Hemp-lime (hempcrete): From hemp to green building materials

1. What is Hemp?

Industrial hemp (Cannabis sativa L.) is grown for bast fibers and shiv/hurd (cellulose-rich wood core). In the hemp-lime system, shiv plays the main role: porous – lightweight – vapor permeable structure, creating the foundation for heat insulation and moisture cushioning; When used as bioaggregates with lime-based binders, the mixture achieves low thermal conductivity and high porosity. The recent review also emphasizes the role of handling and classifying shiv seeds, affecting the mechanical-thermal-moisture effects of the product [2][7].

2. The process of making hemp-lime (hempcrete)

Composition & Grade. Typical hemp-lime consists of shiv + natural hydraulic lime (NHL) or lime–cement + water. Thanks to the ~70–80% porosity, the dry thermal conductivity λ ≈ 0.05–0.12 W·m⁻¹·K (density dependent, humidity, binder type) – significantly lower than regular brick/concrete walls [2][3]. Vapor permeability – moisture buffering is good, contributing to the reduction of condensation and mold [2][6]. However, due to the low density of knitting tissue, hemp-lime is not used as a load-bearing component; it is a fill wall/envelope combined with an independent bearing frame (BTCT/steel/wood), in accordance with the spirit of Appendix BL [1][2].

Construction & finishing. Hemp-lime can be poured in place around the frame, lightly compacted in formwork, or prefabricated block/panel; The finish should be a vapor-permeable lime mortar/mineral paint, avoiding a vapor-tight film that causes "moisture lock". Experimental-simulation shows low thermal diffusivity and moisture stability of hemp-lime walls when the enclosure details are designed correctly [5][6].

3. Why "from hemp" to "green building materials"?

• Outstanding heat-moisture performance.

Thanks to its low λ and capillary structure, the hemp-lime wall soothes day/night thermal fluctuations and has a high MBV, reducing the risk of condensation-mold in hot, humid climates [2][3][6]. Simulations of buildings in temperate/cool climates show that hemp-lime reduces the risk of mold and improves indoor environmental quality; The impact on operating energy depends on the enclosure configuration (thickness,finish) and HVAC system [5][6].

• Life cycle potential – emissions.

Hemp-lime is often seen as a carbon storage material thanks to the bio-CO₂ in hemp biomass and the carbonation of the binder during the use phase. However, "carbon negative" is conditional: LCA 2024 shows that the hemp:binder ratio, energy source, transport, and carbon biogenic calculation standard determines the carbon footprint; Some profiles reach neutral/negative, while a dominant binder configuration may lose its advantage [4].

• Normalized signals.

IRC 2024 – Appendix BL officially includes hemp-lime in the housing code as a non-load-bearing filler wall, regulating the scope of application (scale, seismic, climate) and detailed requirements, paving the way for practical implementation at the scale of low-rise buildings [1].

Access: https://www.ushba.org/post/hemp-lime-appendix-published-in-2024-us-model-residential-housing-codes

4. Practical application of hemp-lime

High-performance enclosure/fill wall. The main application is to cover low-rise buildings and renovation works, aiming at heat insulation – humidity conditioning – sound insulation. Experiments show that λ dry ~0.05–0.12 W·m⁻¹· K in the typical sample; MBV at a "very good" level helps stabilize the humidity of indoor spaces [2][3][6]. Design recommendations: prioritize vapor-permeable finishes, treat rain splashes – moisture bridges – dew spots; Optimal Climate–Energy Target Binder Ratio/Density/Binder Ratio [2][6].

Prefabricated block/panel & upgrading. Hemp-lime can prefabricate blocks/panels to increase uniform quality, shorten construction time; Simulations/field testimonials have shown a significant reduction in the risk of mold and thermal stability when replacing brick/concrete walls with hemp-lime systems of equivalent thickness [5][6].

Note the limitations. Hemp-lime is not load-bearing; requires an independent bearing frame. Mechanical (compression, elastic) and long-lasting (wet–dry, salt, biological) depend on binder type – moisture – maintenance; Mechanical/long-lasting research emphasizes the need to detail the construction – protect the surface appropriately to ensure longevity [7].

5. Conclusion

Hemp-lime is a promising biobuilding shell candidate: good insulation, high humidity cushioning, improved indoor environmental quality, with the ability to reduce life-cycle emissions when the distribution – value chain – biogenic carbon is properly considered. Appendix BL (IRC 2024) creates a normative foundation for application in low-rise housing. For hot and humid markets (such as Vietnam), the feasible route is to pilot hemp-lime fill walls/panels on a small scale, assess heat-energy-LCA humidity on site, optimize the hemp:binder ratio and vapor permeable finish details before replicating [1][2][4][6].

6. Reference

[1] International Code Council (2024). APPENDIX BL—Hemp-Lime (Hempcrete) Construction, 2024 International Residential Code (IRC). Available at: codes.iccsafe.org. (ICC Digital Codes)

[2] Walker, R. and Pavía, S. (2014). Moisture transfer and thermal properties of hemp–lime concretes. Construction and Building Materials, 64, 270–276. https://doi.org/10.1016/j.conbuildmat.2014.04.081. (PDF access) (Ecomat - ecologische bouwmaterialen)

[3] Pochwała, S. (2020). The Heat Conductivity Properties of Hemp–Lime Composites.Materials(MDPI),13(4):1011. https://doi.org/10.3390/ma13041011. (MDPI)

[4] Shanbhag, S.S. et al. (2024). Examining the global warming potential of hempcrete in the United States: a cradle-to-gate LCA. Journal of Building Engineering, 95:109028. (indexed summary) (Directory of Open Access Journals)

[5] Shang, Y. et al. (2021). Hempcrete building performance in mild and cold climates: integrated analysis of carbon footprint, energy, and indoor thermal & moisture buffering. Building and Environment, 206:108370. https://doi.org/10.1016/j.buildenv.2021.108370. (Astrophysics Data System)

[6] Kaboré, A., Maref, W. and Ouellet-Plamondon, C.M. (2024). Hygrothermal Performance of the Hemp Concrete Building Envelope. Energies, 17(7):1740. https://doi.org/10.3390/en17071740. (MDPI)

[7] Walker, R., Pavía, S. and Mitchell, R. (2014). Mechanical properties and durability of hemp-lime concretes. Construction and Building Materials, 61, 340–348. https://doi.org/10.1016/j.conbuildmat.2014.02.065. (PDF access) (awarticles.s3.amazonaws.com)

[8] Tong, W. and Memari, A.M. (2025). State-of-the-Art Review on Hempcrete as a Sustainable Substitute for Traditional Construction Materials for Home Building. Buildings, 15(12):1988. https://doi.org/10.3390/buildings15121988. (MDPI)

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