When used for insulation and nonstructural walling, this promising material offers a lowcarbon alternative to concreteHempcrete is a cultural matter. Theentanglement of design opportunities and social prejudices is manifest in the wayhemp is cultivated and how the crops are regulated, how the material is made andused in architecture, and even in its name. Yet this has not stopped a growing section ofarchitects from embracing the versatile lowcarbon material in their retrofitting andconstruction projects.Hempcrete is a relatively young material; it has only been around since the late 1980s and is made from a mixture of hemp shiv, lime and water. The name evokes concrete, of course. Their mixing techniques and components are quite similar: hempcrete can be projected onto a surface, poured in formwork or cast in block shapes, either onsite or prefabricated and delivered ready to be used. But the resemblance stops here. The first crucial difference isthat hempcrete is not loadbearing, socannot be used structurally. Another difference is the outstanding ecological credentials of hempcrete compared to concrete. The suffix crete does not meananything on its own; hempcrete wasfirstdeveloped in France and called chauxchanvre and bton de chanvre, whichtranslate respectively aslimehemp andhemp concrete. The -creteseems to havebeen a way to add seriousness and robustness to a material that is not only young and biobased, but made from aplant commonly associated with recreational drug use. The plant remains criminalised in the UK, explains architect Paloma Gormley ofMaterial Cultures, where you also need alicence from the Home Office to grow it.In the first few minutes of the interview, Gormley whose practice has designed a number of hempcrete buildings in the UK, including Flat House in Cambridgeshire in 2020 and Block House in Somerset in 2021, and is currently working on a largerscale housing complex of 72 units in Lewes, Sussex lists the qualities of hempcrete. Ithas excellent thermal properties, she explains, keeping interiors warm in the winter and cool in the summer, and its porosity allows the building to breathe byregulating humidity. Hempcrete is alsoverylightweight, and therefore easytomanipulate, in its brick form, onaconstruction site. While research onhempcrete is still in development, currentstudies suggest it is also a great fireretardant and resistant to pests.The magic of hempcrete starts in the field, with soil and sun. Cannabis sativa isa crop that grows quickly and easily onavariety ofgrounds, in a diversity of climates, without pesticides and with an impressive capacity to lock up carbon; some research suggests it is twice as effective as trees insequestering CO2. Bylaw, the cultivars thatare grown in Europe have low levelsofthe psychoactive component tetrahydrocannabinol (THC), which differentiates hemp from cannabis. Hemp,on the other hand, has high levels ofcannabidiol (CBD), now legal in both theEU and the UK. The whole of hemp down to its dust can also be exploited foragroindustrial uses, including animal bedding as well as the production of ropes, insulants, varnishes, fabrics and highend mattresses. Yet due to its association with drugs and crime, it retains a bad reputation and is the target of sustained efforts by public authorities to prevent its growth despite having been legalised in 1993 in the UK and 2018 in the US, after it had become illegal in the 1920s and 1930s.The required thickness ofhempcrete changes the economy of a project in dense urban settingsThe situation is different in the rest ofEurope; France, inparticular, is the largest hempgrowing industry in the EU, and second largest in the world after China. Ontop of the traditional cultivation of hemp, farmers in France can now legally sellCBD products (flowers, resins, oils, capsules, etc), increasing the range of goods that can be produced from the plant and including those that are more profitable; theculture of CBD products has even been nicknamed green gold.It seems logical that architectsin France and nearby Belgium, the Netherlands and Luxembourg have been responsive to the outputs of a mature supply chain connected to the cultivation of hemp. The past 20 years have witnessed a constantly growing list ofincreasingly ambitious projects that rely onhempcrete for insulation and walling. Paris-based architecture practice Barrault Pressacco encountered the promising material a decade ago on their maiden construction project, a private house in GonnevillesurMer, Normandy. The client wanted a lowcarbon home thatwould leavelittle imprint on the field it would bebuilt on, and the engineers that Barrault Pressacco partnered with on the design (LMIngnieur) suggested hemp.Since then, even though their most recentprojects have had different scales andcontexts, they have kept pushing for theuse of hempcrete, including in two sevenstorey social housing buildings in Paris (AR November 2022): one is paired with a concrete structure, and the other with the iconic Parisian Lutetian limestone. The practice is about to release Wallness, abook dedicated to their own experience with hempcrete and research they have conducted into the material. Touse hempcrete in a project, explains Thibaut Barrault, means working on acoherent network of design choices triggered by thematerial that will impact the buildings construction and its life. CyrilPressacco adds: It is not about living behind the insulation any more; it is about living withinit.The housing block at 18 Rue de la Huchette in Paris was renovated and extended byDumont Legrand Architectes with LM IngnieurCredit: Dumont Legrand Architectes and LM IngnieurThis enigmatic sentence is key to analysing the role of hempcrete in architecture on three different levels: sensory, social and economic. All the architects interviewed for this article agreethat entering a room made with hempcrete walls is a peculiar experience. Gormley describes the fresh earthy smell that only fades away a year after completion, once thehempcreteiscompletely dry, but also itsability toregulate moisture and soften acoustics. Indeed, when left bare, theirregular surface of hempcrete makes forapowerful sonic insulant. Pressacco describes buildings made with hempcrete as architecture for the blind, because spatiality is defined by other senses than sight, relying on light. Ona social and economic level, hempcrete isan insulant that needs to be lived in alsobecause itdemands more thickness thanother nonbiogenic mainstream insulating materials: between 300 and 400mm forhempcrete, where 80140mm of polyurethane would deliver roughly equivalent insulation. The impact on the design is logically significant, specifically onthe surface area. While this might notmatter so much for a singlestorey detached country house, it changes the economy of a project in dense urban settings, with more constrained budgets andspace.Lime is also a stain on the green credentials of hempcrete, because it is produced from limestone, a rock mainly composed of calcium carbonate, that goes through a process of calcination. Limestone is heated at a temperature close to 900C, and for a long time; on top of the fuel necessary for the calcination, limestone also releases its sequestered carbon, leading to additional carbon emissions. Emile Deroose of BC Architects & Studies,who have used hempcrete in the transformation of an early 20thcentury former gendarmerie barracks in Brussels, agrees the amount of lime should be limited as much as possible. But, as of today, it is an essential part of the mix. The lime enables vapour transport, protects the hemp shives from rotting, and ensures the mix remains light and retains its insulating characteristics. You could mixwith clay, heexplains, but then theproperties would change drastically. Deroose also adds thatdespite the carbonintensive lime, theimpressive capacity of hemp shives tostoreCO2 still means hempcrete is a carbonnegative material more CO2 is absorbed than released in the atmosphere when it is produced and used.Despite all its qualities, hempcrete isfarfrom having reached mainstream status,even in France and Belgium, where itis slowly becoming more common. The construction industry is very efficient so longas you stay within the current industry standard from architects to engineers to contractors, explains Gormley. We are all trained to deliver buildings in concrete, steeland brick, and there is a lot of builtin resistance to any kind of change. The supply chain is not so much the issue, it is the demand: in the absence of regulation ittakes a lot of effort. All the architects interviewed for this piece expressed that themain hurdleis cultural: a reluctance tochange, todo things differently. As oftenwhen discussing the challenges and opportunities of architecture in addressing climate change, the solutions to human beings selfimposed ecological predicament are right here; they are cheap, beautiful, performing, at the ready, and can be scaled up quickly. When the material, knowledge, tools, builders and designers exist, all that is needed is the will to change.Lead image:France has the second largest hemp-growing industry in the world, and La Chanvrire de lAube is its biggest mill. Credit:Giaime MeloniLa Huchette housing inParis, France, byDumont Legrand ArchitectesOccupying the corner ofRue de la Huchette in the fifth arrondissement of Paris, this project includes the restoration of an existing building which accommodates 10 studio flats and itsextension onto Rue Xavier Privas, where the circulation is inserted.The original structure is from the 18th century; itswalls were built with rubble masonry, which gives them high thermal inertia and permeability moisture transfer has been key to the buildings longevity and thermal comfort. The architects worked with LM Ingnieur toinsulate walls with hemp. Hempcrete is sprayed inside, while the lime-hemp plaster, which reduces thermal bridging, is applied onto the facade with a trowel. The fibrereinforced concrete used for the new volume takes its hue from surrounding plasters.With offices in both Paris and Bordeaux, Dumont Legrand Architectes have used hempcrete in a few residential projects to date including private homes and social housing. They are currently finishing a large-scale development in Biganos, where hemp is mixed with raw earth.Credit: Cyrille WeinerCredit: Dumont Legrand Architectes and LM IngneurCredit: Cyrille WeinerBlock House inSomerset, UK, byMaterial CulturesSituated in the grounds ofaSomerset estate, Block House was built solely from plant and limebased materials. Thesmall retreat, which has a floor area of 90m2, replaced an existing hermitage that had been destroyed by a fallen tree. The new dwelling is located just north of the former building, and its timber structure spans between two simple flint trench foundations both of these design decisions seek to have minimal impact on the surrounding trees. Wood-fibre insulation is supplemented with prefabricated hempcrete blocks, sourced in Buckinghamshire, which provide the structure with thermal mass, helping to maintain a stable internal temperature. Hempcrete blocks were used instead of hempcrete applied insitu to avoid having towaitfor the lime in thehempcrete to cure.The blocks are exposed internally and finished with claybased paint. Externally, the structure is clad with locally sourced larch, complementing the steeply pitched roof of cedar shingles. On the buildings two long sides, the roof extends above a verandah, held aloft byoak columns.Credit: Felix KochCredit: Isabelle YoungCredit: Isabelle YoungUsquare Feder inBrussels, Belgium, byBC Architects & Studies and EVR ArchitectenA former gendarmerie barracks in the Ixelles district of Brussels has been transformed into amixeduse building, primarily for the use of staff and students of the the Universit Libre de Bruxelles (ULB) and Vrije Universiteit Brussel (VUB). The first phase ofthe project involved thereconfiguration andrestoration of six buildings. As well as reusing the existing structures, materials salvaged from neighbouring buildings, such as bricks and glazing, have been reused in the project. New materials are biobased, including clay plasters and hempcrete. As part of the project, two workshops were held onsite in June 2023 tointroduce participants to hempcrete construction the mixing of hempcrete, creating formwork, compressing hempcrete and removing formwork. The workshop contributed to the construction of hempcrete walls enclosing a seminar room.The project continues BC Architects & Studies explorations in hempcrete; the studio ran a twoweek workshop in 2017 to create the public pavilion FORT V, covering the compressed earth block structure in hempcrete.Credit: BC Architects & StudiesCredit: Farah Fervel2024-10-25Justinien TribillonShare AR October 2024Buy Now