CRC · California Residential Code

When are vapor retarders or vapor-permeance limits required for walls?

Whether a vapor retarder is required depends on your building’s climate zone and the wall assembly. The CRC’s **§ R702.7** lets you select a vapor‑retarder class (I, II, III by permeance) using **Table R702.7(2)**, and provides continuous‑insulation exceptions (Tables R702.7(3)–(5)) that may let you omit an interior retarder if specific exterior insulation and interior R‑value limits are met. Follow the tables and installation rules in **§ R702.7** and related subsections to decide and install the correct retarder.

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Last reviewed: July 6, 2026

What the code requires — plain English

A vapor retarder is required on the interior side of frame walls according to § R702.7. The code classifies vapor retarders by permeance (Class I, II, III) and tells you which class (if any) is permitted or required by climate zone and by certain wall assembly conditions (see Tables R702.7(1)–(5)). Exceptions include basement or below-grade walls, assemblies where moisture won’t damage materials, and Climate Zones 1, 2 and 3; Climate Zones 4–8 can also omit an interior retarder when the wall meets the continuous‑insulation options in Table R702.7(5) (§ R702.7) .

The single most important rule: determine the climate zone per § R702.7.3, then apply the vapor‑retarder class options in Table R702.7(2) and the continuous‑insulation alternatives in Tables R702.7(3)–(5) to decide whether an interior vapor retarder (and what class) is required (§ R702.7) .

Requirements in detail

Definitions & class thresholds (quick)

  • Class I — materials with permeance ≤ 0.1 perms (e.g., sheet polyethylene, nonperforated foil) (Table R702.7(1)) .
  • Class II — > 0.1 and ≤ 1.0 perms (e.g., kraft-faced batt facers, vapor‑retarder paint) (Table R702.7(1)) .
  • Class III — > 1.0 and ≤ 10.0 perms (e.g., latex or enamel paint) (Table R702.7(1)) .
  • A responsive vapor retarder (a retarder whose permeance increases with RH) is explicitly allowed as noted in Table R702.7(2) (§ R702.7) .

Which class is allowed/required by climate zone

Use Table R702.7(2) to decide by climate zone whether Class I / II / III is permitted on the interior side. In short:

  • Climate Zones 1–2: Class I & II not permitted; Class III permitted (Table R702.7(2)) .
  • Climate Zones 3–4 (except Marine 4): Class II and III are allowed; Class I not permitted (Table R702.7(2)) .
  • Marine 4, 5, 6, 7, 8: Class I and II are permitted; allowable Class III uses are limited and shown in Table R702.7(3) (Table R702.7(2) and (3)) .

Continuous‑insulation alternatives (when you may omit an interior retarder)

In Climate Zones 4–8, an interior vapor retarder is not required if the wall assembly complies with Table R702.7(5) (walls WITHOUT a Class I, II or III interior vapor retarder) or when other continuous‑insulation/condition limits in Tables R702.7(3) or R702.7(4) apply. These tables set minimum continuous‑insulation R‑values (and other conditions) that allow omission or different classes of interior vapor retarders (§ R702.7 and Tables R702.7(3)–(5)) .

Below is a decision‑summary table with the key numbers you will consult.

Decision factor Threshold / value (key examples) Code Reference
Class I permeance ≤ 0.1 perms Table R702.7(1), § R702.7
Class II permeance > 0.1 and ≤ 1.0 perms Table R702.7(1), § R702.7
Class III permeance > 1.0 and ≤ 10.0 perms Table R702.7(1), § R702.7
CZs with no interior retarder required (automatic) Climate Zones 1, 2, 3 (no interior retarder required in 1–3) Exceptions to § R702.7, § R702.7
Continuous insulation to allow NO interior retarder (CZ 4–8) Minimums by zone: CZ4 R‑4.5; CZ5 R‑6.5; CZ6 R‑8.5; CZ7 R‑11.5; CZ8 R‑14 (Table R702.7(5)) — also interior‑side R to the interior of the continuous insulation must not exceed R‑5 (note) Table R702.7(5), § R702.7
Continuous insulation to permit Class I/II usage Example: CZ6 R‑8.5 (Table R702.7(4) / R702.7(3) vary by wall depth; see tables) Tables R702.7(3) & (4), § R702.7
Spray‑foam special rule (interior side) Spray foam with max permeance 1.5 perms at installed thickness can be treated as meeting continuous‑insulation moisture control under the listed R‑value conditions (R702.7.1) § R702.7.1 (R702.7)
Installation requirement Vapor retarders must be installed per manufacturer or approved design; when functioning as air barrier, install per N1102.5.1.1 (R702.7.2) § R702.7.2 and § R702.7

(Consult each full table for zone‑by‑zone detail — Tables R702.7(1) through R702.7(5) contain the granular rules.)

How the tables interact (brief)

  • First, identify the building’s climate zone per § R702.7.3 and Table R702.7.3. That determines baseline permissions and which tables apply (R702.7(2), (3), (4), (5)) .
  • If the assembly does not meet the continuous‑insulation alternatives, select an interior vapor‑retarder class allowed for that climate zone from Table R702.7(2) (or see the special‑use conditions in Table R702.7(3) for Class III uses) .
  • When using foam plastic sheathing/exterior continuous insulation together with an interior Class I/II, confirm compliance with Table R702.7(4) (minimum continuous R values) and the special notes about facers/permeance (§ R702.7) .

Exceptions & special cases

  • Basement walls and below‑grade portions of walls are excepted from the interior vapor‑retarder requirement (§ R702.7 exceptions) .
  • A vapor retarder is not required at all in Climate Zones 1, 2 and 3 (§ R702.7 exceptions) .
  • Climate Zones 4–8: interior retarder can be omitted if the assembly complies with Table R702.7(5) (continuous‑insulation option) — but that table imposes both minimum exterior continuous‑insulation R‑values and a limit that the total R on the interior side of the exterior continuous insulation must not exceed R‑5 unless an approved design is provided (§ R702.7 and Table R702.7(5)) .
  • Spray foam installed on the interior side of sheathing is given a special treatment: if the spray foam has ≤ 1.5 perms at installed thickness and meets stated R‑value combinations it may be treated as meeting the continuous‑insulation moisture‑control requirement (§ R702.7.1) .
  • Special construction types like strawbale and cob have restrictions in their appendices — e.g., Class I/II vapor retarders are generally not allowed on strawbale or cob walls; specific permeance minima (such as ≥ 5 perms for some water‑resistive barriers) and interior retarder requirements are in Appendix BJ and BK (see BJ105 / BJ104 and BK104) .

Common mistakes

  • Ignoring the climate‑zone requirement: failing to determine the correct California/IECC climate zone mapping per § R702.7.3 leads to wrong class selection (the code uses a specific zone mapping) .
  • Assuming a painted drywall always meets requirements: painted drywall can be Class III (per Table R702.7(1)) but Class III is allowed only under limited conditions in some zones (see Table R702.7(3)), so paint alone may not be adequate in colder/wetter zones .
  • Overlooking the R‑5 interior‑side limit in Table R702.7(5): if cavity insulation plus interior layers exceed R‑5, you cannot use that table to avoid an interior vapor retarder — you must then provide the retarder class required by Table R702.7(2) .
  • Using a non‑responsive Class I on the interior while also having a Class I on the exterior without an approved design — Table R702.7(2) requires special design when non‑responsive Class I is on both faces (§ R702.7 notes) .
  • Misreading the spray foam permeance rule: the special spray‑foam allowance uses 1.5 perms at installed thickness and has R‑value conditions; it does not mean any spray foam is acceptable without checking the installed permeance and R‑value compliance (§ R702.7.1) .

Worked example — concrete scenario with numbers

Scenario: New wood‑frame single‑family house in Climate Zone 6 (California). Wall: 2×4 studs, cavity batt R‑13, interior gypsum, exterior continuous foam insulation R‑8.5.

Step 1 — Can we omit an interior vapor retarder using Table R702.7(5)? Table R702.7(5) for CZ6 requires continuous insulation R‑value ≥ R‑8.5 (we have R‑8.5) — good — but the table also requires that the total insulating value of materials to the interior side of the exterior continuous insulation (including cavity insulation) shall not exceed R‑5. Our cavity insulation is R‑13, so the interior‑side total exceeds R‑5, and therefore we cannot use Table R702.7(5) to omit the interior retarder (§ R702.7 and Table R702.7(5)) .

Step 2 — What vapor retarder class is required? For CZ6 Table R702.7(2) and the zone notes show that Class II vapor retarders are permitted (and Class I is permitted with caveats); Class III is only allowed under the specific conditions listed in Table R702.7(3) (vented claddings or higher continuous‑insulation R‑values). Therefore, on the interior side we must provide at least a Class II vapor retarder (for example, kraft‑faced batt facer or an approved vapor‑retarder paint application) unless an approved hygrothermal design says otherwise (§ R702.7 and Table R702.7(2)) .

Step 3 — Installation note: install the chosen vapor retarder per manufacturer instructions and according to § R702.7.2; if it also serves as part of the continuous air barrier, follow N1102.5.1.1 installation rules (§ R702.7.2) .

Conclusion (example): With cavity R‑13 and exterior R‑8.5 in CZ6, you cannot rely on the continuous‑insulation exception and must install an interior Class II vapor retarder per § R702.7 (unless you provide an engineered hygrothermal design) .

Related provisions (quick list)

  • § R702.7 — Vapor retarders general rules, Tables R702.7(1)–(5) and exceptions (primary controlling section) .
  • § R702.7.1 — Spray foam special rule (1.5 perms / R‑value conditions) .
  • § R702.7.2 — Vapor retarder installation and air‑barrier interface (N1102.5.1.1) .
  • § R702.7.3 and Table R702.7.3 — Climate zone mapping used for R702.7 determinations (compare IECC vs California Energy Code zones) .
  • Appendix BJ (strawbale) — special vapor‑permeance rules for strawbale walls (e.g., no Class I/II on strawbale; minimum permeance for WRBs) .
  • Appendix BK (cob) — cob wall vapor‑retarder/permeance restrictions (e.g., no Class I/II on cob) .

Code references

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

  • CRC § 7.1 High relevance — show source text

    The requirements in this table apply only to insulation used to control moisture in order to allow walls without a Class I, II or III interior vapor retarder. The insulation materi-
    als used to satisfy this option also contribute to but do not supersede the thermal envelope requirements of the_California Energy Code_.|

    R702.7.1 Spray foam plastic insulation for moisture control with Class II and III vapor retarders. For purposes of compliance with Tables R702.7(3) and R702.7(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 is equal to or greater than 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.

    R702.7.2 Vapor retarder installation. Vapor retarders shall be installed in accordance with the manufacturer’s instructions, accepted installation methods 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 Section N1102.5.1.1.

    R702.7.3 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 Table R702.7.3.

    TABLE R702.7.3—IECC VS. CALIFORNIA ENERGY CODE CLIMATE ZONE COMPARISON Col2 Col3
    IECCa CALIFORNIA
    ENERGY CODE    		 DESCRIPTIONb
    _6 _ 16 Includes Alpine, Mono Counties
    _5 _ 11, 12, 16 Includes Siskiyou, Modoc, Lassen, Plumas, Sierra, Nevada Counties
    4 (marine) 1, 2, 16 Includes Del Norte and Humboldt Counties
    _4 _ 2, 12, 13, 16 Includes Inyo, Trinity, Lake, El Dorado, Amador, Calaveras, Tuolumne, Mariposa Counties
    _3 _ 8, 9, 10,11,12,
    13, 14, 15, 16    		 Includes Shasta, Tehama, Butte, Glenn, Colusa, Yuba, Contra Costa, Sutter, Yolo, Sacramento, Placer, San Joaquin,
    Solano, Stanislaus, Merced, Madera, Fresno, Kings, Tulare, Kern, Ventura, Los Angeles, Orange, San Bernardino, River-
    side Counties
    3 (marine) 1, 2, 3, 4, 5, 6,
    9, 12, 16    		 Includes Mendocino, Sonoma, Marin, San Francisco, San Mateo, Alameda, Santa Cruz, Monterey, San Benito,
    San Luis Obispo, Santa Barbara, Ventura, San Diego Counties
    _2 _ _14,
  • CRC § 7.1 High relevance — show source text

    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 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 in this table apply only to insulation used to control moisture in order to allow walls without a Class I, II or III interior vapor retarder. The insulation materi-
    als used to satisfy this option also contribute to but do not supersede the 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 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 in this table apply only to insulation used to control moisture in order to allow walls without a Class I, II or III interior vapor retarder. The insulation materi-
    als used to satisfy this option also contribute to but do not supersede the thermal envelope requirements of the_California Energy Code_.|

    R702.7.1 Spray foam plastic insulation for moisture control with Class II and III vapor retarders. For purposes of compliance with Tables R702.7(3) and R702.7(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 is equal to or greater than 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.

    R702.7.2 Vapor retarder installation. Vapor retarders shall be installed in accordance with the manufacturer’s instructions, accepted installation methods 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 Section N1102.5.1.1.

    R702.7.3 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 Table R702.7.3.

    |TABLE R702.7.3—IECC VS.

  • CRC § 48.9 High relevance — show source text

    ** Exterior wall finishes shall be plasters in accordance with Section BJ104.4, or nonplaster exterior wall coverings in accordance with Section R703 and other finish systems complying with all of the following:

    1. With approved specifications and details showing the finish system’s means of attachment to the wall or its independent support, and a means of draining or evaporating water that penetrates the exterior finish to the exterior.
    2. The vapor permeance of the combination of finish materials shall be 5 perms or greater to allow the transpiration of water vapor through the wall.
    3. Finish systems with weights greater than 10 or less than or equal to 20 pounds per square foot (> 48.9 and ≤ 97.8 kg/m) of wall area require a factor of 1.2 for minimum total length of braced wall panels in Table BJ106.13(3).
    4. Finish systems with weights greater than 20 pounds per square foot (97.8 kg/m) of wall area require an engineered design.

    BJ104.2 Purpose, and where required. Strawbale walls shall be finished so as to provide mechanical protection, fire resistance and protection from weather and to restrict the passage of air through the bales, in accordance with this appendix and this code. Vertical strawbale wall surfaces shall receive a coat of plaster not less than [3] / 8 inch (10 mm) thick, or greater where required elsewhere in this appendix, or shall fit tightly against a solid wall panel or dense-packed cellulose insulation with a density of not less than 3.5 pounds per cubic foot (56 kg/m [3] ) blown into an adjacent framed wall. The tops of strawbale walls shall receive a coat of plaster not less than 3 / 8 inch (10 mm) thick or be tightly covered by gypsum board or a roof-bearing assembly.

    Exception: Truth windows shall be permitted where a fire-resistance rating is not required. Weather-exposed truth windows shall be fitted with a weather-tight cover. Interior truth windows in Climate Zones 5, 6, 7, 8 and Marine 4 shall be fitted with an airtight

    cover.

    BJ104.3 Vapor retarders. Class I and II vapor retarders shall not be used on a strawbale wall, nor shall any other material be used that has a vapor permeance rating of less than 5 perms, except as permitted or required elsewhere in this appendix.

    BJ104.4 Plaster. Plaster applied to bales shall be any type described in this section, and as required or limited in this appendix. Plaster thickness shall not exceed 2 inches (51 mm).

    BJ104.4.1 Plaster and membranes. Plaster shall be applied directly to strawbale walls to facilitate transpiration of moisture from the bales, and to secure a mechanical bond between the skin and the bales, except where a membrane is allowed or required elsewhere in this appendix.

    BJ104.4.2 Lath and mesh for plaster. The surface of the straw bales functions as lath, and other lath or mesh shall not be required, except as required for out-of-plane load resistance by Table BJ105.4 or for structural walls by Tables BJ106.12 and BJ106.13(1).

    BJ104.4.3 Clay plaster. Clay plaster shall comply with Sections BJ104.4.3.1 through BJ104.4.3.6.

  • CRC § 0.5 High relevance — show source text

    • Res4.

    • For installations that may be exposed to staining, specify tile and grout suitable for exposure. Consult product manufacturers; see also Notes and “Product Selection Guides.”

    Limitations

    • Duration of use as a steam shower or steam room as determined by membrane and cement backer board (when used) manufacturers; lower membrane water vapor permeance decreases water vapor transmission.

    • Maximum stud spacing 16" on center. Requirements

    • Steam rooms are highly specialized applications. Design and installation are critical to avoid damage to adjoining materials from vapor migration. Design criteria must include consideration of necessary insulation and temperature and humidity differential.

    • Use a tile contractor knowledgeable in steam applications and experienced with the materials specified.

    • Bonded waterproof membrane (sheet, liquid, and trowelon) must be continuous and must adequately limit vapor transmission into adjacent spaces and building materials, according to intended duration of use as a steam shower.

    • Steam showers designed for continuous use applications should specify a low perm waterproof membrane (a waterproof membrane meeting ANSI A118.10 and with a water vapor permeance rating of 0.5 perms or less when tested per ASTM E96 Procedure E, tested at 90% relative humidity). When a waterproof membrane with a water vapor permeance rating greater than 0.5 perms is specified, a vapor retarder behind the wall assembly is required, and vapor retarder must have a water vapor permeance rating of 0.1 perm or less when tested per ASTM E96 Procedure A, tested at 50% relative humidity. Consult waterproof membrane manufacturer for water vapor permeance rating and vapor retarder requirements.

    • Specifier shall indicate how waterproofing and vapor retarding is to be achieved, including details for membrane penetrations such as penetrations for plumbing, lighting fixtures, fasteners, etc. Specifier shall also indicate where and how to waterproof curbs and jambs and where membrane terminates. Area outside steam shower door is a wet area and should be treated accordingly.

    • Check with membrane manufacturer for suitability for applicable conditions, as not all membranes are suitable for steam, high temperature and/or chemical exposure.

    • Some waterproof membrane manufacturers require use of a vapor retarder membrane in addition to the waterproof membrane. Consult membrane manufacturer for requirements. When used, vapor retarder membrane must weather lap itself and lap into the shower pan membrane.

    • If vapor retarder membrane is required, integrated bonding flange cannot be used.

    • Follow waterproof membrane manufacturers’ directions for interface between drain and membrane(s).

    • Design professional to specify adequate insulation on walls and ceilings to reduce condensation. Consult insulation manufacturer for application suitability.

    • Seal all membrane penetrations with appropriate sealant according to membrane manufacturer’s requirements.

    482 2025 CALIFORNIA PLUMBING CODE

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

    STEAM SHOWER/ROOM; WOOD/METAL STUDS TCNA HANDBOOK

    • Specify slip joints at change in plane, such as where walls and ceiling meet.

    • Slope ceiling minimum 2" per foot to avoid condensate dripping onto occupants; sloping ceiling from center can minimize rundown on walls.

    • When mortar bed walls, studs must provide firm anchorage for metal lath.

    • When mortar bed walls, mortar bed wall thickness— [3] ⁄ 4 " minimum to 1 [1] ⁄ 2 " maximum.

    • When mortar bed walls, cut lath at all slip joints.

  • CRC § 0.625 High relevance — show source text

    875b| |Unit masonry|0.625b| |Cast-in-place or precast concrete|0.625b| |Two-coat work over:|| |Unit masonry|0.5b| |Cast-in-place or precast concrete|0.375b| |Terra cotta (anchored)|1| |Terra cotta (adhered)|0.25| |Vinyl siding|0.035| |Wood shingles|0.375| |Wood siding (without sheathing)a|0.5| |For SI: 1 inch = 25.4 mm, 1 ounce = 28.35 g, 1 square foot = 0.093 m2.
    a. Wood siding of thicknesses less than 0.5 inch shall be placed over sheathing that conforms to Section 2304.6.
    b. Exclusive of texture.
    c. As measured at the bottom of decorative grooves.
    d. 16 ounces per square foot for cold-rolled copper and lead-coated copper, 12 ounces per square foot for copper shingles, high-yield copper and lead-coated high-yield copper.|For SI: 1 inch = 25.4 mm, 1 ounce = 28.35 g, 1 square foot = 0.093 m2.
    a. Wood siding of thicknesses less than 0.5 inch shall be placed over sheathing that conforms to Section 2304.6.
    b. Exclusive of texture.
    c. As measured at the bottom of decorative grooves.
    d. 16 ounces per square foot for cold-rolled copper and lead-coated copper, 12 ounces per square foot for copper shingles, high-yield copper and lead-coated high-yield copper.|

    14-6 2025 CALIFORNIA BUILDING CODE

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

    EXTERIOR WALLS

    1404.3 Vapor retarders. Vapor retarder materials shall be classified in accordance with Table 1404.3(1). A vapor retarder shall be provided on the interior side of frame walls in accordance with Table 1404.3(2) and Tables 1404.3(3) or 1404.3(4) as applicable, or an approved design using accepted engineering practice for hygrothermal analysis. Vapor retarders shall be installed in accordance with Section 1404.3.2. The appropriate climate zone shall be selected in accordance with the California Energy Code .

    Exceptions:

    1. Basement walls.

    2. Below-grade portion of any wall.

    3. Construction where accumulation, condensation or freezing of moisture will not damage the materials.

    4. A vapor retarder shall not be required in Climate Zones 1, 2, and 3.

    5. In Climate Zones 4 through 8, a vapor retarder on the interior side of frame walls shall not be required where the assembly complies with Table 1404.3(5).

  • CRC § 7-8 High relevance — show source text

    The insulation materials used to satisfy
    this option also contribute to but do not supersede the thermal envelope requirements of Chapter 11.|

    7-8 2025 CALIFORNIA RESIDENTIAL CODE

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

    WALL COVERING

    TABLE R702.7(4)—CONTINUOUS INSULATION WITH CLASS I OR II RESPONSIVE VAPOR RETARDER Col2
    CLIMATE ZONE PERMITTED CONDITIONSa
    3 Continuous insulation with_R_-value ≥ 2.
    4, 5 and 6 Continuous insulation with_R_-value ≥ 3 over 2 × 4 wall.
    Continuous insulation with_R_-value ≥ 5 over 2 × 6 wall.
    7 Continuous insulation with_R_-value ≥ 5 over 2 × 4 wall.
    Continuous insulation with_R_-value ≥ 7.5 over 2 × 6 wall.
    8 Continuous insulation with_R_-value ≥ 7.5 over 2 × 4 wall.
    Continuous insulation with_R_-value ≥ 10 over 2 × 6 wall.
    a. The requirements in this table apply only to insulation used to control moisture in order to permit the use of Class II vapor retarders. The insulation materials used to satisfy
    this option also contribute to but do not supersede the thermal envelope requirements of Chapter 11.    		 a. The requirements in this table apply only to insulation used to control moisture in order to permit the use of Class II vapor retarders. The insulation materials used to satisfy
    this option also contribute to but do not supersede the thermal envelope requirements of Chapter 11.
    TABLE R702.7(5)—CONTINUOUS INSULATION ON WALLS WITHOUT A CLASS I, II OR III INTERIOR VAPOR RETARDERa Col2
    CLIMATE ZONE PERMITTED CONDITIONSb, c
    4 Continuous insulation with_R_-value ≥ 4.5
    5 Continuous insulation with_R_-value ≥ 6.5
    6 Continuous insulation with_R_-value ≥ 8.5
    7 Continuous insulation with_R_-value ≥ 11.5
    8 Continuous insulation with_R_-value ≥ 14
    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 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 in this table apply only to insulation used to control moisture in order to allow walls without a Class I, II or III interior vapor retarder. The insulation materi-
    als used to satisfy this option also contribute to but do not supersede the 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.
  • CRC § 6.2 High relevance — show source text

    R702.6.2 Furring strips. Where furring strips are used, they shall be 1 inch by 2 inches or 1 inch by 3 inches (25 mm by 51 mm or 25 mm by 76 mm), spaced a distance on center equal to the desired exposure, and shall be attached to the wall by nailing through other wall material into the studs.

    2025 CALIFORNIA RESIDENTIAL CODE 7-7

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

    WALL COVERING

    R702.7 Vapor retarders. Vapor retarder materials shall be classified in accordance with Table R702.7(1). A vapor retarder shall be provided on the interior side of frame walls of the class indicated in Table R702.7(2), including compliance with Table R702.7(3) or R702.7(4) where applicable. An approved design using accepted engineering practice for hygrothermal analysis shall be permitted as an alternative. Vapor retarders shall be installed in accordance with Section R702.7.2.

    The climate zone shall be determined in accordance with Section R702.7.3 and Table R702.7.3.

    Exceptions:

    1. Basement walls.

    2. Below-grade portion of any wall.

    3. Construction where accumulation, condensation or freezing of moisture will not damage the materials.

    4. A vapor retarder shall not be required in Climate Zones 1, 2 and 3.

    5. In Climate Zones 4 through 8, a vapor retarder shall not be required where the assembly complies with Table R702.7(5).

    TABLE R702.7(1)—VAPOR RETARDER MATERIALS AND CLASSES Col2
    CLASS ACCEPTABLE MATERIALS
    I Sheet polyethylene, nonperforated aluminum foil or other approved materials with a perm rating less than or equal to 0.1.
    II Kraft-faced fiberglass batts, vapor retarder paint or other approved materials applied in accordance with the manufacturer’s instal-
    lation instructions for a perm rating greater than 0.1 and less than or equal to 1.0.
    III Latex paint, enamel paint or other approved materials applied in accordance with the manufacturer’s installation instructions for a
    perm rating greater than 1.0 and less than or equal to 10.0.
    TABLE R702.7(2)—VAPOR RETARDER OPTIONS Col2 Col3 Col4
    CLIMATE ZONE VAPOR RETARDER CLASS VAPOR RETARDER CLASS VAPOR RETARDER CLASS
    CLIMATE ZONE CLASS Ia CLASS IIa CLASS III
    1, 2 Not Permitted Not Permitted Permitted
    3, 4 (except Marine 4) Not Permitted Permittedc Permitted
    Marine 4, 5, 6, 7, 8 Permittedb, c Permittedc See Table R702.7(3)
    a. A responsive vapor retarder shall be allowed on the interior side of any frame wall in all climate zones.
    b. In frame walls, use of a Class I vapor retarder that is not a responsive vapor retarder on the interior side with a Class I vapor retarder on the exterior side shall require an
    approved design.
    c.
  • CRC § 4.2 Medium relevance — show source text
    TABLE R702.4.2—BACKER BOARD MATERIALS Col2
    ** MATERIAL** ** STANDARD**
    Glass mat gypsum backing panel ASTM C1178
    Fiber-reinforced gypsum panels ASTM C1278
    Nonasbestos fiber-cement backer board ASTM C1288 or ISO 8336, Category C
    Nonasbestos fiber mat-reinforced cementitious backer units ASTM C1325

    R702.5 Other finishes. Wood veneer paneling and hardboard paneling shall be placed on wood or cold-formed steel framing spaced not more than 16 inches (406 mm) on center. Wood veneer and hard board paneling less than [1] / 4 -inch (6 mm) nominal thickness shall not have less than a [3] / 8 -inch (10 mm) gypsum board or gypsum panel product backer. Wood veneer paneling not less than [1] / 4 -inch (6 mm) nominal thickness shall conform to ANSI/HPVA HP-1. Hardboard paneling shall conform to CPA/ANSI A135.5.

    R702.6 Wood shakes and shingles. Wood shakes and shingles shall conform to CSSB Grading Rules for Wood Shakes and Shingles and shall be permitted to be installed directly to the studs with maximum 24 inches (610 mm) on-center spacing.

    R702.6.1 Attachment. Nails, staples or glue are permitted for attaching shakes or shingles to the wall, and attachment of the shakes or shingles directly to the surface shall be permitted provided the fasteners are appropriate for the type of wall surface material. Where nails or staples are used, two fasteners shall be provided and shall be placed so that they are covered by the course above.

    R702.6.2 Furring strips. Where furring strips are used, they shall be 1 inch by 2 inches or 1 inch by 3 inches (25 mm by 51 mm or 25 mm by 76 mm), spaced a distance on center equal to the desired exposure, and shall be attached to the wall by nailing through other wall material into the studs.

    2025 CALIFORNIA RESIDENTIAL CODE 7-7

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

    WALL COVERING

    R702.7 Vapor retarders. Vapor retarder materials shall be classified in accordance with Table R702.7(1). A vapor retarder shall be provided on the interior side of frame walls of the class indicated in Table R702.7(2), including compliance with Table R702.7(3) or R702.7(4) where applicable. An approved design using accepted engineering practice for hygrothermal analysis shall be permitted as an alternative. Vapor retarders shall be installed in accordance with Section R702.7.2.

    The climate zone shall be determined in accordance with Section R702.7.3 and Table R702.7.3.

    Exceptions:

    1. Basement walls.

    2. Below-grade portion of any wall.

    3. Construction where accumulation, condensation or freezing of moisture will not damage the materials.

    4. A vapor retarder shall not be required in Climate Zones 1, 2 and 3.

    5. In Climate Zones 4 through 8, a vapor retarder shall not be required where the assembly complies with Table R702.7(5).

  • 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.

  • CRC § 0.5 Medium relevance — show source text

    Notes

    • Test shower pan membrane/waterproof membrane and drainage fitting for leaks before commencing tilework.

    • Materials adversely affected by moisture in areas immediately adjacent to showers, tubs, and roman tubs should be properly protected.

    • All horizontal surfaces, for example shower seats, sills, curbs, etc., must slope towards drain or other surface sloped toward drain. Where present, waterproofing also must be sloped.

    • When glass tile is used, see “Glass Tile Selection and Installation Guide,” and consult manufacturer for recommendations and requirements.

    • Follow applicable plumbing and building codes.

    2025 CALIFORNIA PLUMBING CODE 479

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

    TCNA HANDBOOK COMMON SHOWER CONFIGURATIONS

    480 2025 CALIFORNIA PLUMBING CODE

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

    COMMON SHOWER CONFIGURATIONS TCNA HANDBOOK

    2025 CALIFORNIA PLUMBING CODE 481

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

    TCNA HANDBOOK STEAM SHOWER/ROOM; WOOD/METAL STUDS

    Recommended Uses

    • For steam showers and steam rooms framed with wood or metal studs. Specify mortar bed walls option when wall flatness is critical, such as when tiles with any edge longer than 15" are specified. Environmental Exposure Classifications

    • Res4.

    • For installations that may be exposed to staining, specify tile and grout suitable for exposure. Consult product manufacturers; see also Notes and “Product Selection Guides.”

    Limitations

    • Duration of use as a steam shower or steam room as determined by membrane and cement backer board (when used) manufacturers; lower membrane water vapor permeance decreases water vapor transmission.

    • Maximum stud spacing 16" on center. Requirements

    • Steam rooms are highly specialized applications. Design and installation are critical to avoid damage to adjoining materials from vapor migration. Design criteria must include consideration of necessary insulation and temperature and humidity differential.

    • Use a tile contractor knowledgeable in steam applications and experienced with the materials specified.

    • Bonded waterproof membrane (sheet, liquid, and trowelon) must be continuous and must adequately limit vapor transmission into adjacent spaces and building materials, according to intended duration of use as a steam shower.

    • Steam showers designed for continuous use applications should specify a low perm waterproof membrane (a waterproof membrane meeting ANSI A118.10 and with a water vapor permeance rating of 0.5 perms or less when tested per ASTM E96 Procedure E, tested at 90% relative humidity). When a waterproof membrane with a water vapor permeance rating greater than 0.5 perms is specified, a vapor retarder behind the wall assembly is required, and vapor retarder must have a water vapor permeance rating of 0.1 perm or less when tested per ASTM E96 Procedure A, tested at 50% relative humidity. Consult waterproof membrane manufacturer for water vapor permeance rating and vapor retarder requirements.

  • CRC § 6.1 Medium relevance — show source text

    BJ105.6.1 Water-resistive barriers and vapor permeance ratings. Plastered bale walls shall be constructed without any membrane barrier between straw and plaster to facilitate transpiration of moisture from the bales, and to secure a structural bond between straw and plaster, except as permitted or required elsewhere in this appendix. Where a water-resistive barrier is

    APPENDIX BJ-10 2025 CALIFORNIA RESIDENTIAL CODE

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    APPENDIX BJ — STRAWBALE CONSTRUCTION

    placed behind an exterior finish, it shall have a vapor permeance rating of not less than 5 perms, except as permitted or required elsewhere in this appendix.

    BJ105.6.2 Interior vapor retarders. Wall finishes shall have an equivalent vapor permeance rating of a Class III vapor retarder on the interior side of exterior strawbale walls in Climate Zones 5, 6, 7, 8 and Marine 4, as defined in Chapter 11. Bale walls enclosing showers or steam rooms shall be protected on the interior side by a Class I or Class II vapor retarder.

    BJ105.6.3 Penetrations in exterior strawbale walls. Penetrations in exterior strawbale walls shall be sealed with an approved sealant or gasket on the exterior side of the wall in all climate zones. Penetrations and joints at the floor and ceiling shall be sealed on the interior side of the wall in Climate Zones 5, 6, 7, 8 and Marine 4, as defined in Chapter 11.

    BJ105.6.4 Horizontal surfaces. Bale walls and other bale elements shall be provided with a water-resistant barrier at weatherexposed horizontal surfaces. The water-resistant barrier shall be of a material and installation that will prevent water from entering the wall system. Horizontal surfaces shall include exterior window sills, sills at exterior niches and buttresses. Horizontal surfaces shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain away from bale walls and elements. Where the water-resistant barrier is below the finish material, it shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain to the outside surface of the bale wall’s vertical finish.

    BJ105.6.5 Separation of bales and concrete. A sheet- or liquid-applied Class II vapor retarder shall be installed between bales and supporting concrete or masonry. The bales shall be separated from the vapor retarder by not less than [3] / 4 inch (19.1 mm), and that space shall be filled with an insulating material such as wood or rigid insulation, or a material that allows vapor dispersion such as gravel, or other approved insulating or vapor dispersion material. Sill plates shall be installed at this interface in accordance with Section BJ105.3. Where bales abut a concrete or masonry wall that retains earth, a Class II vapor retarder shall be provided between such wall and the bales.

    BJ105.6.6 Separation of bales and earth. Bales shall be separated from earth by not less than 8 inches (203 mm).

    BJ105.6.7 Separation of exterior plaster and earth. Exterior plaster applied to straw bales shall be located not less than 6 inches (152 mm) above earth or 3 inches (76 mm) above paved areas.

Frequently asked questions

Do I always need a vapor retarder in California homes?

Not always. Climate Zones 1–3 do not require an interior vapor retarder. For Climate Zones 4–8 you may be able to omit one if the wall meets the continuous‑insulation options in Table R702.7(5); otherwise select the class allowed by Table R702.7(2) (§ R702.7) .

What permeance makes a material Class II?

Class II materials have a permeance > 0.1 and ≤ 1.0 perms, per Table R702.7(1) (§ R702.7) .

Can painted drywall act as a vapor retarder?

Painted drywall is typically a Class III material (per Table R702.7(1)), but Class III is only permitted in certain zones and conditions (see Table R702.7(2) and the Class III conditions in Table R702.7(3)) — don’t assume paint alone is sufficient without checking the tables (§ R702.7) .

Is spray foam always a vapor retarder?

No. The code provides a special allowance: spray foam with ≤ 1.5 perms at installed thickness can be treated as meeting certain continuous‑insulation moisture‑control conditions, but spray foams vary in permeance and must meet the R‑value conditions in § R702.7.1 to qualify .

Where do I find which climate zone applies to my project for these rules?

Use § R702.7.3 and Table R702.7.3 to determine the climate zone mapping used by this section (note: this table compares IECC vs California Energy Code zones) (§ R702.7.3) .

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