CRC · California Residential Code

Crawlspace/under-floor access, ventilation and vapor retarder requirements

Every crawlspace must have a permanent access opening (floor openings at least 18"×24", wall openings 16"×24") and meet either venting rules (1 ft² per 150 ft², reducible with a Class I ground vapor retarder) or the unvented‑crawlspace requirements. In specific IECC climate areas a continuous Class I or II vapor retarder with ≤1.5 perms is required on the exposed face of under‑floor insulation; unvented crawlspaces built to § R408.3 are excepted. **§ R408.4** and **§ R408.8** lay out these requirements.

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Last reviewed: July 6, 2026

What the code requires — plain English

Provide permanent access to every under‑floor space (minimum 18 inches by 24 inches for floor openings; 16 inches by 24 inches for wall openings), and sized areaways where the through‑wall opening is below grade. Venting of vented crawlspaces is sized at 1 ft² net vent area per 150 ft² of under‑floor area (with approved reductions when a Class I vapor retarder is used). In specified IECC climate locations a continuous Class I or II vapor retarder must be installed on the exposed face of air‑permeable insulation in the crawlspace (maximum 1.5 perms by ASTM E96 Procedure B). These rules are set out in § R408.4 and § R408.8 of the California Residential Code (CRC) and related under‑floor sections. § R408.4 § R408.8

The most important points: every crawlspace needs an accessible opening sized at least 18"×24" (or 16"×24" through a perimeter wall), vents sized 1 ft² per 150 ft² unless a Class I vapor retarder is installed, and in certain climates a continuous Class I/II vapor retarder is required on the exposed face of under‑floor insulation. § R408.4 and § R408.8

Requirements in detail

Access (who, where, how big)

  • Access required to all under‑floor spaces. See § R408.4.
  • Minimum opening sizes (first mention in bold):
    • Floor openings: 18 inches by 24 inches (457 mm × 610 mm). § R408.4
    • Perimeter wall through‑wall openings: 16 inches by 24 inches (407 mm × 610 mm). § R408.4
    • If any part of the through‑wall access is below grade, provide an areaway at least 16 inches by 24 inches; the bottom of the areaway must be below the access threshold. § R408.4
  • Additional notes: through‑wall access openings must not be located under a residence door; where mechanical equipment is under the floor, follow the California Mechanical Code for access (see § R408.4).

Ventilation (vented crawlspaces)

  • Minimum net vent area: 1 ft² net per 150 ft² of under‑floor area. § R408.2
  • At least one vent must be located within 3 feet (915 mm) of each external corner of the under‑floor space. § R408.2
  • Acceptable vent covers and maximum mesh/opening dimensions are specified in § R408.2 (e.g., perforated sheet metal, hardware cloth, corrosion‑resistant mesh; least dimension of covering not to exceed 1/4 inch unless otherwise noted). § R408.2
  • Exceptions (vent reduction):
    • The total vent area may be reduced to 1/1,500 of under‑floor area when the ground surface is covered with an approved Class I vapor retarder. § R408.2
    • When the ground surface is covered with an approved Class I vapor retarder, vents need not be within 3 ft of each external corner provided cross‑ventilation is achieved. § R408.2

Unvented crawlspaces (alternate approach)

  • Unvented under‑floor spaces are permitted if they comply with § R408.3 — for example, the exposed earth must be covered by a continuous Class I vapor retarder (joints overlapped 6 inches, sealed/taped; extend 6 inches up stem wall and be sealed/attached). Mechanical or conditioned‑air supply or dehumidification sized per the code is required as one of the compliance options. § R408.3

Under‑floor vapor retarder (where required)

  • Who/where: In IECC Climate Zones 1A, 2A and 3A below the warm‑humid line, provide a continuous Class I or II vapor retarder on the exposed face of air‑permeable insulation installed between floor joists and exposed to grade in the under‑floor space. § R408.8
  • Performance: Vapor retarder maximum water vapor permeance 1.5 perms, tested per ASTM E96 Procedure B. § R408.8
  • Exception: This vapor retarder requirement does not apply to unvented crawlspaces constructed in accordance with § R408.3. § R408.8

Key decision table (dimensions / thresholds)

Decision factor Required value / action Code reference
Minimum floor access opening 18 in × 24 in § R408.4
Minimum through‑wall access opening 16 in × 24 in § R408.4
Areaway for below‑grade through‑wall access min 16 in × 24 in; bottom below threshold § R408.4
Net ventilation area for vented crawlspace 1 ft² per 150 ft² of under‑floor area § R408.2
Reduced vent area with Class I vapor retarder 1/1,500 of under‑floor area § R408.2 (Exception)
Vent location (corner requirement) One vent within 3 ft of each external corner (unless Class I VR and cross‑ventilation provided) § R408.2
Vapor retarder class & permeance Class I or II; ≤ 1.5 perms per ASTM E96 Proc. B § R408.8
Vapor retarder overlap/edge seal (unvented option) Overlap 6 in; extend 6 in up stem wall; seal/tape § R408.3

Exceptions & special cases

  • Reduced vent area (to 1/1,500) and corner‑vent exceptions apply when the ground is covered with an approved Class I vapor retarder. § R408.2 (Exceptions)
  • Unvented crawlspaces: If you choose the unvented route per § R408.3, follow its vapor retarder and mechanical/air‑path requirements — the § R408.8 vapor retarder mandate for certain climates is not required for these unvented crawlspaces (R408.8 Exception). § R408.3 § R408.8
  • Access where mechanical equipment is located under floors: the CRC points you to the California Mechanical Code for additional access requirements; that standard covers service clearances and openings for equipment and is not reproduced here. § R408.4

Common mistakes

  • Installing access openings smaller than 18"×24" (floor) or 16"×24" (wall). Inspectors will enforce § R408.4.
  • Assuming any poly sheeting equals an approved vapor retarder — the CRC requires Class I or II and, for R408.8, a maximum 1.5 perms tested per ASTM E96 Procedure B. Don’t confuse mil‑thickness with class/permeance. § R408.8
  • Failing to overlap and seal vapor retarder laps 6 inches and to extend edges 6 inches up stem walls in unvented crawlspace installs. These details are required by § R408.3.
  • Reducing vent area without installing an approved Class I vapor retarder on the ground surface — the CRC allows reduction only when that vapor retarder is present. § R408.2 (Exceptions)
  • Locating a through‑wall access opening directly under a residence door — prohibited by § R408.4.

Worked example

Scenario: a vented crawlspace with a floor area of 900 ft², located in a climate where R408.8 would otherwise apply.

  1. Vent sizing (standard vented): required net vent area = 900 ft² ÷ 150 = 6.0 ft² of net vent opening area. (See § R408.2.)
  2. If you cover the crawlspace ground with an approved Class I vapor retarder, you may reduce vent area to 900 ÷ 1500 = 0.6 ft² net (but ensure cross ventilation or follow the corner‑placement exceptions). (See § R408.2 exceptions.)
  3. Access: at least one permanent opening must be provided. You could use a floor access hatch of 18"×24" (meets § R408.4). If instead you locate the access through a perimeter wall, provide 16"×24" and, if that wall‑opening is below grade, provide an areaway 16"×24" with bottom below threshold. § R408.4
  4. Vapor retarder (if the location is in IECC Climate Zone 2A below the warm‑humid line): installation of a continuous Class I or II retarder on the exposed face of air‑permeable insulation is required, with permeance ≤ 1.5 perms per ASTM E96 Procedure B — unless you instead build the space as an unvented crawlspace that complies with § R408.3. § R408.8 § R408.3

Related provisions (quick links to CRC sections)

  • § R408.1 — Moisture control applicability for under‑floor space.
  • § R408.2 — Openings for under‑floor ventilation (vent sizing, coverings, exceptions).
  • § R408.3 — Requirements for unvented crawlspaces (vapor retarder overlap/seal, mechanical/air options).
  • § R408.4 — Access requirements (opening sizes, areaways).
  • § R408.5 — Removal of debris in under‑floor space (site prep).
  • § R408.6 — Finished grade in under‑floor space (groundwater/ drainage conditions).
  • § R403.1 — Foundation notes that reference R408 for under‑floor ventilation and access.

Code references

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

  • CRC § 2.1. High relevance — show source text

    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. 2.4. Dehumidification sized in accordance with manufacturer’s specifications.

    R408.4 Access. Access shall be provided to all under-floor spaces. Access openings through the floor shall be not smaller than 18 inches by 24 inches (457 mm by 610 mm). Openings through a perimeter wall shall be not less than 16 inches by 24 inches (407 mm by 610 mm). Where any portion of the through-wall access is below grade, an areaway not less than 16 inches by 24 inches (407 mm by 610 mm) shall be provided. The bottom of the areaway shall be below the threshold of the access opening. Through wall access openings shall not be located under a door to the residence. See California Mechanical Code for access requirements where mechanical equipment is located under floors.

    R408.5 Removal of debris. The under-floor grade shall be cleaned of all vegetation and organic material. Wood forms used for placing concrete shall be removed before a building is occupied or used for any purpose. Construction materials shall be removed before a building is occupied or used for any purpose.

    R408.6 Finished grade. The finished grade of under-floor surface shall be permitted to be located at the bottom of the footings; however, where there is evidence that the groundwater table can rise to within 6 inches (152 mm) of the finished floor at the building perimeter or where there is evidence that the surface water does not readily drain from the building site, the grade in the under-floor space shall be as high as the outside finished grade, unless an approved drainage system is provided.

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    FOUNDATIONS

  • CRC § 0.0929 High 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.
  • CRC § 2.4. High relevance — show source text

    2.4. Dehumidification sized in accordance with manufacturer’s specifications.

    R408.4 Access. Access shall be provided to all under-floor spaces. Access openings through the floor shall be not smaller than 18 inches by 24 inches (457 mm by 610 mm). Openings through a perimeter wall shall be not less than 16 inches by 24 inches (407 mm by 610 mm). Where any portion of the through-wall access is below grade, an areaway not less than 16 inches by 24 inches (407 mm by 610 mm) shall be provided. The bottom of the areaway shall be below the threshold of the access opening. Through wall access openings shall not be located under a door to the residence. See California Mechanical Code for access requirements where mechanical equipment is located under floors.

    R408.5 Removal of debris. The under-floor grade shall be cleaned of all vegetation and organic material. Wood forms used for placing concrete shall be removed before a building is occupied or used for any purpose. Construction materials shall be removed before a building is occupied or used for any purpose.

    R408.6 Finished grade. The finished grade of under-floor surface shall be permitted to be located at the bottom of the footings; however, where there is evidence that the groundwater table can rise to within 6 inches (152 mm) of the finished floor at the building perimeter or where there is evidence that the surface water does not readily drain from the building site, the grade in the under-floor space shall be as high as the outside finished grade, unless an approved drainage system is provided.

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    FOUNDATIONS

    R408.7 Flood resistance. For buildings located in flood hazard areas as established in Table R301.2:

    1. Walls enclosing the under-floor space shall be provided with flood openings in accordance with Section R306.2.2.
    2. The finished ground level of the under-floor space shall be equal to or higher than the outside finished ground level on at least one side.

    Exception: Under-floor spaces that meet the requirements of FEMA TB 11.

    R408.8 Under-floor vapor retarder. In Climate Zones 1A, 2A and 3A below the warm-humid line, a continuous Class I or II vapor retarder shall be provided on the exposed face of air-permeable insulation installed between the floor joists and exposed to the grade in the under-floor space. The vapor retarder shall have a maximum water vapor permeance of 1.5 perms when tested in accordance with Procedure B of ASTM E96.

    Exception: The vapor retarder shall not be required in unvented crawl spaces constructed in accordance with Section R408.3.

    Note: The International Energy Conservation Code (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.

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    CALIFORNIA RESIDENTIAL CODE – MATRIX ADOPTION TABLE

    CHAPTER 5 – FLOORS

  • CRC § 0.037. High relevance — show source text

    In wood framed assemblies, compliance with U -factors may be demonstrated by installing wall insulation with an R -value of R-15 in 2x4 assemblies, and R-21 in 2x6 assemblies with 16 inches on center spacing.

    (d) Raised-floor insulation. Raised floors separating conditioned space from unconditioned space or ambient air shall have an overall assembly U -factor not exceeding U-0.037. In a wood framed assembly, compliance with the U -factor may be demonstrated by installing insulation with an R -value of 19 or greater.

    Exception to Section 150.0(d): A building with a controlled ventilation or unvented crawlspace may omit raised floor insulation if all of the following are met: i. The foundation walls are insulated to meet the wall insulation minimums as shown in Table 150.1-A; and

    ii. A Class I or Class II vapor retarder is placed over the entire floor of the crawl space; and iii. Vents between the crawlspace and outside air are fitted with automatically operated louvers that are temperature actuated; and

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    SINGLE-FAMILY RESIDENTIAL BUILDINGS— MANDATORY FEATURES AND DEVICES

    iv. The requirements in Reference Residential Appendix RA4.5.1.

    (e) Installation of fireplaces, decorative gas appliances and gas logs. If a masonry or factory-built fireplace is installed, it shall comply with Section 110.5, Section 4.503 of Part 11, and shall have the following:

    1. Closable metal or glass doors covering the entire opening of the firebox; and
    2. A combustion air intake to draw air from the outside of the building, which is at least 6 square inches in area and is equipped with a readily accessible, operable and tight-fitting damper or combustion-air control device; and Exception to Section 150.0(e)2: An outside combustion-air intake is not required if the fireplace will be installed over concrete slab flooring and the fireplace will not be located on an exterior wall.
    3. A flue damper with a readily accessible control. Exception to Section 150.0(e)3: When a gas log, log lighter or decorative gas appliance is installed in a fireplace, the flue damper shall be blocked open if required by the CMC or the manufacturer’s installation instructions.

    (f) Slab edge insulation. Material used for slab edge insulation shall meet the following minimum specifications:

    1. Water absorption rate for the insulation material alone without facings no greater than 0.3 percent when tested in accordance with Test Method A – 24-Hour-Immersion of ASTM C272.

    2. Water vapor permeance no greater than 2.0 perm/inch when tested in accordance with ASTM E96.

    3. Concrete slab perimeter insulation shall be protected from physical damage and ultraviolet light deterioration.

    4. Insulation for a heated slab floor shall meet the requirements of Section 110.8(g).

    (g) Vapor retarder.

    1. In Climate Zones 1–16, the earth floor of unvented crawl space shall be covered with a Class I or Class II vapor retarder. This requirement shall also apply to controlled ventilation crawl space for buildings complying with the Exception to Section 150.0(d).
  • CRC § 4-9 Medium relevance — show source text

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    FOUNDATIONS

    FIGURE R403.1(1)—PLAIN CONCRETE FOOTINGS WITH MASONRY AND CONCRETE STEM WALLS IN SEISMIC DESIGN CATEGORIES A, B AND C [a, b, c, d, e, f]

    For SI: 1 inch = 25.4 mm. W = Width of footing, T = Thickness of footing and P = Projection per Section R403.1.1.

    NOTES:

    a. See Section R404.3 for sill requirements. b. See Section R403.1.6 for sill attachment.

    c. See Section R506.3.3 for vapor barrier requirements. d. See Section R403.1 for base.

    e. See Figure R403.1.3 for additional footing requirements for structures in Seismic Design Categories D 0, D 1 and D 2 and townhouses in Seismic Design Category C. f. See Section R408 for under-floor ventilation and access requirements.

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    FOUNDATIONS

    FIGURE R403.1(2)—PERMANENT WOOD FOUNDATION BASEMENT WALL SECTION

    PRESSURE-PRESERVATIVE TREATED

    FLASHING

    FIELD-APPLIED 2 � 6 IN. TOP PLATE

    2 � 6 IN. TOP PLATE

    MIN. 3.5-IN. CONCRETE SLAB WITH s

    VAPOR BARRIER AND OPTIONAL

    1-IN. SCREED BOARD

    2 � 6 IN. STUD INSULATED

    AS APPROPRIATE AND WARM SIDE

    VAPOR BARRIER

    FINISH GRADE SLOPE [1] / 2 IN. PER FOOT, MIN. 6 FT. FROM WALL

    PRESSURE-PRESERVATIVETREATED PLYWOOD (SEE TABLE R404.2.3)

    6 MIL POLYETHYLENE FILM

    6 MIL POLYETHYLENE FILM ON CRUSHED STONE OR GRAVEL BACKFILL

    PRESSURE-PRESERVATIVE TREATED 2 � 6 IN. BOTTOM

    Col1 2-IN. AIR GAP
    . BACKFILL) 2-IN. AIR GAP
    4 IN.
    1-IN.-THICK PRESSURE
    TREATED LUMBER OR
    PROTECTING TOP OF P
    PRESS
    TREAT
    PLATE
    PRES
    TREA
    PLATE
    .
    HED
    4 IN.
    16 IN    		 HED
    4 IN.
    16 IN

    STONE FILL UNDER FLOOR (SEE SECTION R403.2)

    6 IN.

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

    FIGURE R403.1(3)—PERMANENT WOOD FOUNDATION CRAWL SPACE SECTION

    PRESSURE-PRESERVATIVE TREATED

    6 MIL POLYETHYLENE FILM

    6 IN.

    6 IN. GRAVEL OR CRUSHED STONE (SEE SECTION R403.2)

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

  • CRC § 25.4 Medium relevance — show source text

    61|68|76|84|91| |34|1|22|29|36|43|50|58|65|72|79|86| |35|1|20|27|34|41|48|55|61|68|75|82| |36|1|19|26|32|39|45|52|58|65|71|78| |37|1|18|25|31|37|43|49|55|62|68|74| |38|1|18|23|29|35|41|47|53|59|64|70| |39|1|17|22|28|33|39|45|50|56|61|67| |40|1|16|21|27|32|37|43|48|53|59|64| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_R309.3.4.2_.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_R309.3.4.2_.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_R309.3.4.2_.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_R309.3.4.2_.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_R309.3.4.2_.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_R309.3.4.2_.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a.

  • CRC § 2.5 Medium relevance — show source text

    Parapet construction R302.2.5 Scope R101.2 Separation R302.2 Structural independence R302.2.6 Trusses

    Steel R505.1.3, R804.3.6 Wood R502.12, R802.10 Tubular Daylighting Device (TDD) (see Skylights) Two-Family Dwellings R302.3

    Under Floor

    Space R408

    Vapor Retarders R702.7 Capillary break R506.2.3.1 Under floor R408.8 Vehicular Gates Appendix BH Veneer

    Masonry R703.7 Ventilation Adjoining rooms R325.1.3 Bathroom and kitchen R325.2

    Habitable rooms R325.1

    Roof R806

    Under floor R408.1 Venting (Mechanical) Chimney and vent connectors R1006 Fireplace R1006 General R325.3

    Violations R113 Visible Transmittance (VT)

    Wall Furnace (see Appliance) Wallboard (see Gypsum) Walls

    Bathtub compartments R327.2 Bracing, steel R603.9 Bracing, wood R602.10 Construction Chapter 6 Covering Chapter 7 Crawl space R202 Cripple R602.9

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    INDEX

    Deflection R301.7

    Exterior, defined R202 Exterior covering R703 Finishes R327.2, R702 Fireplace R1001.5 Foundation R404 Insulating concrete form R608.3, R608.4, R608.5.3 Steel framing R603 Structural insulated panels (SIP) R610 Thickness, masonry chimneys R1003.10 Wood framing R602 Waste

    Construction waste R334

    Water Closet R326.1, R327 Clearances for R327.1 Waterproofing And dampproofing R406 Wells (see Individual Water Supply) Whirlpool Bathtubs R324.3.1, P2720 Wind

    Design criteria R301.2, R301.2.1 Speed maps Figure R301.2(2) Window R609 Emergency Escape and Rescue Openings R319, R321.2.2 Fall protection R321.2 Glazing R324 Light and ventilation R325 Opening limiting devices R321.2.2 Wood

    Floor construction R502

    Foundation walls R404.2 Protection against decay R304 Roof-ceiling construction R802 Shakes R905.8 Shingles R905.7 Trusses R502.12, R802.10 Walls R602

    Yard R318.1, R319.1, R319.2.4, R325.1.1

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    HISTORY NOTE APPENDIX

  • CRC § 1404.3 Medium relevance — show source text

    Where a Class I or II vapor retarder is used in combination with foam plastic insulating sheathing installed as continuous insulation on the exterior side of frame walls, the
    continuous insulation shall comply with Table 1404.3(4) and the Class I or II vapor retarder shall be a responsive vapor retarder.|a. A responsive vapor retarder shall be allowed on the interior side of any frame wall in all climate zones.
    b. In frame walls with a Class I vapor retarder on the exterior side, use of a Class I interior vapor retarder that is not a responsive vapor retarder shall require an_approved_ design.
    c. Where a Class I or II vapor retarder is used in combination with foam plastic insulating sheathing installed as continuous insulation on the exterior side of frame walls, the
    continuous insulation shall comply with Table 1404.3(4) and the Class I or II vapor retarder shall be a responsive vapor retarder.|a. A responsive vapor retarder shall be allowed on the interior side of any frame wall in all climate zones.
    b. In frame walls with a Class I vapor retarder on the exterior side, use of a Class I interior vapor retarder that is not a responsive vapor retarder shall require an_approved_ design.
    c. Where a Class I or II vapor retarder is used in combination with foam plastic insulating sheathing installed as continuous insulation on the exterior side of frame walls, the
    continuous insulation shall comply with Table 1404.3(4) and the Class I or II vapor retarder shall be a responsive vapor retarder.|

    TABLE 1404.3(3)—CLASS III VAPOR RETARDERS Col2
    ZONE CLASS III VAPOR RETARDERS PERMITTED FOR:a, b
    4 Vented cladding over wood structural panels
    Vented cladding over fiberboard
    Vented cladding over gypsum
    Continuous insulation with_R_-value ≥ R-2.5 over 2 × 4 wall
    Continuous insulation with_R_-value ≥ R-3.75 over 2 × 6 wall
    5 Vented cladding over wood structural panels
    Vented cladding over fiberboard
    Vented cladding over gypsum
    Continuous insulation with_R_-value ≥ R-5 over 2 × 4 wall
    Continuous insulation with_R_-value ≥ R-7.5 over 2 × 6 wall
    6 Vented cladding over fiberboard
    Vented cladding over gypsum
    Continuous insulation with_R_-value ≥ R-7.5 over 2 × 4 wall
    Continuous insulation with_R_-value ≥ R-11.25 over 2 × 6 wall
    7 Continuous insulation with_R_-value ≥ R-10 over 2 × 4 wall
    Continuous insulation with_R_-value ≥ R-15 over 2 × 6 wall
    8 Continuous insulation with_R_-value ≥ R-12.5 over 2 × 4 wall
    Continuous insulation with_R_-value ≥ R-20 over 2 × 6 wall
    a. Vented cladding shall include vinyl lap siding, polypropylene, horizontal aluminum siding, brick veneer with airspace as specified in this code, rainscreen systems and other
    approved vented claddings.
    b. The requirements in this table apply only to insulation used to control moisture in order to permit the use of Class III vapor retarders. The insulation materials used to satisfy
    this option also contribute to but do not supersede the building thermal envelope requirements of the_California Energy Code_.    		 a.
  • CRC § 25.4 Medium relevance — show source text

    1|144|157| |27|3/4|37|49|61|73|86|98|110|122|135|147| |28|3/4|34|46|57|69|80|92|103|114|126|137| |29|3/4|32|43|54|64|75|86|96|107|118|129| |30|3/4|30|40|50|60|70|81|91|101|111|121| |31|3/4|28|38|47|57|66|76|85|95|104|114| |32|3/4|27|36|45|54|63|71|80|89|98|107| |33|3/4|25|34|42|51|59|68|76|84|93|101| |34|3/4|24|32|40|48|56|64|72|80|88|96| |35|3/4|23|30|38|45|53|61|68|76|83|91| |36|3/4|22|29|36|43|50|57|65|72|79|86| |37|3/4|20|27|34|41|48|55|61|68|75|82| |38|3/4|20|26|33|39|46|52|59|65|72|78| |39|3/4|19|25|31|37|43|50|56|62|68|74| |40|3/4|18|24|30|35|41|47|53|59|65|71| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section_ R309.3.4.2.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a. Flow rate from Section     R309.3.4.2._|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 gallon per minute = 0.963 L/s.
    a.

  • CRC § 110.7 Medium relevance — show source text
    1. Attic access doors shall have permanently attached insulation using adhesive or mechanical fasteners. The attic access shall be gasketed to prevent air leakage; and
    2. 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.
    4. Bay or bow window roofs and floors shall be insulated to meet the wall insulation requirements of Table 150.1-A.
    5. Masonry/mass walls shall be insulated to meet the wall insulation requirements of Table 150.1-A.
    6. In wood framed assemblies, compliance with U -factors may be demonstrated by installing wall insulation with an R -value of R-15 in 2x4 assemblies, and R-21 in 2x6 assemblies with 16 inches on center spacing.

    (d) Raised-floor insulation. Raised floors separating conditioned space from unconditioned space or ambient air shall have an overall assembly U -factor not exceeding U-0.037. In a wood framed assembly, compliance with the U -factor may be demonstrated by installing insulation with an R -value of 19 or greater.

    Exception to Section 150.0(d): A building with a controlled ventilation or unvented crawlspace may omit raised floor insulation if all of the following are met: i. The foundation walls are insulated to meet the wall insulation minimums as shown in Table 150.1-A; and

    ii. A Class I or Class II vapor retarder is placed over the entire floor of the crawl space; and iii. Vents between the crawlspace and outside air are fitted with automatically operated louvers that are temperature actuated; and

    2025 CALIFORNIA ENERGY CODE 159

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

    SINGLE-FAMILY RESIDENTIAL BUILDINGS— MANDATORY FEATURES AND DEVICES

    iv. The requirements in Reference Residential Appendix RA4.5.1.

    (e) Installation of fireplaces, decorative gas appliances and gas logs. If a masonry or factory-built fireplace is installed, it shall comply with Section 110.5, Section 4.503 of Part 11, and shall have the following:

    1. Closable metal or glass doors covering the entire opening of the firebox; and
  • CRC § 1404.3.1 Medium relevance — show source text

    The exterior continuous insulation shall be permitted to serve as the vapor control layer where, at its
    installed thickness or with a facer on its interior face, the exterior continuous insulation is a Class I or II vapor retarder.
    c. The requirements of this table apply only to continuous insulation used to control moisture in order to allow walls without a Class I, II or III interior vapor retarder. The insula-
    tion materials used to satisfy this option also contribute to but do not supersede the_building_ thermal envelope requirements of the_California Energy Code_.|a. The total insulating value of materials to the interior side of the exterior continuous insulation, including any cavity insulation, shall not exceed R-5. Where the_R_-value of
    materials to the interior side of the exterior continuous insulation exceeds R-5, an_approved_ design shall be required.
    b. A water vapor control material layer having a permeance of not greater than 1 perm in accordance with ASTM E96, Procedure A (dry cup) shall be placed on the exterior side
    of the wall and to the interior side of the exterior continuous insulation. The exterior continuous insulation shall be permitted to serve as the vapor control layer where, at its
    installed thickness or with a facer on its interior face, the exterior continuous insulation is a Class I or II vapor retarder.
    c. The requirements of this table apply only to continuous insulation used to control moisture in order to allow walls without a Class I, II or III interior vapor retarder. The insula-
    tion materials used to satisfy this option also contribute to but do not supersede the_building_ thermal envelope requirements of the_California Energy Code_.|

    �

    EXTERIOR WALLS

    1404.3.1 Spray foam plastic insulation for moisture control with Class II and III vapor retarders. For purposes of compliance with Tables 1404.3(3) and 1404.3(4), spray foam with a maximum permeance of 1.5 perms at the installed thickness applied to the interior side of wood structural panels, fiberboard, insulating sheathing or gypsum shall be deemed to meet the continuous insulation moisture control requirement in accordance with one of the following conditions:

    1. The spray foam R -value meets or exceeds the specified continuous insulation R-value.
    2. The combined R -value of the spray foam and continuous insulation is equal to or greater than the specified continuous insulation R -value.

    1404.3.2 Vapor retarder installation. Vapor retarders shall be installed in accordance with the manufacturer’s instructions or an approved design. Where a vapor retarder also functions as a component of a continuous air barrier, the vapor retarder shall be installed as an air barrier in accordance with the California Energy Code . 1404.3.3 California Energy Code and International Energy Conservation Code Climate Zones. 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 Chapter 12, Table 1202.3.1. 1404.4 Flashing. Flashing shall be installed in such a manner so as to prevent moisture from entering the exterior wall or to redirect that moisture to the surface of the exterior wall covering or to a water-resistive barrier complying with Section 1403.2 and that is part of a means of drainage complying with Section 1402.2.

Frequently asked questions

Can I make the access opening smaller if I have a removable panel?

No. The CRC requires a permanent access opening not smaller than 18"×24" through the floor (or 16"×24" through a perimeter wall). Removable panels do not change the minimum dimension requirement. § R408.4

When can I reduce the required vent area?

You may reduce the total vent area to 1/1,500 of the under‑floor area only when the ground surface is covered with an approved Class I vapor retarder (and other corner‑placement exceptions apply when cross‑ventilation is maintained). § R408.2 (Exceptions)

Do I have to install a vapor retarder if I build an unvented crawlspace?

No — the specific vapor retarder requirement of § R408.8 for certain climates is excepted when the crawlspace is built as an unvented crawlspace in accordance with § R408.3, provided you meet the R408.3 materials and mechanical/air pathway requirements. § R408.3 § R408.8

What permeance rating must the vapor retarder meet?

For under‑floor insulation applications in the climate zones covered by § R408.8, the vapor retarder must have a maximum water vapor permeance of 1.5 perms when tested per ASTM E96 Procedure B. § R408.8

Where do I find access requirements for mechanical equipment under floors?

The CRC directs you to the California Mechanical Code for access requirements where mechanical equipment is located under floors; the CRC’s § R408.4 notes this cross‑reference. § R408.4

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