Using Hemp in Construction

Hempcrete is a natural building material with excellent qualities. It is breathable and has exceptional thermal performance.

| February 2016

Hempcrete is made from lime and hemp shivs (a waste product from hemp fiber growing); it can be used for walls, floors, and for roof insulation; it’s breathable, absorbing and emitting moisture to regulate internal humidity and avoid trapped moisture and mold growth; it provides excellent acoustic and thermal insulation and thermal mass; it’s lightweight and reduces construction costs; and it’s environmentally friendly. The Hempcrete Book (Green Books, 2014) by William Stanwix and Alex Sparrow provides a full explanation of construction techniques, highlighting potential pitfalls and how to avoid them, and includes a comprehensive resources section and examples of completed builds, with design notes.

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, return­ing 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 environ­ment behind us.

9/20/2017 9:16:01 AM

Hempcrete construction looks like a great way to build an eco friendly and sustainable house. I am eager to build one and see how well it withstands the elements in Belize.

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