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The Hempcrete Book
Designing and Building with Hemp-Lime
By William Stanwix, Alex Sparrow
UIT Cambridge LtdCopyright © 2014 William Stanwix and Alex Sparrow
All rights reserved.
History and uses of hemp
The hemp plant, thanks to its many uses and in particular its most famous one, as a widely popular recreational drug, is one of the most instantly recognizable plants in the world. A great deal has been written about hemp's many uses throughout human history and about the politics of its prohibition during the twentieth century, and there is no need for us to reproduce it all here. In the interests of context, however, in this chapter we provide a brief description of the hemp plant, its history and the resurgence in its use today.
Hemp is the English name (from the old English haenep) for the cannabis plant. The words haenep and cannabis are both thought to derive from the Ancient Greek kánnabis, which in turn evolved from an older word in an ancient Iranian language from around 2,500 years ago.
Three varieties of the cannabis plant exist: Cannabis sativa, Cannabis indica and Cannabis ruderalis. Cannabis sativa and C. indica are seen as the more closely related species. Cannabis ruderalis differs from them in that its flowering happens after a predetermined number of days, rather than being dependent on the seasons, and it contains very little tetrahydrocannabinol (THC), the psychoactive substance that gives the drug cannabis its active ingredient.
Hemp is a fast-growing erect annual plant which produces only a few branches, usually at the top of the plant, and grows to a height of between 1.5m and 4m. Its stem is thin and hollow, with a diameter of 4mm to 20mm, depending on the conditions and the specific variety grown. The 'bast' fibres of the hemp plant, which are contained in the bark of the woody stem, range from about 1.2m to 2.1m in length and are extremely strong. Their quality varies depending on the timing of harvesting, and the fibres are graded in terms of their fineness, length, colour, uniformity and strength.
The inner woody stem, the 'shiv' (or 'shive', or 'hurds'), historically has not been used intensively, but this is changing rapidly in the modern world, with new uses being developed all the time: packaging filler and animal bedding, for example. It is hemp shiv that is used in the production of hempcrete.
The seeds of the hemp plant are used as a food source, and ground to produce oils for a wide range of purposes, including technical and industrial applications. The hemp plant in its whole state can be used as a biofuel, and even the cell fluid of the hemp plant is now used in the manufacture of abrasive fluids.
A history of hemp
The common hemp plant, Cannabis sativa, is one of the earliest recorded domestically grown plants, with evidence of its cultivation by humans since Neolithic times. Hemp is found across the world, and has a long history of widespread use for a range of important products: hemp seeds for oils and resin, food, fuel, medicines and cosmetics; hemp fibre for hard-wearing clothing, rope and tough fabrics such as sailcloth (the word canvas derives from cannabis: literally – originally – 'a fabric made from hemp'), and as a pulp from which to make paper.
It is thought that the plant originated in China, and that its cultivation gradually spread westwards through India and into the Middle East, Africa and the Mediterranean, where it formed an essential part of the livelihood and culture of each people who grew it. Surviving writings from the Egyptian, Greek and Roman records show how important the hemp plant was to the lifestyle, trade and expansion of these great civilizations.
The cultivation of hemp in Europe continued throughout modern history, with evidence that its use in Britain, introduced by the Romans, continued thereafter, with the Saxons incorporating it into their medical treatments. Later kings of England promoted the cultivation of hemp, not only for its everyday uses for linen and rope but also for the vital part it played in the military supremacy of Britain as an island nation: Henry VIII passed a law making it compulsory for farmers to grow hemp, such was its importance to the defence of the realm through its use for sailcloth and rigging. Later still, the hemp plant played a not-insignificant part in Napoleon Bonaparte's downfall, since his ill-fated incursion into Russia had as its aim the destruction of Russian hemp plantations. Russia had been supplying the English with hemp, and thus equipping the navy of Napoleon's enemy.
The importance of hemp in British and Irish society throughout the ages is reflected in place names across the land – for example, Hemel Hempstead in the south of England (meaning literally 'place of hemp' or 'hemp pasture'), and Cwm Cywarch in Snowdonia (which translates as 'the steep-walled mountain basin in which hemp is grown'). Street names such as Hemp Mill Walk in Loggerheads in Staffordshire speak for themselves, and Hemp Street in Belfast is at the centre of the area which, until the beginning of the twentieth century, housed the thriving industry around the manufacture of hemp rope and sailcloth for the city's important shipbuilding trade.
The long tradition of hemp growing and processing in Britain can still be seen in the surviving architecture of the hemp and flax industries. Up until the arrival of cheap imported cotton, sisal and jute, towards the end of the nineteenth century, hemp and flax were still widely used for clothing, other textile products, rope and netting. In fact, these two plants are the only fibre crops that are commercially viable in our temperate maritime climate. Many towns in the UK have surviving buildings that were originally a part of these important and widespread industries. The most visible of these are often the 'rope walks' of Victorian, or earlier, times: long buildings where the hemp and flax fibres were stretched out and spun into twine and rope.
For those interested in the history of hemp cultivation in the UK, a good place to start is the quiet coastal towns of Bridport and West Bay in Dorset. The rolling hills of west Dorset and south Somerset, with their well-drained, fertile soil and warm climatic conditions, provide the perfect conditions for cultivating hemp, and the town of Bridport already boasted a thriving hemp- and flax-processing industry by the thirteenth century. The earliest recorded evidence of this is a "payment for a large quantity of sails and cordage in 1211", which was followed by "an order from King John for Bridport rope and cloth to supply the navy in 1213." Although the industry suffered during the sixteenth and seventeenth centuries, owing to competition from other shipyards and local depopulation as a result of plagues, there was a dramatic revival of the town's hemp and flax production during the period from the late eighteenth to late nineteenth centuries. This, combined with the relatively slow growth and development of the town since the beginning of the twentieth century, means that the history of the British hemp industry is preserved in the buildings of Bridport and West Bay today.
Another of the main uses of the plant throughout history has been in religious ceremonies, and more recently as a recreational drug, due to its relaxing and mildly psychoactive effect. This use of cannabis, or marijuana, as a narcotic eventually led to the growing and possession of the hemp plant being outlawed in most Western countries in the early decades of the twentieth century. The prohibition of cannabis remains in force widely today, in the West at least, with some notable exceptions, such as the Netherlands, which was the first Western country to introduce an official policy of tolerating the possession of small amounts of the drug without prosecution. Other countries, including Spain, Portugal, the Czech Republic and Switzerland, have followed its example in recent years by decriminalizing the possession and use of small amounts of the drug. In November 2012 the US states of Washington and Colorado voted to legalize cannabis, although it is still unclear how this will work in practice, since the drug continues to be illegal under US Federal law, which of course has jurisdiction in both of these states.
The unfortunate side-effect of the prohibition of the drug cannabis has of course been the blanket banning of cultivation of all forms of the hemp plant, and its consequent unavailability to Western societies for its many non-drug-related uses. The cultivation of hemp in the UK was outlawed in 1928. Since the 1930s, much effort has gone into developing cultivars of the plant which contain very little THC. The success of this endeavour means that, for some decades now, an industrial hemp plant with little or no THC content has been widely available.
The term 'industrial hemp' refers to cultivars of Cannabis sativa which have been bred to have a THC content of 0.2 per cent or less. These cultivars have been legal to grow in the UK since 1993, and in Canada since 1995. The THC content in a drug-producing plant would be 10-15 per cent or higher, depending on the strain and method of cultivation. Because the industrial hemp plant looks identical to the drug-producing strains, however, the growing of industrial hemp in the UK requires a special licence from the government.
Since the early 1990s, more and more individuals and organizations throughout the Western world are starting to re-embrace the hemp plant, exploring its potential as a natural and sustainable source of materials with great commercial, industrial, agricultural and environmental potential. The table opposite shows some examples of its many uses.
While the hemp plant is not, as is sometimes claimed, a 'miracle plant' that can solve all of the problems of the Western world, it seems that the resurgence in its use will be invaluable in a future where it is essential that we wean ourselves off our dependence on fossil fuels. At the time of writing, hemp has been grown legally again in the UK for 20 years, and the number of farmers cultivating it is steadily increasing, as local and international demand for the crop expands.
In the United States, the commercial growing of hemp has been made legal under State law in the states of Colorado and Washington, although confusingly it remains illegal under Federal law. In late April 2013, Colorado farmer Ryan Loflin planted the first major hemp crop in the USA in more than 60 years. This landmark planting, five months after it was made legal in Colorado, is the result of a huge groundswell of opinion in the USA, which has been pushing for the US government to fall into line with the rest of the world in its attitude to the hemp plant.
Hemp farming in the UK today
Hemp grows easily in a range of soils and climates, providing the soil pH is 6.5 or above (neutral-to-alkaline). The hemp plant is not especially nutrient hungry, although for maximum commercial yields a fertilizer is needed. It's also a deep-rooting plant, helping to break up the soil to some depth, which is beneficial for soil health and condition.
The plant is an effective weed suppressant because it grows very quickly and is very 'competitive', winning out over other plants for growing space and light. For this reason it requires no chemical weedkillers, and in fact hemp is sometimes grown specifically to clear land of chemical-resistant weeds. Another useful property of the plant is its pest resistance – there are virtually no pests or diseases that attack hemp – so there is no need to use pesticides or fungicides during its cultivation.
The speed of growth, together with its natural weed and pest resistance, makes hemp a profitable 'break crop' for UK farmers, providing a useful barrier to pests and diseases in the soil between the sowing of other crops, such as cereals, as part of a crop rotation.
The gradual depletion of nutrients in the UK's soil through the last 60 years or so of ever-more intensive and industrialized farming, and the 'replacement' of these nutrients with increasing applications of chemical fertilizers, has resulted in a very poor soil quality across much of the UK's farming land. Poor soil produces weak plants that are more susceptible to pests and disease, which in turn are targeted with increased levels of chemical pesticides and fungicides. These also indiscriminately kill beneficial insects – both soil-dwellers and airborne pollinators, which are vital for the success of our food crops. Even if the continued use of such chemical fertilizers and pesticides were the best way forward (which it isn't), the fact that many of these products are derived from fossil fuels means that they are a non-sustainable resource.
The situation nationally at the beginning of the twenty-first century, apart from among those farmers who are returning to smaller-scale, organic farming, is soil across the UK's farms that is almost totally devoid of those insects and other organisms which convert decaying organic matter into the nutrients that plants need to grow. These tiny creatures also provide a key source of nutrients at the bottom of the food chain, and their absence is a significant factor in the huge loss of biodiversity in the UK that has occurred over recent decades. The cultivation of a plant such as industrial hemp, which is not nutrient-greedy, can be grown using only organic fertilizers and without chemical weedkillers and pesticides, and breaks up the deep soil, has the potential to bring huge benefits in terms of soil health, food production and the UK's ecosystem as a whole.
Industrial hemp is sown in the late spring, from early May to early June, at a density of around 30-38kg of seed per hectare. The ideal conditions are a warming soil with plenty of moisture present. Prevailing conditions are more important than exact timing when sowing, since the seedlings are not frost hardy.
Seedlings emerge within five days, and grow rapidly, sometimes at a rate of 30cm a week. The crop is harvested in August, having attained a height of 2-4m. As it is cut down, the hemp plant is also cut into shorter lengths and is left on the ground for up to a month for 'retting': a biological process whereby the hemp straw becomes more workable and the bast fibres begin to separate from the shiv. When the retting process is complete, and the sun has dried the hemp, it is baled and stored under cover. The raw hemp straw is then processed to separate it into saleable parts. The different markets for each part are shown in the table below (see also table on page 19).
Hemp can be grown for the straw alone, or allowed to grow on for a seed crop. It is possible to grow a 'dual' crop, for fibre and seed, although this is much less common because the flowering of the plant does lead to some reduction in the overall quality of the stem, from which the bast and shiv fibres are obtained.
Various models of hemp processing have emerged in the UK in recent years, ranging from the super-high-tech, such as the multimillion-pound processing plant in Suffolk built by Hemp Technology Ltd (formerly Hemcore), to small-scale and (relatively) low-tech solutions such as those employed by K J Voase and Son in Yorkshire, who process all of their own hemp using machinery they have built or adapted themselves, on the farm where it is grown.CHAPTER 2
Hemp in construction
Hemp for use in construction forms a relatively small, but growing, proportion of the output from hemp farming in the UK. The main ways in which hemp is used in construction are to make hempcrete and to provide fibres for quilt insulation.
'Hempcrete' is the popular term for a hemp–lime composite building material. It is created by wet-mixing the chopped woody stem of the hemp plant (hemp shiv) with a lime-based binder to create a material that can be cast into moulds. This forms a non-load-bearing, sustainable, 'breathable' (vapour permeable) and insulating material that can be used to form walls, floor slabs, ceilings and roof insulation, in both new build and restoration projects.
Hempcrete was developed in France in the mid-1980s, when people were experimenting to find an appropriate replacement for deteriorated wattle and daub in medieval timber-frame buildings. Across Europe, awareness was growing about the extensive damage that had been done to such buildings in the post-war period through ill-advised repairs using ordinary Portland cement. Using this material to replace the vapour-permeable earth-and-lime mortars and natural cements in historic buildings prevented the buildings' fabric from 'breathing' (see Chapter 4, page 58). This in turn led to the retention of moisture within the fabric, which damaged the timber frames.
Excerpted from The Hempcrete Book by William Stanwix, Alex Sparrow. Copyright © 2014 William Stanwix and Alex Sparrow. Excerpted by permission of UIT Cambridge Ltd.
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Table of Contents
Foreword by Professor Tom Woolley,
Part One: Principles of building with hempcrete,
1 History and uses of hemp,
2 Hemp in construction,
3 An introduction to lime,
4 Key concepts in sustainable building,
5 Getting the basics right,
6 Variations on the hemp–lime mix,
7 Performance of hempcrete in a building,
8 Tools and equipment,
9 Health and safety,
10 Planning the build,
Focus on self-build 1: Agan Chy,
Part Two: Hempcrete construction,
11 The hempcrete wall: an overview,
12 Foundations and plinth,
13 The structural frame,
15 Mixing hempcrete,
16 Placing hempcrete,
17 Floors, ceilings and roof insulation,
18 Finishes for hempcrete,
19 Practicalities on a hempcrete build,
20 Restoration and retrofit,
Focus on self-build 2: Hemp Lime House,
Part Three: Designing a hempcrete building,
21 Design fundamentals,
22 Indicative detailing Focus on self-build 3: Bridge End Cottage,
23 A look to the future,
Also by Green Books,