Getting the dope on hemp building


When we hear the word hemp we usually think of clothing, rope or some other use. But hemp has many other purposes, including as a building material. Lance Turner explains.

All modern building materials seem to have some drawbacks, even the eco-friendly ones. Mudbricks are heavy and don’t insulate well, strawbales are heavy and result in very thick walls, AAC (autoclaved aerated concrete, such as Hebel) has poor impact resistance and low thermal mass, wood is often unsustainably sourced and burns rather well.

But what if there was a building material that could be used like concrete, but was light, strong, flexible, carbon neutral and could be produced almost anywhere? Well, there is, and it’s derived from the hemp plant.

Hemp has become notorious for its use as a drug, but low-THC (less than 0.03%) hemp called industrial hemp is now being grown in many countries throughout the world. Australia has been slow to legalise industrial hemp farming until recently. Industrial hemp is an excellent agricultural crop, taking 14 weeks to grow for maximum crop rotation, uses little pesticides and will revitalise poor soils. Industrial hemp is finding many uses, including clothing and other fabrics, rope and as a replacement for glass fibre in reinforced plastics. It is the outer fibrous sheath of the hemp plant stem that is used for these purposes, but it’s the inner core, often called hurd or shiv, that is of most interest. Sixty per cent of the hemp plant is hurd which often is deemed waste material and either burnt or used as animal bedding. Using it as aggregate in hempcrete better utilises this byproduct, adding value to a ‘waste’ material.

Hemp hurd is unique in that, when mixed with lime and water (plus sand, cement and other optional additives if desired), over time, it undergoes a chemical reaction that converts it into a concrete like material. In effect, the hemp hurd petrifies, due to the very high silica content of the hemp. This is why hemp has been successful in binding with lime in lieu of other agricultural stalks such as straw and flax. This petrification process occurs over the lifetime of the building through the carbonation of lime and is estimated to ultimately absorb over 200kg of CO2 per square metre of wall.

Advantages of hempcrete
However, unlike concrete, hempcrete, as it has become known, is non-structural, lightweight (around 15% to 20% the weight of concrete), flexible (so it resists earthquake damage and needs no reinforcing), is fire resistant, termite and rodent resistant and actually locks up more carbon than is required to make it, making it carbon negative. It is cast like concrete and is easy to work and can be poured onsite or prefabricated into bricks and blocks, or indeed into almost any shape. Hempcrete is also a good insulator, and has a long thermal lag time, so it can assist in keeping buildings thermally stable without the need for much, if any, heating and cooling, provided the rest of the building is designed appropriately. Published technical literature from the UK shows a 300mm hempcrete wall to have an R-value of 4.2 and if used in between floor joists as insulation, will achieve an R-value of 4.0 for 200mm thickness. Hempcrete, being cast in position, is also highly draughtproof which stops heat from entering or leaving the building. Having good airtightness, once the room is at a comfortable level, there is little need to continually run heating or cooling to maintain that comfort.

While hempcrete is a good insulator, it is also water resistant yet allows air to permeate through it, so buildings made with hempcrete walls will actually breathe, improving air quality and reducing dampness buildup. For this reason, hempcrete walls should not be sealed with non-permeable paints or cementitious renders.

Read the full article in ReNew 115