Hemp in Construction
Key Properties of Hempcrete
No Toxic Chemicals - fire-, pest- and rot-resistance given by lime binder, no chemical treatment required
Vapour-permeable/Breathable - for excellent indoor air quality and longevity of building fabric
Exceptional Thermal Performance - hempcrete combines insulation and thermal mass in the same material
Better-than-zero-carbon - a net sequestration of atmospheric carbon in hempcrete
Flexibility - suitable for domestic or commercial new build and retrofit projects
English Heritage Approved - often used in traditional and listed buildings
Materials and Process
The stalk of the hemp plant contains very strong fibres and a woody cellulose core, both of which are used in building. Its woody core (previously a waste material from the seed and fibre crop) is used as an aggregate in hempcrete (a.k.a. hemp-lime); a medium-density insulation material which also has thermal mass.
Hempcrete is made by wet-mixing hemp “shiv” (the chopped stalk) with natural lime or formulated lime binders to produce a loose fill material, which hardens and sets. It is a vapour-permeable; better-than-zero- carbon; chemical-free, insulation material, which is used to form monolithic walls, roof insulation and floor insulation layers.
Hemp fibres are used to make “hemp wool” insulation batts, which, like sheep wool or recycled timber batts, provide a natural, low-embodied-energy, vapour- open alternative to synthetic, high-embodied-energy insulations such as mineral or glass wool. Unlike synthetic insulations, natural insulations such as hemp, timber or wool are able to absorb and release water vapour and retain their shape (and therefore their performance) in the presence of moisture.
Hemp fibres or shiv can also be mixed into a lime plaster to provide some insulation or improved tensile strength.
Main Benefits and Uses
Wet Mix - Hempcrete can be mixed on site and “poured” (it’s actually a damp solid) into shuttering, or spray applied using a specially adapted render pump.
Pre-cast - Hempcrete can be pre-cast into blocks and panels for assembly on site.
Either way, hempcrete is always applied around a structural frame as it is non load-bearing. The cast-on- site method provides the best thermal performance as the whole thermal envelope of the building can be cast in one piece, however the drying time on site means this method is more suited to single domestic dwellings. For commercial-scale projects the predictability that pre-dried hempcrete blocks or panels bring to the schedule usually outweighs the extra design work needed to resolve the thermal bridging and air tightness issues which an assemble- on-site system always brings.
Finishes for hempcrete should be vapour permeable, and include; lime or clay plasters, lime render, and timber, brick or stone cladding. In fact any cladding finish could be used together with a vented cavity, and internally a gypsum-board service void is sometimes used in commercial buildings, although this increases the overall embodied energy of the build, and reduces the moisture buffering and indoor air quality benefits that hempcrete brings.
Hempcrete’s vapour-permeability, the loose fill casting method, and the typical finishes described above, all make it the ideal material for sensitively adding thermal performance to traditional and historic buildings, ensuring our built environment heritage remains usable, and therefore is protected for future generations. The main uses in this sector are casting hempcrete infill panels to historic timber frame properties, and as internal or external solid wall insulation in brick and stone buildings.