Hemp as a building material is an encouraging development when the number of concerning news items about irreversible climate change is on the rise.
When we think about climate change, often overlooked is the substantial chunk of carbon emissions linked to housing construction. New residential dwellings and renovations typically involve the use of materials with a high embodied energy, such as aluminium, steel, and the cement that alone represents about five per cent of global CO2 emissions.
Solutions are needed, involving greater use of more environmentally sound, renewable materials. Even better is to shift the paradigm entirely, and to build in such a way that each new house removes carbon from the atmosphere rather than adding more, helping to reverse climate change. This is technically possible right now.
One solution: hemp as a building material
Carbon-negative building material options include strawbale, bamboo, and hemp. Of the three, hemp perhaps has the greatest range of benefits. It is waterproof, non-flammable, termite-resistant, a good thermal insulator, and offers superior acoustic insulation. Due to its breathable quality, in cooler climates hemp discourages condensation on inside wall surfaces, inhibiting the development of mould and its associated health problems. After demolition, hemp walls are even biodegradable. Being durable, a hemp house will sequester carbon for as long as it stands, and has an expected lifespan in excess of more conventional construction materials.
While they are growing, hemp stems absorb between 1.3-1.6 times their weight in carbon, and depending on the technique used, this translates into 110-165kg of carbon take-up per cubic metre of hemp wall. Typically combined with lime and water, after construction the lime slowly reacts with the silica-rich hemp, causing it to turn to stone. This petrification process results in further carbon being removed. Although the lime is carbon-positive, the net balance of the mixture is carbon-negative, so long as the hemp is not transported a long distance. While much of the hemp currently used in Australia is grown domestically, some is imported.
The history of hemp for construction
Hemp use for construction goes back a long way, with a hemp masonry material discovered on a bridge in France that dates back to the 6th century. The modern hemp building sector is comparatively young, with the first project completed as recently as 1989. Since then, most hemp constructions have been built in Europe, one recent example being a development of 42 homes, known as The Triangle, in the British town of Swindon. While this project has had some teething problems, these are linked to issues other than the use of hemp.
In Australia, low-THC industrial hemp is permitted to be grown everywhere other than the Northern Territory. Hemp has the advantages of needing little irrigation water, requiring few fertilisers, avoiding pesticide use, and improving soil structure. It grows to maturity in about three to four months, and two to three crops per year are possible here, depending on the climate. Following a decision in May to legalise human consumption of hemp foods, this extra yield will be shifting the economics of the industry onto a healthier footing. The part of the plant used for construction is the shredded inner woody core (known as hurd or shiv), which is separated from the long outer fibres via an industrial process known as decortication.
Hemp construction in Australia
Among the companies building with hemp is Hemp Homes Australia, based in Margaret River, Western Australia. It focuses on passive solar, custom-designed hemp houses, with the first example having recently been completed, and five others in the pipeline. To secure a local hemp supply, Western Australia’s growers are calling for investment in a processing plant that would allow locally sourced hemp to be used, or better still, for a network of them dotted around the state. Colin Steddy, director of The Hemp Corporation, has plans to invest in getting one off the ground.
A couple of Australian ecovillages under construction will be using hemp. These are Shepherds Ground, near Maitland in the Hunter region, and Witchcliffe Ecovillage near Margaret River in Western Australia. So far, dozens of hemp houses have been built across the country.
Hempcrete is the name for the most commonly used type of hemp houses building material. Because a hemp-lime mixture is not dense enough to be load-bearing, hempcrete is commonly used in conjunction with post-and-beam construction, and is applied inside a formwork. Encasing the timber frame, it takes on the roles of cladding, insulation, and Gyprock. Another construction option involves hempcrete blocks, but due to the gaps these have an inferior thermal insulation compared to cast hempcrete. Hempcrete can also be used in the insulation of flooring, the under-floor area, and roofs.
If you are looking to build a new house from hemp, it is important to recognise that hempcrete construction is a mature technology that has been tested in Europe for the past 30 years. Roughly speaking, you can anticipate that hemp will be approximately the same cost as brick and other commonly used materials. For non-owner-builders, it is recommended to use a company that has previously built hemp houses, or is committed to attending trainings or workshops such as those held regularly by the Australian Hemp Masonry Company. This is particularly important because hemp materials behave differently from their conventional equivalents. Specifying hemp in the contract is one way to ensure that it is not substituted for something else that lies within a mainstream house builder’s comfort zone.
With a regular stream of concerning news items about the future impacts of ‘irreversible’ climate change, it is encouraging that there is a practical carbon-negative means of helping to reverse some of the damage.
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