Hempcrete Building Codes:
Appendix BL Explained
The 2024 International Residential Code includes Appendix BL for hemp-lime construction. That is a major code-development milestone—but it does not mean hempcrete is automatically approved under the same prescriptive rules everywhere in the United States. This guide explains what Appendix BL covers, what it does not cover, and how to check the code path for a real project.
Is hempcrete in the U.S. building code?
Yes. Hemp-lime construction has its own appendix in the 2024 IRC. But Appendix BL is a model-code appendix, and IRC Section R101.2.1 says appendix provisions do not apply unless they are specifically adopted.
What Is Appendix BL?
Appendix BL — Hemp-Lime (Hempcrete) Construction is part of the 2024 International Residential Code. It creates a dedicated residential code framework for hemp-lime used as a nonstructural building material and wall-infill system.
Before a dedicated appendix existed, many U.S. hemp-lime projects had to rely heavily on alternative-material approval, project-specific engineering, testing, or other documentation acceptable to the building official. Appendix BL gives jurisdictions a model set of provisions they can adopt instead of building every review from scratch.
Appendix BL addresses materials, wall construction, framing relationships, moisture detailing, finishes, thermal performance, fire-related requirements, mechanical performance, and referenced standards.
Why “Hempcrete Is Legal in 49 States” Is Misleading
The IRC is a model code. State and local governments decide which edition, amendments, and optional appendices become enforceable.
Hempcrete is approved in 49 states because it is in the IRC.
The 2024 IRC includes Appendix BL, but Section R101.2.1 states that appendix provisions do not apply unless specifically adopted. A jurisdiction can adopt the 2024 IRC without BL, adopt BL with amendments, or remain on an older code edition.
Published
Appendix BL exists in the 2024 model IRC and can be referenced as a recognized framework.
Adopted
A state or local jurisdiction must put the appendix into its enforceable code for the BL prescriptive pathway to apply directly.
Alternative approval
Even where BL is not adopted, a project may still have an engineered or alternative-material approval path.
Four Questions Before You Say “Code Approved”
Which code edition?
Determine which residential code edition is currently adopted for the project address.
Was BL adopted?
Read the jurisdiction’s adopting ordinance, amendments, or appendix schedule.
Does the project fit BL?
Check structure, seismic category, height, density, wall design, materials, finishes, and other limits.
If not, what path?
Ask whether engineered design, alternative-material approval, testing, or another code route is accepted.
What Appendix BL Actually Covers
The appendix establishes a bounded prescriptive pathway. It is not a universal authorization for every hemp-lime building or wall system.
Nonstructural infill
BL governs hemp-lime as nonstructural material installed between or around wall framing. The frame carries the structural loads.
Defined density range
The general construction provisions use a hemp-lime infill density range of 12.5 to 25 lb/ft³ (about 200 to 400 kg/m³).
Prescriptive building limits
The standard BL prescriptive provisions limit buildings using hemp-lime infill to no more than one story above grade plane and a maximum building height of 25 feet.
Seismic design can trigger engineering
Appendix BL requires registered-design-professional engineering for specified higher seismic design categories.
Wall weight matters
Prescriptive provisions cap average unit wall weight, so wall thickness cannot be separated from density and finish weight.
Materials must match evidence
Hurd, binder, water, and additives are tied to specifications represented by approved testing. “Hemp + lime” is not a complete code specification.
What a Plan Reviewer Will Actually Care About
Appendix BL connects the material to framing, bracing, moisture control, finishes, thermal performance, fire provisions, and testing.
| Topic | Why it matters | What to document |
|---|---|---|
| Hemp hurd | Hurd quality must correspond to approved test samples and be sufficiently free of dust and fiber clumps. | Supplier specification and project mix documentation. |
| Binder | The binder system is part of the tested/approved material combination. | Binder manufacturer, formulation, proportions, additives, test evidence. |
| Density | Density affects wall weight and the tabulated thermal resistance. | Target density, project samples, measurement method and results. |
| Framing | Hemp-lime is infill; the frame remains responsible for structural loads. | Framing drawings, anchorage, connections, bracing. |
| Moisture | Water management is integrated into the appendix. | Foundation separation, flashing, openings, penetrations, finishes. |
| Thermal | BL provides density-based unit R-values and allows approved lab testing. | Density, thickness, R-value path, energy-code calculations. |
| Fire | The 2024 appendix does not give every wall a generic fire-resistance rating. | Required flame/smoke evidence and rated-assembly testing where needed. |
Hemp-Lime Infill Does Not Replace Structural Bracing
Appendix BL’s prescriptive bracing language is one reason a builder should not treat hemp-lime as ordinary cavity insulation. The hemp-lime infill is not the shear-wall system. Framing and bracing still have to be designed and documented as the code requires.
Projects outside the prescriptive conditions may move to an engineered design.
Does Appendix BL Give Hempcrete a Fire Rating?
This is one of the most important places to distinguish the 2024 appendix from broader claims about hemp-lime fire performance.
2024 Appendix BL
The 2024 language states that hemp-lime walls do not automatically have a fire-resistance rating. Where a rating is required, the wall assembly must establish it through an applicable approved testing route such as ASTM E119 or UL 263.
Flame / smoke is separate
The appendix also coordinates hemp-lime infill with flame-spread and smoke-developed requirements for insulation materials. That is not the same as assigning a one-hour or two-hour wall rating.
Appendix BL R-Values Are Density Dependent
The code does not assign one universal R-value per inch to every hemp-lime mix.
| Density | 2024 BL unit R-value | Interpretation |
|---|---|---|
| 12.5 lb/ft³ | R-2.10 per inch | Lowest tabulated density and highest tabulated unit R-value. |
| 15 lb/ft³ | R-1.86 per inch | Intermediate density. |
| 20 lb/ft³ | R-1.54 per inch | Denser mix, lower tabulated unit R-value. |
| 25 lb/ft³ | R-1.20 per inch | Highest tabulated density and lowest tabulated unit R-value. |
Hempcrete Permit Checklist
Use this before ordering hurd, binder, or committing to a wall assembly.
What Adoption Can Look Like in Practice
These examples show why a national yes/no answer is unreliable.
Austin, Texas
Austin adopted Appendix BL with its 2024 technical-code updates in 2025, creating a local code pathway for hemp-lime construction.
California
California’s triennial rulemaking record states that HCD did not adopt Appendix BL statewide. Seeing the appendix printed in a code publication does not itself prove statewide enforceability.
Washington
Washington’s 2026 rulemaking makes Appendix BL available for local adoption, with the provisions not mandatory unless specifically referenced by the adopting jurisdiction.
Hempcrete Building Code Questions
Is hempcrete legal in the United States?
There is no single nationwide residential permitting answer. Hemp-lime now has a model-code pathway in 2024 IRC Appendix BL, but local enforceability depends on the adopted code and amendments. Projects may also be approved through alternative-material or engineered routes where BL itself is not adopted.
Does every state using the IRC automatically allow Appendix BL?
No. IRC Section R101.2.1 states that appendix provisions do not apply unless specifically adopted.
Is hempcrete structural under Appendix BL?
No. Appendix BL treats hemp-lime as a nonstructural material and wall-infill system. Structural loads are carried by the framing system.
Can I build a two-story hempcrete house prescriptively under 2024 Appendix BL?
The standard BL prescriptive limitations include no more than one story above grade plane and a maximum height of 25 feet. A design outside the prescriptive path may require an approved engineered design.
What R-value does Appendix BL give hempcrete?
It is density-dependent: R-2.10 per inch at 12.5 lb/ft³, R-1.86 at 15 lb/ft³, R-1.54 at 20 lb/ft³, and R-1.20 at 25 lb/ft³. Approved laboratory testing of a specific mix is also allowed.
Does 2024 Appendix BL give hempcrete a one-hour fire rating?
Not automatically. The 2024 appendix says hemp-lime walls do not have a generic fire-resistance rating and directs rated assemblies to approved testing such as ASTM E119 or UL 263. Future 2027 IRC provisions are separate.
What if my city has not adopted Appendix BL?
Ask whether the building department accepts an alternative-material approval, engineered design, testing package, or another code-compliance route.
Sources Used for This Guide
Use these sources to verify the current edition and local adoption rather than relying on a static blog post alone.
Planning a Hemp-Lime Project?
Confirm the locally adopted code before treating Appendix BL as the permit path. Coordinate the building department, designer, material supplier, and required testing before ordering material or finalizing wall details.