2,000 square feet or less are not required to comply with the solar ready requirements of Section 160.8. 2. Mechanical ventilation for indoor air quality. Additions to existing buildings shall comply with Section 160.2 subject to the requirements specified in Subsections A and B below. When field verification and diagnostic testing are required by Section 180.1(a)2, buildings with three habitable stories or fewer shall use the applicable procedures in the Residential Appendices, and buildings with four or more habitable stories shall use the applicable procedures in Nonresidential Appendices NA1 and NA2.

    Exception to Section 180.1(a)2: A dwelling unit air leakage test is not required for additions. A. Whole-dwelling unit mechanical ventilation . i. Dwelling units that meet the conditions in Subsection a or b below shall not be required to comply with the whole-dwelling unit ventilation airflow specified in Section 160.2(b)2Aiv or 160.2(b)2Av. a. Additions to an existing dwelling unit that increase the conditioned floor area of the existing dwelling unit by less than or equal to 1,000 square feet. b. Junior accessory dwelling units (JADU) that are additions to an existing building. ii. Additions to an existing dwelling unit that increase conditioned floor area by more than 1,000 square feet shall have mechanical ventilation airflow in accordance with Section 160.2(b)2Aiv or 160.2(b)2Av, as applicable. The mechanical ventilation airflow rate shall be based on the conditioned floor area of the entire dwelling unit comprising the existing dwelling unit conditioned floor area plus the addition conditioned floor area.

  • CRC § 180.1 Medium relevance — show source text

    Exception 7 to Section 180.1 : Dwelling unit space heating system. New or replacement space heating systems serving an addition may be a heat pump or gas heating system.

    (a) Prescriptive approach. The envelope and lighting of the addition; any newly installed space-conditioning or ventilation system, electrical power distribution system, or water-heating system; any addition to an outdoor lighting system; and any new sign installed in conjunction with an indoor or outdoor addition shall meet the applicable requirements of Sections 110.0 through 110.12; 160.0, 160.1, and 160.2(c) and (d); and 160.3 through 170.2.

    1. Envelope. A. Additions that are greater than 700 square feet shall meet the requirements of Section 170.2(a), with the following modifications:

    i. Framed walls extension. Extensions of existing wood-framed walls may retain the dimensions of the existing walls and shall install cavity insulation of R-15 in a 2x4 framing and R-21 in a 2x6 framing. ii. The maximum allowed fenestration area shall be the greater of 175 square feet or 20 percent of the addition floor area.

    iii. When existing siding of a wood-framed wall is not being removed or replaced, cavity insulation of R-15 in a 2x4 framing and R-21 in a 2x6 framing shall be installed and continuous insulation is not required. iv. Additions that consist of the conversion of existing spaces from unconditioned to conditioned space shall not be required to perform the air sealing part of QII when the existing air barrier is not being removed or replaced. B. Additions that are 700 square feet or less shall meet the requirements of Section 170.2(a), with the following modifications.

    i. Roof and ceiling insulation in a ventilated attic shall meet one of the following requirements: a. In Climate Zones 1, 2, 4, and 8 through 16, achieve an overall assembly U -factor not exceeding 0.025. In wood framed assemblies, compliance with U -factors may be demonstrated by installing insulation with an R -value of R-38 or greater.

    2025 CALIFORNIA ENERGY CODE 271

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

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

    b. In Climate Zones 3 and 5 through 7, achieve an overall assembly U- factor not exceeding 0.031. In wood framed assemblies, compliance with U- factors may be demonstrated by installing insulation with an R- value of R-30 or greater. ii. Radiant barrier. For buildings three habitable stories or less, radiant barriers shall be installed in attics with exposed attic deck undersides in Climate Zones 2–15. iii. Extensions of existing wood-framed walls may retain the dimensions of the existing walls and shall install cavity insulation of R-15 in a 2x4 framing and R-21 in a 2x6 framing; and iv. Fenestration products must meet the U- factor, RSGHC and VT requirements of Table 180.2-B. v. Quality insulation installation (QII) requirements of Section 170.2(a)6 do not apply. Exception to Section 180.1(a)1B: Insulation in an enclosed rafter ceiling shall meet the requirements of Section 160.1(a).

  • CRC § 2.5 Medium relevance — show source text

    iv. Attic ventilation shall comply with the California Residential Code, Title 24, Part 2.5, Section R806. Exception 1 to Section 150.2(b)1J: Dwelling units with at least R-38 existing insulation installed at the ceiling level. Exception 2 to Section 150.2(b)1J: Dwelling units where the alteration would directly cause the disturbance of asbestos.

    Exception 3 to Section 150.2(b)1J: Dwelling units with knob and tube wiring located in the vented attic.

    194 2025 CALIFORNIA ENERGY CODE

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

    SINGLE-FAMILY RESIDENTIAL BUILDINGS—ADDITIONS AND ALTERATIONS TO EXISTING RESIDENTIAL BUILDINGS

    Exception 4 to Section 150.2(b)1J: Where the accessible space in the attic is not large enough to accommodate the required R -value, the entire accessible space shall be filled with insulation provided such installation does not violate Section 806.3 of Title 24, Part 2.5. Exception 5 to Section 150.2(b)1J: Where the attic space above the altered dwelling unit is shared with other dwelling units and the requirements of Section 150.2(b)1J are not triggered for the other dwelling units. K. Lighting. The altered lighting system shall meet the lighting requirements of Section 150.0(k). The altered luminaires shall meet the luminaire efficacy requirements of Section 150.0(k). Where existing screw-base sockets are present in ceiling-recessed luminaires, removal of these sockets is not required provided that new JA8-compliant trim kits or lamps designed for use with recessed downlights or luminaires are installed. L. Mechanical ventilation for indoor air quality—entirely new or complete replacement ventilation systems. Entirely new or complete replacement ventilation systems shall comply with all applicable requirements in Section 150.0(o). An entirely new or complete replacement ventilation system includes a new ventilation fan component and an entirely new duct system. An entirely new or complete replacement duct system 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 filtration devices and duct material, if the reused parts are accessible and can be sealed to prevent leakage. M. Mechanical ventilation for indoor air quality—altered ventilation systems. Altered ventilation system components or newly installed ventilation equipment serving the alteration shall comply with Section 150.0(o) as applicable subject to the requirements specified in Subsections i and ii below. i. Whole-dwelling unit mechanical ventilation. a. Whole-dwelling unit airflow. If the whole-dwelling ventilation fan is altered or replaced, then one of the following Subsections 1 or 2 shall be used for compliance as applicable.

    1. Dwellings that were required by a previous building permit to comply with the whole-dwelling unit airflow requirements in Section 150.0(o) shall meet or exceed the whole-dwelling unit mechanical ventilation airflow specified in Section 150.0(o)1C, 150.0(o)1E or 150.0(o)1F as confirmed through field verification and diagnostic testing in accordance with the applicable procedures specified in Reference Residential Appendix RA3.7.
  • CRC § 150.2 Medium relevance — show source text

    Exception 5 to Section 150.2(b)1Biib: Tapered insulation with thermal resistance less than prescribed at the drains and other low points may be used provided that the thickness of insulation is increased at the high points of the roof so that the average thermal resistance equals or exceeds the required value. J. Ceiling. Vented attics shall meet the following: i. In Climate Zones 1 through 4, 6, and 8 through 16 insulation shall be installed to achieve a weighted U -factor of 0.020 or insulation installed at the ceiling level shall result in an insulated thermal resistance of R-49 or greater for the insulation alone; and Exception to Section 150.2(b)1Ji: In Climate Zones 1, 3 and 6, dwelling units with at least R-19 existing insulation installed at the ceiling level. ii. In Climate Zones 2, 4, and 8 through 16, air seal all accessible areas of the ceiling plane between the attic and the conditioned space in accordance with Section 110.7; and Exception 1 to Section 150.2(b)1Jii: Dwelling units with at least R-19 existing insulation installed at the ceiling level. Exception 2 to Section 150.2(b)1Jii: Dwelling units with atmospherically vented space-heating or water-heating combustion appliances located inside the pressure boundary of the dwelling unit. iii. In Climate Zones 1 through 4 and 8 through 16, recessed downlight luminaires in the ceiling shall be covered with insulation to the same depth as the rest of the ceiling. Luminaires not rated for insulation contact must be replaced or retrofitted with a fireproof cover that allows for insulation to be installed directly over the cover; and Exception 1 to Section 150.2(b)1Jiii: In Climate Zones 1 through 4 and 8 through 10, dwelling units with at least R-19 existing insulation installed at the ceiling level. iv. Attic ventilation shall comply with the California Residential Code, Title 24, Part 2.5, Section R806. Exception 1 to Section 150.2(b)1J: Dwelling units with at least R-38 existing insulation installed at the ceiling level. Exception 2 to Section 150.2(b)1J: Dwelling units where the alteration would directly cause the disturbance of asbestos.

    Exception 3 to Section 150.2(b)1J: Dwelling units with knob and tube wiring located in the vented attic.

    194 2025 CALIFORNIA ENERGY CODE

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

    SINGLE-FAMILY RESIDENTIAL BUILDINGS—ADDITIONS AND ALTERATIONS TO EXISTING RESIDENTIAL BUILDINGS

    Exception 4 to Section 150.2(b)1J: Where the accessible space in the attic is not large enough to accommodate the required R -value, the entire accessible space shall be filled with insulation provided such installation does not violate Section 806.3 of Title 24, Part 2.5. Exception 5 to Section 150.2(b)1J: Where the attic space above the altered dwelling unit is shared with other dwelling units and the requirements of Section 150.2(b)1J are not triggered for the other dwelling units. K. Lighting. The altered lighting system shall meet the lighting requirements of Section 150.0(k).

  • CRC § 0.184. Medium relevance — show source text

    (a) Roof deck, ceiling and rafter roof insulation. The opaque portions of roof decks separating attic spaces from ambient air, and ceilings or rafter roofs separating conditioned spaces from unconditioned spaces or ambient air, shall meet the requirements of Items 1 through 4 below:

    1. In Climate Zones 4 and 8 through 16, roof decks in newly constructed attics that are above conditioned space shall be insulated to achieve an area-weighted average U -factor not exceeding U-0.184. Exception 1 to Section 150.0(a)1:

    i. The space-conditioning system air handler and ducts are located entirely in conditioned space below the ceiling separating the occupiable space from the attic; or ii. The space-conditioning system air handler is located in unconditioned space and has 12 linear feet or less of supply duct, including the length of the air handler and the plenum, located in unconditioned space, with all other portions of the supply ducts located in conditioned space below the ceiling separating the occupiable space from the attic; or iii. The space-conditioning system is a ductless system. Exception 2 to Section 150.0(a)1: Space-conditioning duct systems buried within insulation in an attic that complies using Section 150.1(b) and is verified according to RA 3.1.4.1. 2. Ceilings and rafter roofs shall be insulated to achieve an area-weighted average U -factor not exceeding U-0.043 or shall be insulated between wood-framing members with insulation resulting in an installed thermal resistance of R-22 or greater for the insulation alone. For vented attics, the mandatory insulation shall be installed at the ceiling level; for unvented attics, the mandatory insulation shall be placed at either ceiling or roof level; and Exception to Section 150.0(a)2: Ceilings and rafter roofs in an alteration shall be insulated to achieve an area-weighted average U-factor not exceeding 0.054 or shall be insulated between wood-framing members with insulation resulting in an installed thermal resistance of R-19 or greater. 3. Attic access doors shall have permanently attached insulation using adhesive or mechanical fasteners. The attic access shall be gasketed to prevent air leakage; and 4. Insulation shall be installed in direct contact with a roof or ceiling which is sealed to limit infiltration and exfiltration as specified in Section 110.7, including but not limited to placing insulation either above or below the roof deck or on top of a drywall ceiling.

    (b) Loose-fill insulation. When loose-fill insulation is installed, the minimum installed weight per square foot shall conform with the insulation manufacturer’s installed design weight per square foot at the manufacturer’s labeled R -value.

    (c) Wall insulation. Opaque portions of above grade walls separating conditioned spaces from unconditioned spaces or ambient air shall meet the following requirements:

    1. 2 × 4 inch framing shall have an overall assembly U -factor not exceeding U-0.095 Exception to Section 150.0(c)1: Existing walls already insulated to a U -factor not exceeding U-0.110 or already insulated between framing members with insulation having an installed thermal resistance of R-11 or greater.
    2. 2 × 6 inch or greater framing shall have an overall assembly U -factor not exceeding U-0.069.
    3. Opaque nonframed assemblies shall have an overall assembly U -factor not exceeding U-0.102.
  • CRC § 1.1 Medium relevance — show source text

    1.1 Note||||X|||||||||||||||||||| |R802.10.2||||X|||||||||||||||||||| |R803.2.1.2|||X||||||||||||||||||||| |R806 Note||||||||||||||||||||X|||| |R806.1.1|||X|X|||||||||||||||||||| |R807.1||||X||||||||||||||||||||

    The state agency does not adopt sections identified with the following symbol: †

    2025 CALIFORNIA RESIDENTIAL CODE 8-1

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

    8-2 2025 CALIFORNIA RESIDENTIAL CODE

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

    8 ROOF-CEILING CONSTRUCTION

    User notes:

    About this chapter: Chapter 8 addresses the design and construction of roof-ceiling systems. This chapter contains two roof-ceiling framing systems: wood framing and cold-formed steel framing. Allowable span tables are provided to simplify the selection of rafter and ceiling joist size for wood roof framing and cold-formed steel framing. Chapter 8 also provides requirements for the application of ceiling finishes, the proper ventilation of concealed spaces in roofs (for example, enclosed attics and rafter spaces), unvented attic assemblies and attic access.

    SECTION R801—GENERAL

    R801.1 Application. The provisions of this chapter shall control the design and construction of the roof-ceiling system for buildings.

    R801.2 Requirements. Roof and ceiling construction shall be capable of accommodating all loads imposed in accordance with Section R301 and of transmitting the resulting loads to the supporting structural elements.

    R801.3 Roof drainage. In areas where expansive soils or collapsible soils are known to exist, all dwellings and townhouses shall have a controlled method of water disposal from roofs that will collect and discharge roof drainage to the ground surface not less than 5 feet (1524 mm) from foundation walls or to an approved drainage system.

    SECTION R802—WOOD ROOF FRAMING

    R802.1 General. Wood and wood-based products used for load-supporting purposes shall conform to the applicable provisions of this section.

    R802.1.1 Sawn lumber. Sawn lumber shall be identified by a grade mark of an accredited lumber grading or inspection agency and have design values certified by an accreditation body that complies with DOC PS 20. In lieu of a grade mark, a certificate of inspection issued by a lumber grading or inspection agency meeting the requirements of this section shall be accepted.

    Note: See Section R301.1.1.1 for limited-density owner- built rural dwellings.

    R802.1.1.1 End-jointed lumber. Approved end-jointed lumber identified by a grade mark conforming to Section R802.1.1 shall be permitted to be used interchangeably with solid-sawn members of the same species and grade. End-jointed lumber used in an assembly required elsewhere in this code to have a fire-resistance rating shall have the designation “Heat-Resistant Adhesive” or “HRA” included in its grade mark.

    R802.1.2 Structural glued-laminated timbers. Glued-laminated timbers shall be manufactured and identified as required in ANSI A190.1, ANSI 117 and ASTM D3737.

    R802.1.3 Structural log members. Structural log members shall comply with the provisions of ICC 400.

Frequently asked questions

Can I use a single ridge vent without soffit vents?

No. The code requires cross ventilation (vents that provide an air path in and out of the attic). A ridge vent alone does not provide the intake side; soffit or other low vents are typically required to create cross flow per § R806.1 and § R806.2 .

How do I calculate net free area (NFA) when using louvers/screens?

Use the manufacturer’s listed net free area for each product (the open free area after louvers/screens). If unknown, determine the free‑area fraction and multiply by gross opening; sum product NFAs to meet the required total per § R806.2 .

Do I need vents if I build an unvented attic?

Unvented attics are permitted only if they meet the specific conditions in § R806.5 (e.g., attic inside the thermal envelope, no interior Class I vapor retarder on the attic floor, etc.). You cannot assume unvented—check § R806.5 before omitting vents .

What mesh size is required for WUI vents?

WUI vents have specific mesh/performance requirements (and must be listed/approved). See § R806.1.1 and the California Wildland‑Urban Interface Code (Part 7) for the ember/flame testing and permitted mesh sizes .

Is the 1‑inch airspace mandatory everywhere?

Where eave or cornice vents are installed, the code requires a minimum 1‑inch space between insulation and roof sheathing at the vent location so airflow is not impeded (see § R806.3) .

More in California Residential Code

Ask about the CRC

Get cited, plain-English answers on the California Residential Code for your project — any code section, any scenario.

Start Free Trial

Related in the CRC

When are unvented attic assemblies permitted and what conditions apply?

What are the required attic access opening size, location and headroom requirements?

Are there special venting rules for Wildland‑Urban Interface (WUI) areas?

Roof ventilation, unvented attics and attic access

California Residential Code