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Appendix BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

Section BL103 — HEMP-LIME CONSTRUCTION

2025 California Residential Code (Title 24, Part 2.5) · 2025 edition · updated 2026-07-27 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

BL103.1 General.

Hemp-lime construction shall be limited to the nonstructural, solid infill mix of hemp hurd and its binder between or around structural and nonstructural wall framing. Hemp-lime infill shall have a density ranging from 12.5 lb/ft [3] to 25 lb/ft [3] (200 kg/m [3] to 400 kg/m [3] ). Hemp-lime walls shall be designed and constructed in accordance with Section BL103 and with Figures BL103.1(1) through BL103.1(4) or an approved alternative design.

FIGURE BL103.1(1)—TYPICAL HEMP-LIME WITH INTERIOR STUD FRAMING

FIGURE BL103.1(2)—TYPICAL HEMP-LIME WITH CENTER STUD FRAMING

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APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

FIGURE BL103.1(3)—TYPICAL HEMP-LIME WITH EXTERIOR STUD FRAMING

FIGURE BL103.1(4)—TYPICAL HEMP-LIME WITH DOUBLE STUD WALL FRAMING

BL103.2 Materials.

Materials to be used in hemp-lime construction shall be in accordance with Sections BL103.2 through BL103.2.3.

BL103.2.1 Hemp hurd. Hemp hurd shall match the specifications of the approved test samples in Sections BL106.3 and BL107.1. Hemp hurd shall be substantially free from dust and fiber clumps such that the installed hemp-lime maintains its integrity.

BL103.2.2 Binders. Acceptable binders, singular or in combination, include hydraulic lime, hydrated lime, pozzolans, natural cements or other binders that match the specification of the approved test samples in Sections BL106.3 and BL107.1.

BL103.2.3 Water and water additives. Water and any water additives shall match the specifications of the approved test samples in Sections BL106.3 and BL107.1.

BL103.3 Structure.

The structure of buildings using hemp-lime infill shall be in accordance with this code and Sections BL103.3.1 through BL103.3.9 or with an approved engineered design by a registered design professional.

BL103.3.1 Limitations and requirements for buildings using hemp-lime infill. Buildings using hemp-lime infill shall be subject to the following limitations and requirements:

  1. Number of stories: not more than one story above grade plane.
  2. The building height shall not be more than 25 feet (7620 mm).

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APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

  1. Braced wall panel lengths: in accordance with Sections R602.10.3 and BL103.3.2.
  2. Unit wall weight: Hemp-lime walls shall not exceed an average unit wall weight of 65 psf (317 kg/m [2] ).

BL103.3.2 Bracing. Bracing for buildings with hemp-lime infill in Seismic Design Categories A, B and C shall be in accordance with Section R602.10 and in accordance with the following. Walls with hemp-lime infill shall use Method LIB and shall not be braced with solid sheathing. Hemp-lime infill walls utilizing Method LIB shall not require gypsum board to be installed and shall use the minimum braced wall lengths listed in Section R602.10. Adjustment factors in Table R602.10.3(4) shall be used as applicable. Alternatively, hemp-lime infill walls shall comply with Section R301.1. Walls or wall sections without hemp-lime infill shall be permitted to use any bracing method allowed in Section R602.10.

BL103.3.3 Connection of light-frame walls to hemp-lime walls. Light-frame walls perpendicular to, or at an angle to, a hemplime wall assembly shall be fastened to the hemp-lime wall in accordance with Section R602 or R603.

BL103.3.4 Hemp-lime thickness. Hemp-lime infill shall be not less than 3 inches (76 mm) thick between face of framing and finish. Maximum hemp-lime wall thickness is limited by the average unit wall weight limit of 65 psf (317 kg/m [2] ) in Section BL103.3.1, Item 4.

BL103.3.5 Contact with structural metal. Structural metal members and components in contact with hemp-lime shall be protected in accordance with Section BL103.4.

BL103.3.6 Contact with wood members. Hemp-lime shall be permitted to be in contact with untreated wood members.

BL103.3.7 Openings in walls. Doors, windows and similar openings in hemp-lime walls shall be in accordance with the following:

  1. Rough framing for doors and windows shall be part of, or be fastened to, the wall framing in accordance with this code.

  2. An approved water-resistive barrier shall be installed at openings in hemp-lime walls in accordance with Sections BL103.7.4 and BL104.5.1.

  3. Header size and their maximum span above openings in bearing walls with hemp-lime infill shall be determined with Tables R602.7(1) and BL103.3.7 or an approved design by a registered design professional.

  4. Cast-in-place hemp-lime over and overhanging the face of a header more than 3 inches (76 mm) shall require an approved design of its support by a registered design professional.

  5. Hemp-lime blocks overhanging headers shall require an approved design of their support by a registered design professional.

TABLE BL103.3.7—ALLOWABLE HEADER SPAN MULTIPLIERa

WALL HEIGHT ABOVE HEADER UNIT WALL WEIGHT (psf) UNIT WALL WEIGHT (psf) UNIT WALL WEIGHT (psf) UNIT WALL WEIGHT (psf)
WALL HEIGHT ABOVE HEADER 15 30 45 65
1′-0″ 1.00 1.00 1.00 1.00
1′-6″ 1.00 1.00 0.90 0.90
2′-0″ 1.00 0.90 0.90 0.85
2′-6″ 1.00 0.90 0.90 0.85
3′-0″ 1.00 0.90 0.90 0.80
For SI: 1 foot = 304.8 mm, 1 pound per square foot = 4.882 kg/m2.
a. Multiply the maximum allowable spans from Table R602.7(1) by the applicable factor to determine the adjusted maximum allowable header span.

BL103.3.8 Voids. Voids shall be filled with hemp-lime or other approved material before application of finish.

BL103.3.9 Anchorage of hemp-lime. Hemp-lime for interior and exterior stud walls shall be anchored or shall be in accordance with an approved design by a registered design professional. Horizontal anchorage rails shall be installed at not more than 24 inches (610 mm) on center and in accordance with Figures BL103.1(1) and BL103.1(3). Horizontal anchorage rails shall be no less than 1 inch by 2 inches (25 mm by 51 mm). Anchorage rails shall be wood, metal per Section BL103.4, or other approved material. Anchorage rails should be attached to the side of the stud facing the interior of the wall with one 8d box nail to each stud and run the entire length of the wall.

BL103.4 Contact with metal.

Metal in contact with hemp-lime shall be stainless steel or primed and painted with a coating in accordance with Section BL103.4.1.

BL103.4.1 Protective coatings. Metal shall be painted with an epoxy, oil, bituminous paint or other approved coating. Waterbased paints shall not be used.

Exception: Heads of pneumatically driven hot-dip galvanized nails.

BL103.5 Mechanical, electrical and plumbing in hemp-lime infill.

Electrical and telecommunication wiring, panels and boxes, mechanical ducts, plumbing pipes and other mechanical, electrical and plumbing components in or in contact with hemp-lime infill shall be isolated in sleeves, pipes, conduits or tubing made of plastic, or of metal in accordance with Section BL103.4, or separated from hemp-lime with an approved alkaline-resistant material.

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APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

BL103.6 Hemp-lime installation methods.

Hemp-lime shall be installed in accordance with Sections BL103.6.1 and BL103.6.2, and one of Sections BL103.6.3 through BL103.6.7.

BL103.6.1 Mix and mixing. The materials and ratio of hemp hurd to binder to water shall match the specifications of the approved test samples in Sections BL106.3 and BL107.1. The water-to-binder ratio shall be not less than 1:1 and not greater than 2:1 by weight or by binder manufacturer’s recommendations. The hemp hurd, binder and water shall be thoroughly and uniformly mixed by manual or mechanical means.

BL103.6.2 Formwork for hand-cast and spray-applied methods. Forms shall be removable or permanent and shall not deform under the lateral pressure of the installed wet hemp-lime.

BL103.6.2.1 Permanent forms. Permanent forms shall be permitted to be installed on only one side. Permanent forms shall be reed mats or other approved materials with an open weave. Sheet materials shall not be used as permanent forms. Permanent forms remain after curing as a finish or substrate for another finish.

Exception: Permanent forms of any material shall be permitted at the jambs, heads and sills of openings.

BL103.6.2.2 Removable forms. Removable forms shall be removed within 24 hours after hemp-lime placement or per the binder manufacturer’s specifications.

Exception: Removable forms temporarily supporting hemp-lime infill above wall openings shall not be removed for a minimum of 3 days or per binder manufacturer’s specifications.

BL103.6.3 Hand-cast. Hand-cast hemp-lime infill shall be installed in uniform lifts not greater than 4 inches (102 mm) in height. Each lift shall be tamped to achieve stable walls free of voids.

BL103.6.4 Spray-applied. Spray-applied hemp-lime infill shall be installed in accordance with Sections BL103.6.4.1 through BL103.6.4.4.

BL103.6.4.1 Forms. Forms shall be installed on one side in accordance with Section BL103.6.2 or BL103.6.7.2 for lined applications.

BL103.6.4.2 Mixing. Mixing shall be in accordance with Section BL103.6.1 or the spray equipment manufacturer’s instructions.

BL103.6.4.3 Installation. Hemp-lime shall be sprayed from the base up and per the spray equipment manufacturer’s and/or binder manufacturer’s instructions.

BL103.6.4.4 Screeding. Excess hemp-lime shall be removed by screeding per the spray equipment manufacturer’s and/or binder manufacturer’s instructions.

BL103.6.5 Precast blocks. Precast hemp-lime blocks shall be cast and installed in accordance with Sections BL103.6.5.1 through BL103.6.5.5 or per manufacturer’s specifications.

BL103.6.5.1 Block dimensions. Hemp-lime blocks shall be a minimum thickness of 3 inches (76 mm) in all dimensions and shall not exceed the maximum thickness in accordance with Section BL103.3.4.

BL103.6.5.2 Casting. Hemp-lime blocks shall be cast in accordance with Sections BL103.6.1 through BL103.6.6, as applicable, or by other means that produce approved blocks.

BL103.6.5.3 Mortar. Mortar shall consist of lime and sand or other aggregate with a ratio of not less than 1:1 and not greater than 1:3, or other approved mortar. The lime shall be hydrated Type N or S, or hydraulic lime.

BL103.6.5.4 Installation. Hemp-lime blocks shall be installed in a running bond between and around wall framing members. Mortar shall fill all voids between blocks and shall be not less than [1] / 8 inch (3 mm) thick. Spaces between blocks and framing shall be not more than [3] / 4 inch (19 mm) and shall be filled with mortar. BL103.6.5.5 Hemp-lime block veneer. Hemp-lime block veneer shall not exceed 50 psf (244 kg/m [2] ) of veneer only unit wall weight, shall be limited to 5-inch (127 mm) thickness, and shall be anchored to the supporting wall studs in accordance with Section R703.8.4 or secured with approved ties and fasteners to an approved backing. Metal ties and fasteners shall be protected in accordance with Section BL103.4.

BL103.6.6 Hemp-lime panels. Hemp-lime panels shall require an approved design by a registered design professional.

BL103.6.7 Lined application. Interior and exterior hemp-lime lined applications shall be installed in accordance with Sections BL103.6.7.1 through BL103.6.7.6 and Sections BL103.6.3 through BL103.6.6, as applicable.

BL103.6.7.1 General. Prior to installation, the concrete or masonry walls receiving the installation shall be clean and free of loose mortar. Lined installations on basement walls shall require an approved design by a registered design professional. Exterior applications shall be in accordance with Section BL103.7.6. Attachment of precast blocks to the receiving wall shall be in accordance with Section BL103.6.5.5. Attachment of hemp-lime panels to the receiving wall shall be in accordance with Section BL103.6.6.

BL103.6.7.2 Formwork. Forms shall be in accordance with Section BL103.6.2. Permanent formwork shall not be allowed on the nonreceiving wall side. BL103.6.7.3 Thin lining. Thin linings are from 3 inches to 4 [3] / 4 inches (76 mm to 121 mm) thick. Hand troweled hemp-lime shall be installed over a bonding coat. BL103.6.7.4 Medium lining. Medium linings exceed 4 [3] / 4 inches (121 mm) and are not greater than 6 [1] / 2 inches (165 mm) thick. For hand-cast or spray-applied, 1 [1] / 2 -inch by 1 [1] / 2 -inch (38 mm × 38 mm) dovetail-shaped vertical anchorage rails shall be attached with the narrowest face to the receiving wall, spaced not less than 20 inches (508 mm) and not greater than 32 inches

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APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

(813 mm), with fasteners not less than 2 feet (610 mm) and not greater than 3 feet (914 mm) apart. Hand-cast medium linings shall be installed over a bonding coat on the receiving wall. See Figure BL103.6.7.4.

FIGURE BL103.6.7.4—TYPICAL HEMP-LIME MEDIUM LINING

BONDING COAT, WHERE OCCURS, PER SECTION BL103.6.7.4
WALL WALL

VERTICAL ANCHORAGE RAILS

PER SECTION BL103.6.7.4

HEMP-LIME INFILL PER SECTIONS

BL103.2 AND BL103.6

BL103.6.7.5 Thick lining. Thick linings exceed 6 [1] / 2 inches (165 mm) and shall not be greater than 8 inches (203 mm) thick or per the binder manufacturer’s specifications. For hand-cast or spray-applied, 1 [1] / 2 -inch by 2 [1] / 2 -inch (38 mm × 64 mm) vertical anchorage rails shall be attached with the 2 [1] / 2 -inch (64 mm) face parallel to the receiving wall and spaced not less than 20 inches (508 mm) and not greater than 32 inches (813 mm). The anchorage rails shall be fastened to and separated from the receiving wall with 2-inch (51 mm) spacers not less than 3 feet (914 mm) and not greater than 4 feet (1219 mm) apart. Handcast thick linings shall be installed over a bonding coat on the receiving wall. See Figure BL103.6.7.5.

FIGURE BL103.6.7.5—TYPICAL HEMP-LIME THICK LINING

BONDING COAT, WHERE

BL103.6.7.6 Minimum thickness at anchorage rails. The minimum thickness of hemp-lime between the exterior face of vertical anchorage rails and the finished face of hemp-lime shall be 3 inches (76 mm) or in accordance with the binder manufacturer’s specifications.

BL103.7 Moisture control.

Hemp-lime assemblies shall be protected from water intrusion and damage in accordance with Sections BL103.7.1 through BL103.7.9.

BL103.7.1 Water-resistive barriers. Water-resistive barriers are prohibited on hemp-lime walls, except as permitted or required elsewhere in this appendix.

BL103.7.2 Vapor retarders. Vapor retarders are prohibited on hemp-lime walls, except as permitted or required elsewhere in this appendix.

BL103.7.3 Penetrations in hemp-lime walls. Penetrations in exterior hemp-lime walls shall be sealed with an approved sealant or gasket on the exterior side of the wall in all climate zones and on the interior side of the wall in Climate Zones 5, 6, 7, 8 and Marine 4, as defined in Chapter 11.

BL103.7.4 Horizontal surfaces. Hemp-lime walls and other hemp-lime assemblies shall be provided with a water-resistive barrier at weather-exposed horizontal surfaces. The water-resistive barrier shall be of a material and installation that will prevent water from entering the wall system. Horizontal surfaces shall include exterior window sills and sills at exterior niches. Horizontal surfaces shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain away from hemp-lime

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APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

walls and other assemblies. Where the water-resistive 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 exterior surface of the hemp-lime wall’s vertical finish.

BL103.7.5 Air barrier. Exterior hemp-lime walls shall have a vapor-permeable air barrier on all exterior and interior surfaces, except as permitted or required elsewhere in this appendix. Plaster in accordance with Section BL104.3 shall be acceptable as an air barrier.

BL103.7.6 Separation of hemp-lime and earth or paved areas. Hemp-lime shall be not less than 8 inches (203 mm) above exposed earth or paved areas.

BL103.7.7 Separation of exterior plaster and earth or paved areas. Exterior plaster applied to hemp-lime shall be not less than 8 inches (203 mm) above exposed earth or paved areas.

BL103.7.8 Separation of hemp-lime and exterior plaster from foundation. Hemp-lime and exterior plaster shall be separated from the foundation with an approved moisture barrier.

BL103.7.9 Base of wall flashing. The outer face of exterior walls shall be flashed to prevent water intrusion at the base of the wall.

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▸ Contents — 2025 California Residential Code (Title 24, Part 2.5)
2025 California Residential Code (Title 24, Part 2.5)
  1. Chapter 1 — ADMINISTRATION
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — BUILDING PLANNING
  4. Chapter 4 — FOUNDATIONS
  5. Chapter 5 — FLOORS
  6. Chapter 6 — WALL CONSTRUCTION
  7. Chapter 7 — WALL COVERING
  8. Chapter 8 — ROOF-CEILING CONSTRUCTION
  9. Chapter 9 — ROOF ASSEMBLIES
  10. Chapter 10 — CHIMNEYS AND FIREPLACES
  11. Chapter 44 — REFERENCED STANDARDS
  12. Appendix AA — BOARD OF APPEALS
  13. Appendix AB — PERMIT FEES
  14. Appendix AC — RESERVED
  15. Appendix BA — RESERVED
  16. Appendix BB — TINY HOUSES
  17. Appendix BC — RESERVED
  18. Appendix BD — RESERVED
  19. Appendix BE — RADON CONTROL METHODS
  20. Appendix BF — PATIO COVERS
  21. Appendix BG — SOUND TRANSMISSION
  22. Appendix BH — AUTOMATIC VEHICULAR GATES
  23. Appendix BI — LIGHT STRAW-CLAY CONSTRUCTION
  24. Appendix BJ — STRAWBALE CONSTRUCTION
  25. Appendix BK — COB CONSTRUCTION (MONOLITHIC ADOBE)
  26. ▸ Appendix BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION
    Overview
    1. Section BL101 — GENERAL
    2. Section BL102 — DEFINITIONS
    3. Section BL103 — HEMP-LIME CONSTRUCTION
    4. Section BL104 — FINISHES
    5. Section BL105 — FIRE RESISTANCE
    6. Section BL106 — THERMAL PERFORMANCE
    7. Section BL107 — MECHANICAL PERFORMANCE
    8. Section BL108 — REFERENCED STANDARDS
  27. Appendix BM — 3D-PRINTED BUILDING CONSTRUCTION
  28. Appendix BN — EXTENDED PLATE WALL CONSTRUCTION
  29. Appendix BO — EXISTING BUILDINGS AND STRUCTURES
  30. Appendix CA — RESERVED
  31. Appendix CB — RESERVED
  32. Appendix CC — RESERVED
  33. Appendix CD — RESERVED
  34. Appendix CE — RESERVED
  35. Appendix CF — RESERVED
  36. Appendix CG — NONSEWERED SANITATION SYSTEMS
  37. Appendix CH — PRIVATE SEWAGE DISPOSAL
  38. Appendix CI — SWIMMING POOL SAFETY ACT
  39. Appendix CJ — EMERGENCY HOUSING
  40. Appendix CK — AREAS PROTECTED BY THE FACILITIES OF THE CENTRAL…

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