17 July 2015


Hemp mortar was discovered in bridge abutments (Merovingian bridges) in France built in the 6th century (the Merovingians ruled the Franks in much of the territory then known as Gaul). Since its rediscovery, and redevelopment in France in the 1980's, it has seen growing use as 'hempcrete' (hemp lime composite) across Europe, where hemp cultivation was never criminalised. France is Europe's biggest producer of hemp, but only about 5% of hurds are used in the building sector. In 2010/2011 the total area cultivated with hemp in the EU was about 11,000 ha, a slight decline from the previous year. Given it has already survived so many centuries, people expect hempcrete buildings to have a long life and across Europe and the United Kingdom (UK), hundreds of buildings now use hempcrete, including a seven-story office tower in France and a Marks and Spencer department store in the UK.

The host of the UK television series 'Grand Designs' waxed lyrical about hempcrete for the documentary 'Bringing it Home' (2011) during construction of over 40 hempcrete homes at The Triangle, Swindon, Kevin McCloud said; "I look at the range of materials out there ... I cannot find one to match hemp ... such a low environmental impact, that can be grown locally and harvested with minimum input in a matter of just a few months, it's an incredibly expedient building method" he said. "This field of hemp is absorbing all this CO2 and dirt and we are going to build houses with it ... the project manager here on site said to me he loved the material ... instead of pouring out heavy, wet, toxic, caustic concrete, it would instead pour out this sort of fluffy, lightweight stuff that was like sawdust; it didn't require any power tools ... so all we had was buckets and barrows and blokes and because there were no power tools there were no cables everywhere, and it actually makes for a very, very healthy, very safe building site. Hemp, it's a very, very builder-friendly material. I mean, you know, it's a no-brainer." Yes, building with hemp seems a complete no-brainer in the current world climate.

In 2014 the Hemp Industries Association (HIA) in the United States (US) estimated the total retail value of all hemp products sold in the US at $620 million (a 22% increase). Sadly for the US hemp industry, all the raw hemp materials were imported from other countries. Over 30 countries grow hemp, including Spain, Austria, Canada, China, UK, France, Russia and Australia. 

Though it lacks the structural stability its name might suggest, hempcrete does provide natural insulation and is virtually fireproofIn 2015, according to The New York Times, hemp-based building materials could usher in a new era of 'green' building in the US. While hemp has had a long history as a fibre used in ropes, sails and paper products - US Presidents Washington and Jefferson both grew it - a small number of US entrepreneurs have turned to hempcrete. 

Hempcrete is a bio-composite made of the inner woody core ('shiv', a short length, stalk by-product) of the hemp plant (bast and hurd fibres) mixed with a lime-based binder. The shiv has a high silica content which allows it to bind well with lime, a property unique to hemp among all natural fibres. Lime protects the shiv from biological decay, mainly through its ability to wick water away, its high alkalinity, as well as providing essential fire resistance.

Hempcrete is environmentally sustainable due to:

a) Being a carbon-sequestering building material from the growth of the hemp crop to the carbonation of lime in the walls for the life of the building. The breakdown of the carbon sequestration is as follows:
In one (1) cubic metre of hempcrete, emitted CO2 is;
110kg of hemp hurd =  -202 kg (CO2 absorbed)
220 kg of lime binder = +94 kg (CO2 released)
Total sequestration = -108 kg/m3 of wall built

b) Reducing energy consumption due to excellent insulation and airtightness. There is little heating or cooling loss from a hempcrete building which means constant energy output to cool or warm the building is not required. Hempcrete has low effusivity and high thermal inertia, it does not take as long to heat and once heated will slowly release heat when the temperature drops, at night, so a heater is rarely needed.

c) Being fully recyclable; any waste on-site can be re-used in the next mix. If a building is being demolished, the hempcrete can be easily broken down and re-used in a new build. As landfill, being a natural product, it will break down over the course of time and add lime and organic matter to the soil.

CaCo3 (limestone) goes the full circle from being broken down to carbonation when hempcrete is formed. 
As the hempcrete hardens from carbonation, it will eventually petrify the hemp and form limestone.

A lightweight, cementitious insulating material weighing around an eighth of comparable concrete (fully cured hempcrete blocks float in water) it is not used as a structural element, only as insulating infill of walls, roof and under-floor as part of a timber-framed building with all loads carried by internal wood stud framing. Hempcrete is ideally suited to low-rise construction and offers good thermal and acoustic performance with the ability to regulate internal relative humidity through hygroscopic material behaviour, contributing to healthier building spaces and providing effective thermal mass.

Hempcrete is mixed in a concrete or mortar mixer for 1-2 minutes and 'hand delivered' into the wall cavities. Walls are formed with temporary wooden or plastic 'shuttering' for the inner/outer surfaces. Hempcrete is lightweight and can be moved easily about in buckets and passed 'bucket-brigade' fashion to workers filling the cavities. Site clean-up is easy too, simply till it into the soil! Hempcrete is finished with an exterior hard render coating about 20 mm thick and further protected with a final coloured topcoat; the end result is stucco-like. The interior can be left natural or finished with lime plaster for a more traditional look. Hempcrete is toxin-free, airtight yet breathable, unaffected by mould and pests and practically fireproof. Best of all, hempcrete is a sustainable building material because hemp can be grown and replenished relatively quickly.

Hempcrete restoration work in the UK
So why haven't we seen it adopted in the US and Australia just like in Europe? Prohibition! While hemp is legal to use and import, the US has been prohibited from growing it for nearly 70 years, purely because it resembles its psycho-active cousin 'marijuana'. President Obama signed the Agricultural Act of 2014 which removed federal restrictions on growing industrial hemp, good news for US growers, although it will take time to cultivate the plant. Until then the US must rely on hundreds of millions of dollars worth of hemp imports (mostly from Canada and China) to meet its growing need for hemp. Hemp grown legally in Europe, the UK and Canada through a system of licensed growers produces seed certified low in THC. Currently, added shipping reduces the carbon-negative feature of hempcrete in North America and adds to production costs. In 2015, Hemp, Inc. released a video update on the progress of its decortication plant in Spring Hope, North Carolina. According to the footage, Hemp, Inc.’s decortication line is about 60% complete. To date, there are only five decorticators of this magnitude in the world. "Once in operation, the American hemp farmers will have access to the largest natural fibre manufacturing and processing facility in North America for their crops", said Bruce Perlowin, CEO of Hemp, Inc. 


The number of acres of hemp planted by US farmers has increased over 1,000% in 2015, as compared to the number of acres planted in 2014, the first year of legal hemp growing in the United States. The green squares in the map below represents hemp farmers who have registered with their state.

It’s been legal to grow hemp in Canada since 1998, though farmers still need to apply for a license and authorisation from Health Canada. In 2009, 9,725 ha of hemp were planted and it is expected that hemp will continue to contribute more than $100 million to Canada's economy annually. Several Canadian companies have advanced plans to start developing the fibre side of hemp but the $30 million cost to build a processing plant has been a barrier. Alberta Innovates has a small processing plant where they are testing production and manufacturing options. “We are just at the verge of seeing hempcrete buildings across the province and beyond”. Canadian entrepreneurs will have to act soon, though, if they don’t want to deal with American competition. Several US states have already started to relax regulations, and earlier this year a bill was introduced in the US Senate that seeks to end the federal ban on cultivation entirely. The bill is currently in committee.

Australia's prohibition on growing industrial hemp was lifted in the late 1990's, but farmers are restricted to growing fibre and construction materials and politics has prevented them from gaining access to booming hemp food markets. In 2014 Australian hemp producer Ecofibre said of the industry: "What we are producing presently is just low-level, low-value market material such as pet bedding, horse bedding, erosion control mediums, oil spill containment products, garden mulch - you know, basic things like that ... there's lots of products we could get to eventually, but ... the industry isn't at that stage yet. It's hampered". 

Growing industrial hemp under license is already legal in Queensland, New South Wales, Victoria, Tasmania and Western Australia. However, the industry is very restricted because of archaic law/s not allowing hemp for human consumption (only two countries ban it, Australia and New Zealand) based on missguided concerns by law enforcement that hemp consumption could somehow affect roadside saliva drug testing or would 'send the wrong message' about Cannabis consumption. 

Hemp is Cannabis sativa, a member of the Cannabaceae family. Cannabis is the plant genus, sativa (Latin for 'cultivated') is the species. A high-resin crop, Cannabis is generally planted about 1.2-1.5m (four to five feet) apart and mostly used for its medicinal/therapeutic leaves and buds. Hemp, however, is a low-resin crop, generally planted about 10 cm (four inches) apart, mostly used for its versatile stalk and seed. Different types of Cannabis are classified as strains and different kinds of hemp are classified as varieties and cultivars.

Tasmanian hempcrete house under construction
In New South Wales (NSW) the introduction of a licensing scheme under the Hemp Industry Act 2008 was designed to allow farmers to grow low THC hemp crops for fibre and oil production while limiting the risk to law enforcement. It is an offence under the Drug Misuse and Trafficking Act 1985 to possess hemp unless it's cultivated or supplied under authority. Those wishing to grow should be aware that an environmental assessment and approval process applies in addition to NSW Department of Primary Industry’ licensing requirements. Low THC hemp plant material cannot be fed to livestock.

Queensland has allowed industrial production under license since 2002, with issuance controlled under the Drugs Misuse Act 1986. If you intend to grow or research industrial hemp, you must have a license. All activities carried out under a license are subject to monitoring at the licensee’s expense by inspectors who, among other things, sample plants before harvest to test for THC content.

In Western Australia (WA),  The Department of Agriculture and Food acts as the Registrar for the Industrial Hemp Act 2004. This legislation enables licensees to cultivate, harvest and process industrial hemp on a commercial scale. AGWEST Plant Laboratories handles the compliance side of the licensing process including inspection and sampling of crops and reporting to the Registrar. The Industrial Hemp Act 2004 allows a person wishing to participate in the industry to apply for a license, usually valid for three years, and be heavily scrutinised to ensure suitablity and eligibility to participate in the industry.

In Tasmania, industrial hemp industry regulations include five year licenses. In January 2015 the Tasmanian Government announced reforms to simplify the regulation and support growth in the industry. Under the new laws, the allowable THC content will rise from 0.35% to 1.0%, bringing it in line with New South Wales, Queensland and the Australian Capital Territory. Tasmanian research data from the Forthside Research Station showed that up to 15 t/ha of dry stem can be produced from one planting. It is also possible to grow two crops in the same season if an early (September) and a late (December) sowing are carried out on the same site. 

In 1998, Victoria became the first Australian State to pass legislation permitting growers, under license, to grow industrial hemp. However, since then, the industry has not progressed. Hemp is defined under the Drugs, Poisons and Controlled Substances Act 1981 as a narcotic plant. Persons wishing to grow industrial hemp must obtain authorisation from the Department of Environment and Primary Industries (DEPI) to do so.

Ruth Trigg, IHA SA and Greens
MP Tammy Franks 
The South Australia Controlled Substances Act lumps all types of Cannabis together too, but still prohibits all of them, effectively preventing any form of industrial hemp cultivation. A special license was given in 1995 and trial crops were planted in South Australia (SA). In 2015, in South Australia, hemp is still not legally grown. Legalising hemp to make clothes, building materials and beauty products could be part of the solution to South Australia’s looming post-Holden manufacturing crisis said the newly formed Industrial Hemp Association (IHA) of SA and Greens MLC Tammy Franks. “It can revolutionise the building industry using the qualities of hempcrete; fire retardant, insulation, acoustics, absorbing CO2 for the rest of its life, and for ease of use as a pourable slurry.” The Manufacturing and Innovation Minister revealed in State Parliament that he was planning to meet with the Industrial Hemp Association of SA to discuss its proposal to legalise hemp production in the state.

Hemp itself is a beneficial crop requiring no fertiliser, weed killer, pesticide or fungicide. It grows so thickly that weeds cannot grow. Farmers grow it in rotation with other crops such as barley or rye. The crop following hemp requires no weed killer because hemp drives the weeds out.

It is encouraging to see a trend in some developed countries of accepting the use of energy and resources to fuel a wasteful and profligate construction industry is way beyond the ability of the planet to continue to support. Mass materials like cement and concrete cause significant pollution, use a lot of energy and non-renewable resources. Many of the materials such as insulation and finishes contain toxic chemicals like brominated fire retardants which can seriously damage our health as well as the ecosystem. It has long been recognised that buildings and their use contribute significantly to CO2 emissions however it seems there are two main approaches to creating a lower impact on the environment. There are those focussed on getting mainstream construction to be more energy efficient, even though they still rely on high-embodied energy products such as cement, bricks, concrete and steel. This focus is based on retaining the same style of construction using the same materials with more insulation (petrochemical-based) and solar technology to reduce the impact. It would seem the intent is to appease the environmental conscience without pain. In the UK, the construction and use of buildings accounts for over 50% of the carbon dioxide produced. Studies have shown that up to 200kg of CO2 is emitted in the production of each square metre of walling for houses alone, equating to 40 tonnes for the walls of a typical house using double brick. The other approach has been to look at low impact alternatives that also are healthier and less polluting in both manufacture and construction technique.

Hemp Cottage, County Down, Northern Ireland
Although the US still has a long way to go towards ending Cannabis prohibition, it's both encouraging and inspiring to see how far they've come in a relatively short amount of time. Four states and Washington DC have legalised Cannabis, two dozen states offer legal medical 'marijuana' and hemp production could be a game-changer across numerous industries while helping to keep the planet healthy. As government policy becomes increasingly concerned with reducing carbon emissions and finding more efficient ways of meeting carbon reduction targets, it seems possible that hempcrete can make a major contribution to this, offering a genuinely zero-neutral solution to sustainable construction. As for Australia? The people are still preparing for the revolution to start, the hemp revolution, that is.

Resources included; Hempcrete Could Be Putting the Green in Green Building, What is Hempcrete,  National Hemp Association, HempcretePolitics of Pot, Hempcrete Sustainability, Hempcrete Best Concrete, Hemp Now Seen As Growth Industry, DPI NSW Summer Crops/Fibres, Qld Industry Agriculture Niche HempIndustrial Hemp Queensland, Industrial Hemp Western Australia, DPIW TasmaniaNew Laws Aim to Streamline Tasmanias Industrial Hemp IndustryHemp VictoriaDPI Victoria, Australian Hemp

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