Go back to article: A sustainable storage solution for the Science Museum Group

Hemp as a building material

Hemp shiv has characteristics that make it particularly suitable as a building material. It is more resistant to biological decay than similar bio-based materials such as straw and its silica content makes it fire-retardant. Shiv is a very porous material, being approximately sixty per cent air by volume, which makes it very hygroscopic, attracting (adsorbing) moisture quickly and releasing it (desorbing) slowly. It is hydrophilic, absorbing water up to four times its weight. Thermal conductivity is also low due to the internal porosity, which retards the rate of heat transfer as air is less conductive (Arnaud and Gourlay, 2012).

However, the fibres require a binding material to produce a mortar. As hempcrete was initially used as a sympathetic repair material for historic oak framed buildings infilled with a mixture of straw and lime (wattle and daub) (Hirst et al, 2010) lime-based binders have been used in the mix in preference to Portland cement. Portland cement, which sets hard and relatively quickly and requires less skill to use, replaced the use of lime mortars from the middle of the nineteenth century (Carran et al, 2011). But, because cement’s hardness and density is incompatible and damaging when used with any ‘soft’ and porous building material such as traditional brick or porous sedimentary stone, lime mortar production has recently been revived for the conservation and restoration of heritage buildings and for use with other natural building materials (Labesse, 2005). Commercially available lime binders typically contain a varying proportion of natural hydraulic lime, hydrated or slaked lime, a hydraulic component such as Portland cement and/or other silica-containing materials such as fly ash, a residue of coal burning. Fly ash improves the workability of the mix due to its fine round grains and reduces the required water content (De Bruijn and Johansson, 2013). 

Component DOI: http://dx.doi.org/10.15180/150405/007