Hempcrete is a bio-composite building material gaining popularity for its sustainable properties and versatility. It’s composed of the inner woody core of the hemp plant, known as the “shiv,” combined with a lime-based binder. The unique characteristic of hemp shiv is its high silica content, which allows it to bind effectively with lime, setting it apart from other natural fibers. This combination results in a lightweight, cementitious insulating material.
Composition and Properties of Hempcrete
The key components of hempcrete are:
- Hemp Shiv: The inner woody core of the hemp plant, providing bulk and insulation.
- Lime Binder: A natural binder that hardens and binds the hemp shiv together.
Hempcrete is significantly lighter than concrete, typically weighing about one-seventh or one-eighth as much. A fully cured hempcrete block will even float in water. While not used as a structural element, it serves as excellent insulating infill between framing members, contributing to reduced racking in the structure. The primary load-bearing responsibility rests on the internal framing, commonly wood studs, making it well-suited for low-rise construction. However, in Europe, hempcrete buildings have reached up to ten stories in height, showcasing its potential for larger projects.
Applications and Construction
Hempcrete is mixed on-site using mortar mixers for a short duration (1-2 minutes). The mixture is then manually packed into wall cavities. During construction, slip-forming techniques using temporary wooden or plastic “shuttering” are employed to create the inner and outer wall surfaces. Due to its lightweight nature, the material can be easily transported around the construction site in tubs and passed along to workers filling the wall cavities. Site cleanup is simplified, as excess material can simply be tilled into the soil.
Externally, hempcrete walls are typically finished with a hard render coating, approximately 20mm thick, providing protection. A final colored topcoat is then applied, giving the building a stucco-like appearance. Internally, the walls can be left in their natural state or finished with lime plaster for a more traditional aesthetic.
History and Sustainability
The use of hempcrete dates back centuries, with evidence of its existence found in a 6th-century bridge abutment in France. Its rediscovery has led to increased adoption in Europe. Industrial hemp used in hempcrete is grown by certified commercial growers to ensure it has very low THC levels and is non-psychoactive. The longevity of existing hempcrete structures suggests a long lifespan for new buildings constructed with this material.
Hemp cultivation is environmentally friendly, requiring no fertilizers, weed killers, pesticides, or fungicides. Its dense growth naturally suppresses weeds, making it an excellent rotation crop with plants like barley or rye. The subsequent crop benefits from the weed-suppressing effects of hemp. Furthermore, hemp seeds are a nutritious food supplement rich in Omega-3 oil, amino acids, protein, and fiber, often considered a “super food.” The outer fibers are used in textile production, often blended with cotton. Traditionally, the woody inner core is used as animal bedding.
Hempcrete in North America
For over 60 years, hemp cultivation was illegal in America due to its resemblance to marijuana. However, it’s legally grown in Europe, the UK, and Canada under strict licensing and certification procedures that ensure low THC levels (<0.05%). Farmers are required to purchase seeds from certified growers, and their fields undergo periodic inspections. Currently, hemp for hempcrete in North America is often imported. A new processing plant for hemp core is under construction in Canada, which will provide a closer supply source for the continent.
Importing hemp adds shipping costs, slightly diminishing hempcrete’s carbon-negative impact and increasing production expenses. Logistics are also affected, requiring the purchase of a large container of material and on-site storage. This necessitates having personnel and equipment ready to unload the truck quickly, ideally in dry weather. The material must be kept dry until use, so adequate tarps are essential.
Frequently Asked Questions about Hempcrete
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Can I use smaller foundations for a hempcrete house?
Yes, potentially, but it depends on engineering recommendations and local building codes.
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How thick do the walls need to be to comply with building regulations?
Generally, 300mm is recommended for most domestic applications.
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Can hempcrete be used in a load-bearing capacity?
Typically, it is used with a timber, steel, or concrete frame that bears the vertical load. Hempcrete can add racking strength if needed.
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Is hempcrete available as blocks?
Yes, structural and thermal blocks are available.
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Is hempcrete fireproof?
Yes.
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Can hempcrete be used for purposes other than walls?
Yes.
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Does hempcrete need weather protection?
Yes, usually a lime render or rain-screen cladding is used. Weatherproof hempcrete is under development.
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Does it need a cavity when used as an external wall?
No, hempcrete provides insulation as a monolithic walling material.
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Does it need expansion joints in long walls?
No, movement joints are generally not required due to hempcrete’s mechanical characteristics.
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How airtight is hempcrete?
Extremely airtight due to its monolithic application.
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Does hempcrete have a warranty?
Manufacturers typically warranty the material. The installer warrants the work.
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How much does it cost to build with hempcrete?
Costs vary, but spray-applied hempcrete can be comparable to conventional construction costs.
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Can hempcrete be used in the renovation of old buildings?
Yes, it can be used as infill panels for historic timber-framed buildings.
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How high can a hempcrete building be designed?
The structural frame limits the height, as hempcrete is non-load-bearing.
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How much CO2 is locked up in hempcrete?
Spray-applied hempcrete locks up around 110kg of CO2 per cubic meter. Shuttered and cast hempcrete locks up around 110kg to 165kg of CO2 per cubic meter, depending on compaction.
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Does hempcrete breathe?
Yes, it performs well in moisture management, providing a healthy internal environment.
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Is hempcrete resistant to pests?
The lime binder protects against rodent and insect infestation.
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How long will hempcrete last?
The combination of plant-based materials and mineral binders suggests a lifespan measured in centuries.
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When is it better to cast or spray hempcrete?
Casting is suitable for smaller projects or volunteer labor. Spraying is more economical for larger projects (over 70 cubic meters).
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Can hempcrete be installed in the winter?
Yes, if the temperature is above 5°C and the material is protected from frost and heavy rain.
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How do I find a contractor to use hempcrete?
Suppliers can provide lists of contractors.
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How high can one build in a day with hempcrete?
There is no real limit besides practical site arrangements.
Conclusion
Hempcrete offers a sustainable and versatile building solution with numerous benefits, including excellent insulation, breathability, and carbon sequestration. While some challenges exist, such as sourcing materials in certain regions, its growing popularity and increasing availability suggest a promising future for this innovative building material.
