The mixture of hemp shiv and a lime-based binder with the addition of water produces a bio-composite material often called hemp-lime (9). As with the early use of hemp within concrete, in hemp-lime the shiv performs as a lightweight aggregate and the lime as binder and preservative. This composite material is rigid, lightweight and durable, while achieving high levels of thermal insulation and vapour permeability.
Lime’s unique chemical and physical properties are ideal for its combination with hemp. Throughout the lime’s curing process it retains a high permeability which allows vapour to migrate through the composite, therefore allowing the hemp chips to dry out completely. This coupled with the lime’s alkalinity, acting as a natural biocide, ensures the hemp shiv is safe from mould and bacterial attack.
The amount of CO2 produced during the manufacturing and transportation of lime raises questions about its environmental impact. However, Rachel Bevan suggests that for a hemp-lime wall ‘the environmental burden of cement production is significantly compensated for by the CO2 sequestration of hemp’(10).
A hemp-lime wall can be built using a variety of methods. However due to its low compressive strength it is, so far as the author is aware, always applied as infill to a load-bearing frame such as timber or as a surface coating to masonry(11). Hemp-lime is tamped or sprayed into formwork on site, which can be removed immediately as the mixture will hold its shape. Alternatively, bespoke pre-cast hemp-lime blocks have been developed to slot around a timber frame, providing a solution which is less dependent upon the weather during construction. A third variant is hemp-lime and sand blocks which can be load-bearing but the resulting ‘block walls are denser and thus will not have such good thermal insulation properties although they will provide excellent thermal mass.’(12)
When cast and left open to the air the moisture in the hemp allows the material to absorb carbon dioxide which then combines with the lime during its curing process. As the lime cures, reverting back to calcium carbonate, the wall construction gains both rigidity and compressive strength. Depending on local conditions, the hemp-lime takes between eight to ten weeks to dry out(13) and continues to gain strength over the following years.(14)
Bevan notes that ‘there is often no need to use timber sheathing boards, breather membranes, internal finishes like plasterboard and external cladding which can make normal timber frame construction rather complicated’(15). Instead the hemp-lime performs all these roles including thermal and acoustic insulation. An external render is required and, as long as it is breathable, any external finish may be specified, even a hemp-lime or lime chalk render.
Hemp-lime has a low impact on the environment and its carbon sequestration and storage capacity are a significant benefit of the material.(16) During photosynthesis, the hemp plant absorbs carbon dioxide from the atmosphere during the day, converting carbon to biomass and exhaling oxygen. When the hemp is subsequently used in construction, the carbon within it is locked away for the life of the building. Therefore, using hemp within the structure of a building can be better than zero carbon, sometimes referred to as carbon negative. 108kg of CO2 can be locked away in 1m3 of lime-hemp.
A lime-hemp wall of 300mm depth provides levels of thermal and acoustic insulation well above regulation standard. It also regulates the internal relative humidity and temperature swings ‘through hygroscopic material behaviour, contributing to healthier building spaces and providing effective thermal mass’.(17) The building envelope is vapour permeable which allows moisture in the building to migrate out and eliminates the likelihood of condensation on internal faces.
Simplification of the construction process with fewer materials and layers is also a benefit. It is usually where different layers or materials meet that failure occurs and it is also more cost effective to have one material performing many functions rather than many materials each performing a single role. For example, in a standard timber frame construction you may expect to find a layer of insulation, a vapour barrier layer, a breather membrane layer and a sheathing layer to name but a few. However hemp-lime itself can perform all of these functions and, due to the reduction of connections, a more air tight envelope can be achieved.
Unlike concrete, lime’s mechanical flexibility allows movement without cracking. If a crack does appear the lime acts in a manner which can self heal. Any moisture ingress re-activates the lime around the crack, filling the void and sealing the material. This flexibility also allows movement with the frame, reducing the risk of gaps appearing between hemp-lime and structure.
The main limitation of hemp-lime is its load-bearing capacity. The highly aerated material compresses under heavy loads. The technique of using lime-hemp materials as load-bearing elements by forming compressed blocks has been developed. However, they require the addition of sand and this drastically reduces their thermal performance as well as increasing the embodied energy of the material through extra processing and increased density.