Robert Celt
New Member
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'. This in turn led to the retention of moisture within the fabric, which damaged the timber frames.
A replacement was sought that would not only preserve the vapour-permeable nature of a building's fabric, thereby keeping it in good health, but also provide insulation. It was discovered that the stem of the hemp plant, highly durable and comprised of strong cellulose (capable of going from wet to dry and vice versa almost indefinitely without degrading), was the ideal aggregate to add to lime mortars to achieve this effect. Thanks to the cell structure of the hemp stalk and the matrix structure created by the individual pieces of hemp inside the wall, together with the properties of the lime binder itself, a hempcrete wall has a good ability to absorb and release moisture. Also, since a great deal of air is trapped inside a hempcrete wall (both within the hemp itself and within the matrix of the hemp shiv in the cast material), it is a surprisingly good insulating material, and the density which the lime binder adds gives the finished material a good amount of thermal mass. Almost as soon as this technique was developed for the repair of historic buildings, people started experimenting with its use in sustainable new build — and found that it was equally suitable for this application.
Is building with hemp a new phenomenon? It hardly seems likely that human civilizations would have cultivated the plant for millennia for such a wide range of uses without using it in their buildings. It is unlikely, however, that physical evidence of any such use in ancient times would survive, since plant-based building materials will of course eventually decay, return-ing to the soil from whence they came. After all, that is one aspect of the very reason that we are interested in them today: a low-impact building material will allow us to house ourselves 'lightly', without leaving a legacy of adverse effects on the environ-ment behind us.
There is some evidence, however, that building with hemp did not start in the twentieth century and, further, that properly maintained hemp buildings can last for centuries. A historic hemp house in Miasa village, in the Nagano prefecture of Japan, now recognized as a Japanese national heritage site, was built in 1698 and survives in good condition to this day.
At the time of writing, hempcrete has been used in building for around 30 years — since its 'invention' in the 1980s. The use of hempcrete has gradually spread, first across Europe and more recently around the world, and the number of people using it, both in new build and in the repair of older buildings, continues to grow.
In the UK a great many buildings, both commercial and residential, have now been built with hempcrete. A notable upsurge in the commercial use of hempcrete came with the Renewable House Programme, funded by the UK government between 2007 and 2010. Under this scheme, a range of developers received varying levels of public funding to build social housing using natural renewable materials, resulting in the construction of around 200 homes. Of the twelve projects funded, seven used hempcrete as an insulation material.
Since the UK construction industry is notoriously slow to adopt new practices, and has been largely sceptical of the need for (or even the possibility of) using natural materials, State-funded and -driven programmes such as the Renewable House Programme are invaluable in facilitating investigation into issues relating to the large-scale adoption of natural materials within the construction sector. The programme certainly had its challenges, and many of the projects undertaken suffered to some extent from the effects of contractors being given novel materials to work with. However, the overall results were encouraging, and no problems were encountered to suggest that hempcrete, along with other natural materials, would not be suitable for adoption on an increasing scale within the mainstream construction industry.
There are also some other uses of the hemp plant in construction, primarily of the bast fibres in the manufacture of fibre quilt insulation materials, and both shiv and bast fibre as an addition to lime plasters, providing additional strength and some insulation to the plaster. While these materials are not the main focus of this book, they are occasionally referred to throughout, so a brief overview of them is included later in this excerpt. First, however, we look at the different ways in which hempcrete can be used.
Hemp shiv for building
Currently in the UK there are no agreed standards for the characteristics of hemp shiv for the construction industry, nor for its production or processing. In France, where the hemp building industry is more established, there are strict guidelines for hemp farmers that govern the quality and colour of hemp shiv to be used in construction, and it is hoped that, in time, similar standards for the nature and quality of the product can be agreed for the UK industry.
The processing of hemp shiv for use as a building aggregate (once all the leaves, seeds and bast fibres have been removed) involves breaking it up into small pieces and removing any remaining fibre and dust. Hemp shiv for building should be as dry and clean as possible, with a minimum of fines (small pieces of bast fibre) and dust present.
The length of the pieces should be between about 10mm and 25mm, but this is not absolutely critical: successful walls have been made with shiv that contains shorter pieces, but it is generally acknowledged that pieces of this length produce a good matrix structure within the wall, which is beneficial for its thermal performance (a material's 'success' in conserving heat and power in a building) and vapour permeability (the degree to which a material allows water vapour through it). Walls are also regularly built with hemp shiv that contains a certain amount of fines, although the proportion must be low, otherwise the fines can soak up too much water and potentially affect the setting of the binder. The absence of dust from the shiv is far more important, since excessive dust can have an even more significant impact on the structural integrity of the wall — in extreme cases leading to collapse. This is because the dust soaks up a very high proportion of the water added at the mixing stage, causing the binder to fail. The only way to avoid this is to compensate by adding a lot more water, but this will significantly extend the drying time of the hempcrete. The presence of excessive amounts of dust in hemp shiv for building is to be avoided at all costs.
Hemp should be stored dry, although it comes in plastic-wrapped bales, so there is a certain tolerance of these being left outside on-site in the short term if suitable storage is not available. The shiv should go in the mixer as dry as possible, to avoid excess water entering the mix, but if some areas of a bale do get wet it will not affect the quality of the finished hempcrete. However, if the shiv has been subject to prolonged exposure to moisture and starts to show signs of rotting (the colour changing to black), all black areas should be scraped out and removed from the bale before adding it to the mix.
There is no need for any treatment of hemp shiv with fire retardants or preservatives as long as it is being used with a lime-based binder for hempcrete. Once cast as hempcrete, the lime in the binder acts to effectively inhibit insect attack and protect from dampness and fire.
News Moderator: Robert Celt 420 MAGAZINE ®
Full Article: Using Hemp In Construction
Author: William Stanwix and Alex Sparrow
Contact: Mother Earth News
Photo Credit: None found
Website: Mother Earth News
'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'. This in turn led to the retention of moisture within the fabric, which damaged the timber frames.
A replacement was sought that would not only preserve the vapour-permeable nature of a building's fabric, thereby keeping it in good health, but also provide insulation. It was discovered that the stem of the hemp plant, highly durable and comprised of strong cellulose (capable of going from wet to dry and vice versa almost indefinitely without degrading), was the ideal aggregate to add to lime mortars to achieve this effect. Thanks to the cell structure of the hemp stalk and the matrix structure created by the individual pieces of hemp inside the wall, together with the properties of the lime binder itself, a hempcrete wall has a good ability to absorb and release moisture. Also, since a great deal of air is trapped inside a hempcrete wall (both within the hemp itself and within the matrix of the hemp shiv in the cast material), it is a surprisingly good insulating material, and the density which the lime binder adds gives the finished material a good amount of thermal mass. Almost as soon as this technique was developed for the repair of historic buildings, people started experimenting with its use in sustainable new build — and found that it was equally suitable for this application.
Is building with hemp a new phenomenon? It hardly seems likely that human civilizations would have cultivated the plant for millennia for such a wide range of uses without using it in their buildings. It is unlikely, however, that physical evidence of any such use in ancient times would survive, since plant-based building materials will of course eventually decay, return-ing to the soil from whence they came. After all, that is one aspect of the very reason that we are interested in them today: a low-impact building material will allow us to house ourselves 'lightly', without leaving a legacy of adverse effects on the environ-ment behind us.
There is some evidence, however, that building with hemp did not start in the twentieth century and, further, that properly maintained hemp buildings can last for centuries. A historic hemp house in Miasa village, in the Nagano prefecture of Japan, now recognized as a Japanese national heritage site, was built in 1698 and survives in good condition to this day.
At the time of writing, hempcrete has been used in building for around 30 years — since its 'invention' in the 1980s. The use of hempcrete has gradually spread, first across Europe and more recently around the world, and the number of people using it, both in new build and in the repair of older buildings, continues to grow.
In the UK a great many buildings, both commercial and residential, have now been built with hempcrete. A notable upsurge in the commercial use of hempcrete came with the Renewable House Programme, funded by the UK government between 2007 and 2010. Under this scheme, a range of developers received varying levels of public funding to build social housing using natural renewable materials, resulting in the construction of around 200 homes. Of the twelve projects funded, seven used hempcrete as an insulation material.
Since the UK construction industry is notoriously slow to adopt new practices, and has been largely sceptical of the need for (or even the possibility of) using natural materials, State-funded and -driven programmes such as the Renewable House Programme are invaluable in facilitating investigation into issues relating to the large-scale adoption of natural materials within the construction sector. The programme certainly had its challenges, and many of the projects undertaken suffered to some extent from the effects of contractors being given novel materials to work with. However, the overall results were encouraging, and no problems were encountered to suggest that hempcrete, along with other natural materials, would not be suitable for adoption on an increasing scale within the mainstream construction industry.
There are also some other uses of the hemp plant in construction, primarily of the bast fibres in the manufacture of fibre quilt insulation materials, and both shiv and bast fibre as an addition to lime plasters, providing additional strength and some insulation to the plaster. While these materials are not the main focus of this book, they are occasionally referred to throughout, so a brief overview of them is included later in this excerpt. First, however, we look at the different ways in which hempcrete can be used.
Hemp shiv for building
Currently in the UK there are no agreed standards for the characteristics of hemp shiv for the construction industry, nor for its production or processing. In France, where the hemp building industry is more established, there are strict guidelines for hemp farmers that govern the quality and colour of hemp shiv to be used in construction, and it is hoped that, in time, similar standards for the nature and quality of the product can be agreed for the UK industry.
The processing of hemp shiv for use as a building aggregate (once all the leaves, seeds and bast fibres have been removed) involves breaking it up into small pieces and removing any remaining fibre and dust. Hemp shiv for building should be as dry and clean as possible, with a minimum of fines (small pieces of bast fibre) and dust present.
The length of the pieces should be between about 10mm and 25mm, but this is not absolutely critical: successful walls have been made with shiv that contains shorter pieces, but it is generally acknowledged that pieces of this length produce a good matrix structure within the wall, which is beneficial for its thermal performance (a material's 'success' in conserving heat and power in a building) and vapour permeability (the degree to which a material allows water vapour through it). Walls are also regularly built with hemp shiv that contains a certain amount of fines, although the proportion must be low, otherwise the fines can soak up too much water and potentially affect the setting of the binder. The absence of dust from the shiv is far more important, since excessive dust can have an even more significant impact on the structural integrity of the wall — in extreme cases leading to collapse. This is because the dust soaks up a very high proportion of the water added at the mixing stage, causing the binder to fail. The only way to avoid this is to compensate by adding a lot more water, but this will significantly extend the drying time of the hempcrete. The presence of excessive amounts of dust in hemp shiv for building is to be avoided at all costs.
Hemp should be stored dry, although it comes in plastic-wrapped bales, so there is a certain tolerance of these being left outside on-site in the short term if suitable storage is not available. The shiv should go in the mixer as dry as possible, to avoid excess water entering the mix, but if some areas of a bale do get wet it will not affect the quality of the finished hempcrete. However, if the shiv has been subject to prolonged exposure to moisture and starts to show signs of rotting (the colour changing to black), all black areas should be scraped out and removed from the bale before adding it to the mix.
There is no need for any treatment of hemp shiv with fire retardants or preservatives as long as it is being used with a lime-based binder for hempcrete. Once cast as hempcrete, the lime in the binder acts to effectively inhibit insect attack and protect from dampness and fire.
News Moderator: Robert Celt 420 MAGAZINE ®
Full Article: Using Hemp In Construction
Author: William Stanwix and Alex Sparrow
Contact: Mother Earth News
Photo Credit: None found
Website: Mother Earth News
