Hemp lime is a lightweight construction material that can be used for walls, insulation of roofs and floors and as part of timber-framed buildings. It provides very good thermal and acoustic performance and offers a genuinely zero-carbon contribution to sustainable construction.
On 12 September 2008, BRE published Hemp lime construction: A guide to building with hemp lime composites (EP 85). It was written by Rachel Bevan, principal architect at Rachel Bevan Architects and Tom Woolley, professor of architecture at the Centre for Alternative Technology and chairman of the UK Hemp Lime Construction Products Association. It is the output of a Department for Environment, Food & Rural Affairs (DEFRA) funded study commissioned by the National Non-Food Crops Centre.
Hemp lime is a composite material used for walls, insulation of roofs and floors and as part of timber-framed buildings. It is most commonly a mix of renewably-sourced hemp shiv, a specially-formulated lime binder and water.
Whilst there is a growing awareness of the need to reduce the energy use of buildings in operation, there is still a tendency to construct them from materials that have a high embodied energy, or are in some way damaging to the environment. Hemp lime construction offers a real alternative.
Hemp lime construction offers 120 pages of comprehensive guidance on the use of hemp lime for housing and low-rise buildings, providing practical information on materials, design and construction. It includes case studies and design details, and explains how the use of hemp-based material can capture and store carbon dioxide in the fabric of buildings.
Cannabric was invented by Monika Brümmer in 1999. A bio-construction block with hemp already mentioned in another article on our website. In summary, this material can be used in single-layer load-bearing walls, high thermal comfort, bioclimatic, acoustic and with a negative carbon footprint.
Ryerson University's 2014-2015 lecture series continues tomorrow with a lecture titled "How to Make Sustainable Architecture and How Not to!", featuring architect Tom Woolley of Northern Ireland-based Rachel Bevan Architects. Woolley specializes in the field of sustainable architecture, and is known for his pioneering of natural materials, specifically hemp and lime, in construction projects.
Hempcrete or hemplime is biocomposite material, a mixture of hemp hurds (shives) and lime, sand, or pozzolans, which is used as a material for construction and insulation. It is marketed under names like Hempcrete, Canobiote, Canosmose, Isochanvre and IsoHemp. Hempcrete is easier to work with than traditional lime mixes and acts as an insulator and moisture regulator. It lacks the brittleness of concrete and consequently does not need expansion joints. The result is a lightweight insulating material ideal for most climates as it combines insulation and thermal mass.
Hempcrete is developed from a mixture of hemp shives, and a lime based binder. The lime-based binder typically consists of either hydrated lime or natural hydraulic lime. Hydrated lime is made from pure limestone and sets through the absorption of CO2 during the carbonation process. When dealing with time constraints, hydraulic binders are used in combination with regular hydrated lime because the set time for hempcrete will be less than that of regular limes (about two weeks to a month to gain adequate strength). For example, a small fraction of cement and/or pozzolanic binder is added to speed up the setting time as well. The overall process creates a mixture that will develop into a solid, but light and durable product.
The typical production process of hempcrete is contained in a few steps. At the manufacturing location, hemp is stored in a storage room, and the lime is stored in silos. The hemp and lime are added to water and thus mixed. The mixture then travels through a conveyor belt where a machine shapes it into blocks. The blocks are then taken to an area to cure. Once cured, the blocks are separated into 2 m3 batches and loaded on pallets. They are wrapped up with polyethylene packaging film and polypropylene straps to be transported to a construction site.
Hempcrete has been used in France since the early 1990s, and more recently in Canada, to construct non-weight bearing insulating infill walls, as hempcrete does not have the requisite strength for constructing foundation and is instead supported by the frame.Hempcrete was also used to renovate old buildings made of stone or lime. France continues to be an avid user of hempcrete, and it grows in popularity there annually. Canada has followed France's direction in the organic building technologies sector, and hempcrete has become a growing innovation in Ontario and Quebec. There are two primary construction techniques used right now for implementing hempcrete. The first technique consists of using forms to cast or spray hempcrete directly in place on the construction site. The second technique consists of stacking prefabricated blocks that are delivered to the project site similar to masonry construction. Once hempcrete technology is implemented between timber framing, drywall or plaster is added for aesthetics and increased durability.
The typical compressive strength is around 1 MPa, around 5% that of residential grade concrete. It is a low density material and resistant to cracking under movement, thus making it suitable for use in earthquake-prone areas. Hempcrete walls must be used together with a frame of another material that supports the vertical load in building construction, as hempcrete's density is 15% that of traditional concrete. Studies in the UK indicate that the performance gain between 230 mm (9 in) and 300 mm (12 in) walls is insignificant.[clarification needed] Hempcrete walls are fireproof, transmit humidity, resist mould, and have excellent acoustic performance. Limecrete, Ltd. (UK) reports a fire resistance rating of 1 hour per British/EU standards.
The fact that the mixture contains a plant-based compound introduces the caution against water and rising damp levels. Hempcrete walls need to be built with a joint between the wall and the ground in order to avoid capillary rising as well as water runoff at the wall base. Moreover, hempcrete block can only be installed above the ground level. External walls need to avoid rotting of shives by implementing protection by the rain gale with sand and lime plaster. The exterior of a hempcrete based assembly needs these protections, but the interior side of an assembly can stay exposed.
The main cause of the environmental footprint for hempcrete comes from the production of the binder. Reports have estimated that 18.5% - 38.4% of initial emissions from binder production can be recovered through the carbonation process. The binder is produced by the calcination of lime which takes place in kilns at very high temperatures. The transport phase poses an embodied energy footprint since it involves the consumption of diesel. The diesel consumption also occurs due to the functioning of machineries used for hemp shives production. Abiotic depletion is caused from the consumption of lead and cadmium in the electricity generation process, which is largest in the manufacturing of the hempcrete block inside the company.
The hempcrete homes did not perform as well acoustically and were more expensive to build, costing £526 per square metre of floor space compared with £478 for the masonry homes. But the hemp houses met sound regulations and the cost uplift was to be expected given hempcrete was a new and unknown form of construction at the time.
The material can be placed between shuttering manually or sprayed in. Alternatively, blocks are available in a range of different sizes; these have the advantage of being familiar to those with block laying experience. Blocks also eliminate the drying times of insitu hempcrete construction and make hempcrete construction easier in winter when temperatures are low. Hempcrete can be placed between timber studs or externally in situations where clients want to express the structural frame. The Hemp Block company says its 350mm thick blocks have a U value of 0.2 W/m²K, 450mm thick blocks have a U value 0.11 W/m²K which makes these suitable for Passivhaus projects. Hempcrete is also used for insulating solid walls internally and externally and it can be used for floor screeds too.
Woolley says there is a lot of international interest and he is part of a group developing building codes for hemp construction in the USA. Hempcrete has Local Building Standards Scotland (LBSS) certification north of the border and according to Woolley, LABC, the professional body for local authority building control which also certifies products, has said it would certify hempcrete providing someone is prepared to stump up the £3,000 registration fee. Part of the reason no one has done so is because building control is largely supportive of the material which means certification is not essential.
The company is also talking about supplying hempcrete to a small office developer and another one interested in using the material for a three-storey, 120-unit later living development. Hempcrete may not have hit mainstream construction quite yet, but it has come a long way since Ralph Carpenter built his extension 20 years ago.
Hemp buildings store the carbon that has been harvested through the farming of the hemp in the building envelope. The thickness of the walls and whether or not there is also subfloor and roofing insulation all influence the amount of carbon stored. The lime in the AHMC Binder draws in further carbon dioxide as it slowly achieves a full cure. This combination more than balances out the energy emissions in making the AHMC Binder.
O'FLAHERTY, Fin, KHALAF, Faraj Jabber and STARINIERI, Vincenzo (2019). Influence of nanomaterials on properties of lime and hemp/lime composites for energy efficient wall design. Advances in building energy research. 1e1e36bf2d