BASICS
Hempcrete is a bio-composite building material that is created by coating and mixing particles of hemp hurd (also known as shiv – the inner woody core of a hemp plant) with water and lime and then allowing the materials to dry and harden into a natural insulation material. Although builders can use clay as a binder in a hempcrete mix, hempcrete users generally prefer lime, as it can enable the dried mixture to resist wetness, mold, and decay. With these features, hempcrete is often referred to as hemp-lime, a name commonly used in Europe or as “bonded cellulose insulation.”
Like traditional insulation, hempcrete can be the infill material to a wall structure or the insulation material for floors, ceilings, roofs, or windows. When it comes to walls that are framed using studs or heavy timbers, builders can use temporary formwork or permanent, permeable sheathing that is situated around framing to cast monolithic hempcrete walls. Molding and casting walls involve a steady process of creating and mixing hempcrete in conjunction with a bucket brigade of helpers pouring, spreading, pressing, or tamping the hempcrete into formwork. Prior to casting and upon being dumped out of a mixer, hempcrete mixes typically look like crumbly oatmeal with no cohesion until builders squeeze and tamp the mixtures. When builders spread and press the mixture in formwork, they typically do it at a depth of four to eight inches at a time. When the formwork is full, the team of helpers can then remove and raise the slip forms for further assembly to complete the wall. For hempcrete installation that goes beyond the framing of one or both sides of the wall, builders can use spacers attached to the slip forms to extend the depth of the wall.
After hempcrete is cast, several factors will affect drying times, with the largest factor being the moisture content of the mix. Overall, cast hempcrete walls will need a range of two to eight weeks or sometimes more for sufficient drying. Once fully dried, hempcrete can serve two components of a wall by being the insulation and substrate for finished plaster, very similar to a light straw-clay wall system.
Other than casting hempcrete, builders can also apply hempcrete to structures by spraying or by installing precast bricks, blocks, or complete structural wall panels. Precast materials eliminate the need for on-site mixing and placing. Production of precast materials also allows the curing and drying process to occur in controlled conditions. The drawback for precast blocks and panels is that they will need to be joined by mortar or an overlapping method. Depending on project design conditions, mortar joints, under the characteristic of thermal bridging, may reduce the overall thermal performance of the hempcrete insulation.
GALLERY
HISTORY
Humans have recognized hemp’s useful properties for building for thousands of years as structures using hemp date back to the Romans. Archaeologists confirmed Merovingians used hemp shiv in the construction of a bridge, in 6th century AD, when France was still Gaul. The more recent modern use of hempcrete was first developed in France in the 1980s. French restorationists developed the method when replacing wattle and daub as they attempted to develop better methods for restoring medieval timber-framed buildings. Since its beginnings as a restoration material, the use of hempcrete as a building material has grown in Europe and Canada along with formal studies examining it.
Although the United States has relaxed restrictions for industrial hemp, proponents of hempcrete say it still faces hurdles as many building professionals are not aware of the material and how they can apply it to buildings under safe and proper uses. In order to counter hempcrete’s underutilization and for it to gain acceptance by building professionals and permitting officials, the US Hemp Building Association achieved acceptance in 2022 of its residential code proposal submitted to the International Code Council (ICC), which sets standards in the construction industry. The ICC will incorporate the association’s proposal as a new appendix to the 2024 International Residential Code (IRC).
MATERIAL PROPERTIES
Thermal Characteristics:
The thermal performance of hempcrete can vary and depends on the density of the mixture, compaction, and setting. When a mixture has more binder, the density will increase and the hempcrete will likely have a lower R insulation value. While there is no consistent R value for all hempcrete mixes, mixes tested in the 275 to 350kg/m³ range gave a value of R 1.9 per inch, requiring a 12-inch wall to reach R-24. For a colder climate, a wall between 12 to 16 inches wide may be necessary to achieve code compliance. Experts are resisting claims by hempcrete proponents who say the material has very high R insulation values based on the concept of blending thermal resistance with thermal mass, a concept known as “mass enhanced R-value.” According to experts, thermal mass does not improve thermal resistance and the claims are not based on lab testing protocols recognized in the building industry. Although steady-state R values for hempcrete are lacking and are lower than conventional insulation, building researchers are attempting to come up with new metrics that show hempcrete’s real world thermal performance, such as considering the material’s hygrothermal performance. Hempcrete’s hygrothermal performance is based on its pore structure and how it absorbs and releases thermal effects such as heat and moisture.
Fire Resistance:
Hempcrete with lime has shown a high degree of flame or fire resistance. According to a 2008 book by UK architects and hempcrete experts Rachel Bevan and Tom Woolley, when researchers carried out a fire test on a 250 mm thick wall of hemp lime blocks laid in lime mortar, the wall remained undamaged for 1 hour and 40 minutes without failure. A 2009 fire test done in the United Kingdom also showed that a 3×3 meter cast assembly of hempcrete with 12 inches of thickness and vertical load met fire resistance for 73 minutes. According to the US Hemp Building Association website, although the association did not submit a fire test with its international residential code proposal, the association is planning an ASTM E119 test soon. Another favorable feature of hempcrete is that when it does burn, it will not release toxic compounds because of its natural components, whereas traditional petrochemical wall insulation will.
Durability and Strength:
Don’t let the name fool you… hempcrete is quite different than conventional concrete. It will not be the structural component of a building’s foundation or frame. Hempcrete insulation is not suitable for supporting loads by itself, but the density of hempcrete under a framed load can prevent framing from bending or buckling and can also help increase the load each framing member carries. Hempcrete also can increase in durability over time as the lime content in the binder undergoes carbonization for several years. According to a 2018 white paper from a Canadian hempcrete company, compressive strength for hempcrete depends on the casting process and it ranges from 116 to 145 per square inch.
Carbon Sequestration:
Hemp-shiv in hempcrete has a long-term biogenic carbon storage characteristic, making the material favorable for reducing carbon dioxide emissions in the air. Along with shiv, lime binders can also achieve additional carbon storage as builders set and cure a hempcrete mixture. This is because the curing of a lime binder mix involves a chemical process of carbonation, meaning the hempcrete will reabsorb carbon dioxide (CO₂) from the air
Acoustical Performance:
A 2002 acoustic test done in the United Kingdom shows hempcrete had a sound reduction of 57 to 58 dB, exceeding a 53 dB code requirement.
ADDITIONAL NOTES
Variation of Binders and Agents:
Lime-based binders for hempcrete can consist of many formulations, but they will generally all have hydrated lime. Experts recommend not to use 100% hydrated lime for a mixture as it will require a long drying period and the middle volume of hempcrete will probably not have access to air for its curing process. Different manufacturers of hempcrete have honed in on their own separate proprietary formulas and additives for making hempcrete, including the use of setting agents such as hydraulic and pozzolanic binders. The range of admixture substances for hydraulic setting agents can include natural hydraulic lime, natural cement, Portland cement, and magnesium cement. Hempcrete binders rarely consist of 100% hydraulic agents as builders favor hydrated lime as being less expensive, having a lower density, and having more moisture handling properties, including antifungal and antimicrobial qualities. For pozzolanic binders, the substances can include metakaolin, fly ash, blast furnace slag, and zeolite.
Moisture Regulation and Indoor Air Quality:
For indoor air quality, set and finished hempcrete will not release toxins or gas, although builders should vet commercial products prior to use. Because of the porous structure of hemp hurds, hempcrete can absorb large amounts of moisture vapor while maintaining integrity during humid conditions and the material can also release the stored vapor when conditions allow. Besides the favorable qualities of hemp hurds, the lime in hempcrete will also make the material less susceptible to mold growth and moisture damage, especially as lime mineralizes over time. Lime has a high pH characteristic and is inherently antimicrobial and antifungal, making hemp coated in lime more resistive to mold. Because of these hurd and lime features, expert Chris Magwood says hempcrete is a good choice for buildings in varying climates and where humidity levels are often high. Even though hempcrete has a leg up compared to other insulation materials in its ability to handle moisture and prevent mold, owner-builders with mold sensitivities should consult with professionals to ensure they are building a hempcrete house correctly and that the house has proper moisture management, including vapor permeable walls and systems to avoid bulk water and heavy rain.
FAQ
Using hemp hurd or shiv can be favored over other cellulose materials because of its durable core structure and moisture handling properties. For builders that are environmentally conscious, they may also favor hempcrete for its carbon sequestering attributes.
Experts recommend using air-tight finishes (e.g. lime) rather than vapor-tight ones, as an air-tight layer will allow for moisture movement and regulation. Despite the vapor permeability a lime plaster might have, lime users typically intend for the external plaster to be the first line of defense to resist moisture.
Hempcrete is relatively fragile and will likely be damaged without protection. All exterior walls required plastering or equivalent exterior covering. Plastering an interior hempcrete wall will help protect it from furniture and other inside activity along with keeping dust from coming off the hempcrete face.
Because hempcrete will soften and degrade if exposed to groundwater, standing water, or conditions where it cannot release water vapor or dry out, hempcrete experts suggest limiting its use to above grade applications.
Additional Reading
Reference:
Building with Hemp by Steve Allin, Seedpress, 2nd Edition, 2005.
Website by Steve Allin –
https://hempbuilding.com/
Hemp lime construction: A Guide to Building with Hemp Lime Composites by Rachel Bevan and Tom Woolley, BRE Electronics Publications, 2008.
(Downloadable version https://www.brebookshop.com/details.jsp?id=325431 )
Lime Hemp and Rice Husk-Based Concretes for Building Envelopes by Morgan Chabannes, Eric Garcia-Diaz, Larent Clerc, Jean-Charles Bénézet, Frédéric Becquart, Springer, 2018.
Essential Hempcrete Construction: The Complete Step-by-Step Guide by Chris Magwood, New Society Publishers, 2016.
The Hempcrete Book: Designing and Building with Hemp-Lime by William Stanwix and Alex Sparrow, Greenbooks, 2014.
Hemp + Lime, Examining the Feasibility of Building with Hemp and Lime in the USA, Design and Demonstration, by the Parsons Healthy Materials Lab, May 2020.
International Residential Code (IRC 2024) Appendix: AY Hemp-Lime (Hempcrete) Construction
Organizations:
International Hemp Building Association – Kenmare, County Kerry, Ireland
https://internationalhempbuilding.org/
US Hemp Building Association – Nashville, TN
Endeavour Sustainable Building School – Peterborough, OT
