Reducing emissions in the construction industry, with its dependence on carbon-intensive concrete, is notoriously difficult and extraordinarily important in tackling the climate emergency.

When the power generation relied on by buildings and construction is included together they account for a huge 39% of global energy-related carbon dioxide emissions. Reducing energy use in homes and the broader built environment is a major priority of governments planning to meet the climate targets. So how did it happen that one of the oldest, proven and greenest building materials known to humanity came to be virtually forgotten and overlooked? In Britain, growing the plant which produces the material was once considered a ‘patriotic duty’ and now at least one flagship building project is showing how a fast plant could lead to rapid transitions to a more sustainable construction sector.

When the British brewery Adnams built its huge carbon neutral distribution centre in the rural county of Suffolk in the UK in 2006, most people noticed the stunning arched roof covered in greenery. At 0.6 hectares, it is the biggest sedum “green” roof in Britain, home to over half a million bees, with its own beekeeper. But the building also uses a construction material that could be of enormous use to house builders in the future struggling to provide for growing populations while also reducing carbon emissions. The walls are built entirely from over 90,000 lime & hemp blocks, making it the biggest building in the UK to use this material. The lime comes from the northern county of Derbyshire and the hemp was locally grown in the East Anglian region, meaning that the equivalent of 100 to 150 tonnes of CO² are locked up in its blockwork. A conventional brick and block building of the same size would have generated about 300 to 600 tonnes of CO² emissions.

Hemp is light, good at regulating moisture and heat, and is a good insulator. It’s also cheap, easy and fast to grow, and non-toxic to handle. The hemp construction enables the firm to save 50% on electricity and gas through its strong insulation qualities and its natural ability to hold a constant cool temperature is perfect for the storage of beverages. Air locks and active airflow management are all that is required to keep the beer at the desired storage temperature; no artificial refrigeration or heating is needed.

Because of its fast growth (hemp fibre is usually ready for harvest in just two months) – structural blocks and panels made from hemp and “hempcrete” – one of several specific hemp building products – could become an alternative to more traditional cement blocks and timber framing. Hemp oil from the seeds is already widely used in a range of industrial and commercial products and provides additional revenue for farmers. Its fast growth rate allows it to out-compete other raw materials because it is rapidly renewable, needs few inputs to grow well and can be used in a wide variety of diverse industries. For example, hemp can produce about four times as much paper as trees per acre.

So why is it not used more widely? After all, hemp was grown and used in a huge range of products across the world for centuries. In the modern context, its growth absorbs carbon and replenishes the soil, suppressing weeds without chemicals. It can be used to detoxify poisoned land and to make organic alternatives to plastic. But despite its great qualities, hemp – even just as a building material – remains a niche idea suited for individual eco-builds rather than mainstream construction. The answer lies in the fact that hemp is a variety of the cannabis plant. And, it’s lack of take-up has nothing to do with the plant’s thermal properties and more to do with the US-sponsored global crackdown throughout the 20th century on all things cannabis-related, known as “the war on drugs”. There are various explanations for why this situation arose – from xenophobia and racism – the original prohibitions on drugs were surrounded with racist language and aimed variously at Chinese immigrants, black Americans and Mexicans – to pressure from the oil industry and one lawmaker’s desire to hold onto power after the end of alcohol prohibition. Whatever the reasons, global prohibition meant that US allies had little choice but to ban the growth of all varieties of cannabis – whether or not the plants actually produced a high when consumed. Growing hemp in most countries – apart from China, where they continued regardless – became and remains an expensive process dogged by obstacle-strewn licences, layers of inspection and the constant fear of arbitrary closure and crop destruction. It makes a difficult context in which to produce a promising business model.

However, cannabis regulation for personal and medical use gradually became more acceptable in the US and several states, such as Colorado, have legalised production in recent years. The game changer is the 2018 Farm Bill, which means there is no longer a requirement for Drug Enforcement Administration (DEA) registration for growing and/or manufacturing certain varieties of the cannabis plant. Regular planters and farmers are now considering growing the crop, which could prove itself in reactivating and strengthening rural economies. After all, hemp is quick-growing, requires few pesticides or fertilisers and there is a lot of money to be made: the global industrial hemp market size was already estimated at USD 4.71 billion in 2019 and is growing fast metaphorically as a market as well as a plant. In 2018, over 77,000 acres of industrial hemp were grown across the US alone. That number is expected to surge in the coming years as regulations ease and the benefits of hemp are realised. The recent approval of the United States Department of Agriculture (USDA) hemp rules by the White House is already expected to further boost demand.

Wider relevance

This example is interesting because using this old material in a new way could help solve many of the issues currently facing the construction industry whilst helping to reduce carbon at the same time. As a natural material it is breathable and good at maintaining a steady temperature and humidity – all highly valuable qualities in building materials. Using hemp in construction is a relatively low-tech, high skill process. It’s just one material providing stability, non-load bearing structure and insulation – replacing the multiple layers of bricks, mortar blocks, insulation, cavity, cavity ties and plasterboard required in conventional buildings. It is manufactured from the woody, pithy core of the stem of the hemp plant, which is often called ‘shiv’ – a word from the textile industry (where hemp has been used for centuries) to describe the waste from the fibre-making process.

This is then further chopped into inch long pieces and mixed with a hydraulic lime binder and a small amount of water to make a kind of lightweight concrete that is usually called hempcrete. This material is then either fabricated into blocks or used loose to fill up – by hand – a hollow plywood framework. The hempcrete is tamped down lightly, the shuttering is removed and the process repeated to cast the next section. It needs time to dry, but the overall build time compares to conventional processes – just in a different order. The hempcrete can be left with a slightly rough finish, or rendered and plastered and painted.

Hempcrete offers significant opportunities to replace concrete – an enormously carbon-intensive substance. The cement industry is one of the largest contributors to global warming, accounting for 5 percent of all CO² emissions. Cement manufacturing is highly energy and carbon-intensive because of the extreme heat required to produce it.  A ton of cement requires 4.7 million BTU of energy, equivalent to about 400 pounds of coal, and generates nearly a ton of CO². Its production is growing and expected to rise to 3.7-4.4 billion tons by 2050.

Hempcrete, on the other hand, does not require widespread chemical processes or the burning of fuels that release greenhouse gas emissions to make it usable. The carbon in the plant’s natural structure remains intact, while the lime utilised in hempcrete as a binder can capture carbon dioxide from the atmosphere, as the lime cycle’s natural chemical processes allow hydrated lime to harden into limestone. Its light weight means it requires less fuel for transport, and shallower foundations, which can mean a 75% reduction in the amount of concrete required – or even different kinds of foundations that don’t require any concrete at all.

Using hemp could also see a reduction in other chemicals used as standard in construction and maintenance. Hemp is also naturally fire-resistant and mould resistant, which could reduce our reliance on chemical fire retardants that have been linked to several health problems. Studies in laboratory animals and humans have linked the most scrutinised flame retardants, called polybrominated diphenyl ethers, or PBDEs, to thyroid disruption, memory and learning problems, delayed mental and physical development, lower IQ, advanced puberty and reduced fertility. Other flame retardants have been linked to cancer.

The way we think about insulation can also change. Some 75 percent of all homes in the US still rely on fibreglass insulation – a mixture of plastic and recycled glass. The high thermal qualities of hemp will make much of this redundant. The R-value is a measure of resistance to heat flow through a given thickness of material. So the higher the R-value, the better its insulating properties. Hempcrete has an R-value of 2.4 to 4.8 per inch (depending on thickness), compared to concrete at 0.1 to 0.2 per inch, and traditional fibreglass system at 3.8.

Hemp is also a suitable material for the renovation and improvement of existing buildings. This will become increasingly important as we seek to upgrade the housing stock to the necessary standards to reduce the need for heating and its associated carbon emissions. As a safe, non-toxic material, it can be used to manufacture structural blocks and prefabricated panels for exterior walls, for doubling the thickness of existing walls from the inside or from the outside, as well as industrial partitioning and dividing existing buildings into apartments.

Unlike regular bricks or cinder blocks, hemp blocks actually increase the R-value by incorporating thermal performance into the structural component of the building. Hempcrete also makes an excellent insulation alternative to subfloors, walls, attics, a plaster alternative and even a material to build cabinets and countertops. Hempcrete breathes, allowing moisture to evaporate (which means no mould), does not let off-gas as many oil-based products do (safe for sleeping areas), and is avoided by insects such as termites (good for timber-frame houses). The non-toxic and lightweight properties of hempcrete also ensure it is easy to move around a job site. Hempcrete does not shrink or crack and can even increase in strength over time.

Context and background

Industrial hemp is a variety of the cannabis sativa plant species and is – along with bamboo –  one of the fastest growing plants on Earth. It was one of the first plants to be spun into usable fibre approximately 50,000 years ago. Evidence of hemp based string used in pottery in Mesopotamia has been found dating back 5000 years. In China, hemp has been farmed consistently from around 1100BC for a variety of uses, including for fabric, food, paper and for medicinal purposes. France, Russia and Spain having grown the plant consistently for the last 700 years. In the UK, hemp used to be a common crop and it was considered a patriotic duty to grow it; in 1535 Henry VIII required all farmers to sow quarter of an acre of hemp for every 60 acres they owned. Its rot-resistance and salt-water resistant properties made it good for making sails and rope for the navy. Hemp was also stronger than cotton and grew more quickly with fewer inputs and less nurturing. It can be refined into a variety of commercial items, including paper, rope, textiles, clothing, biodegradable plastics, paint, insulation, biofuel, food, and animal feed.

Although the drug cannabis and industrial hemp both derive from the species cannabis sativa and contain the psychoactive component tetrahydrocannabinol (THC), they are distinct strains with unique phytochemical compositions and uses. Hemp has lower concentrations of THC and higher concentrations of cannabidiol (CBD), which decreases or eliminates its psychoactive effects. The legality of industrial hemp varies widely between countries, with some governments permitting the growing only of hemp that is bred with an especially low THC content.

Hempcrete was first developed in France in the 1980s as a method of adding thermal performance to medieval timber frame buildings, whilst allowing the historic building fabric to continue working in the way it was intended to. In fact, the ancient Gauls used a Hempcrete-like material to build a bridge over two-thousand years ago. The fact that hempcrete is a vapour permeable material, with the hemp plant part of it made from hard cellulose material that is very similar to timber, means that hempcrete controls moisture at a similar rate to existing wooden elements. This prevents a build-up of trapped moisture in historic buildings where timbers may be vulnerable to rot if modern building materials are used.

Enabling factors

When Adnams Brewery were looking to build a new distribution centre, hempcrete was still an innovative product in the UK and had not been used in such a large commercial building. It was still largely used by self-builders or specialists in sustainable building in individual and unusual houses. The managing director of Adnams at the time spoke about being driven equally by business sense and environmental credentials, and the company has made a point of taking a stance on sustainability. They expected energy prices to soar in the future and that this energy efficient building would make the company substantial savings. They also wanted to be “shaping the path for other businesses to follow.”

The firm is also aware of its place in the local rural community. The Mayor of Southwold at the time said residents would be glad to see the plant’s traffic going through the town being cut, as the new site is on the edge, adding: “It is refreshing to see a business developing such a unique building that will not cause an unsightly blot on the landscape.” The move saw a reduction of approximately 60 heavy goods vehicle movements and 20 van deliveries per day, in and out of Southwold town centre and a reduction of approximately 60 employee cars, entering, leaving and parking in the town.

Although gigantic, the buildings are almost invisible from the road, being sunken seven metres underground and with green roofs that not only merge with the natural environment but use reed beds to catch and recycle rainwater. The recycled water is then used in the plant to water the grass roof, flush toilets and wash goods vehicles. The firm remains a big local employer, however, and its future success remained important to local people. The new building replaced the Victoria Street site in the small coastal town of Southwold, which was built in 1980, but Adnams have been brewing cask ales since 1872. They consider themselves a family business (with a plc listing), employ 536 people (as of June 2020), and their heritage is a part of their brand. During the lockdown in Spring/Summer 2020, Adnams worked with a local university (UEA) to produce hand sanitiser from their unused alcohol, provided celebratory drinks for Mercedes employees after they produced their 10,000th ventilator (another example of rapid industrial conversion in the face of an immediate crisis), and dropped off free drinks to local people making scrub sets for the NHS and 5,000 care packages.

There is now a huge opportunity to scale up this kind of building, while also bringing a decent living to farmers from a crop that is environmentally sustainable to grow. The timescale to build a hempcrete house is about the same as a conventional build house although the production sequence is different to allow the material to dry. In terms of the embodied energy, a study in Finland suggested the amount of energy being used in the development and renewal of a large Helsinki suburb using conventional materials and methods is never recovered over time. By using natural materials such as hemp in construction, there’s an immediate reduction in levels of CO² emissions, not just a hope for reducing energy use in future years.

Scaling the use of hemp in building will require a less hands-on method, and research is currently being carried out by a not-for-profit organisation, Material Cultures, in partnership with architect Summer Islam, the engineers Arup, the University of the Arts London, University College London, and the brick manufacturers HG Matthews. They aim to research building systems and components using hemp and other natural materials, to manufacture them at HT Matthews’s traditional Buckinghamshire brick factory.

Scope and evidence

  • At 0.6 hectares,Adnam’s carbon neutral distribution centre it is the biggest sedum “green” roof in Britain, home to over half a million bees, with its own beekeeper.
  • The walls are built entirely from over 90,000 lime & hemp blocks, making this the biggest building in the UK to use this material.
  • The woody, pithy core of the stem of the hemp plant is chopped into inch long pieces and mixed with a hydraulic lime binder and water to make a kind of lightweight concrete usually called hempcrete.
  • The lime comes from the northern country of Derbyshire and the hemp was locally grown in the East Anglian region, meaning that the equivalent of 100 to 150 tonnes of CO² are locked up in its blockwork.
  • A conventional brick and block building of the same size would have generated about 300 to 600 tonnes of CO² emissions.
  • The hemp construction enables the firm to save 50% on electricity and gas through its strong insulation qualities.
  • Air locks and active airflow management are all that is required to keep the beer at the desired storage temperature; no artificial refrigeration or heating is needed.
  • Hempcrete could become an alternative to more traditional cement blocks and timber framing. This could reduce pressure on forests worldwide and cut back on the estimated 15 billion trees cut down each year.
  • Hemps needs few inputs – low water and no chemicals – to grow well and can be used in a wide variety of diverse industries. For example, hemp can produce about four times as much paper as trees per acre.
  • Hemp’s growth gobbles carbon and replenishes the soil, suppressing weeds without chemicals. It can be used to detoxify poisoned land and to make organic alternatives to plastic.
  • The global industrial hemp market size was already estimated at USD 4.71 billion in 2019 and is growing fast. In 2018, over 77,000 acres of industrial hemp were grown across the US.

Lessons for rapid transition

  1. Hemp – whose many properties including fast growth have led it to be dubbed a ‘super crop’ – could solve some entrenched problems in the construction industry and offer a lot to a range of industries if it were freed from prohibition. Fast, tried and tested, low-skill materials could aid us in building low carbon homes that are good for the construction industry, for employment, for peoples’ health and for our environment.
  2. New is not always better, early technologies and materials looked at with fresh eyes can have much to offer today’s sustainability challenges.
  3. Be careful what you ban and beware cultural prejudices that prevent progress: placing broad restrictions driven by cultural and political motivations, on products which have multiple positive uses and applications, can undermine wider efforts at progress. Achieving rapid carbon cuts means reviewing many things that we do and established habits and prejudices in all economic sectors.

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