First published September 17, 2019.
Hemp will not replace the concrete in foundations, bridges or tower frames, but it can replace some of the cement-heavy materials used in walls, floors and insulation. The best-known product, hempcrete, is a mix of chopped hemp stalk, a lime-based binder and water. It is light, insulating and breathable, and it locks up carbon as it cures, yet it is far too weak to carry a building's weight on its own. Understanding that difference is the key to reading headlines that say builders are "swapping concrete for hemp".
Here is what hemp building materials are, where they work well, what the evidence says about their environmental benefits, and why the construction industry is looking for alternatives in the first place.
Why builders want an alternative to cement
Concrete is the most widely used building material on Earth, and its key ingredient, cement, carries a heavy climate cost. Making clinker, the core of Portland cement, means heating limestone in kilns, which releases carbon dioxide from both the fuel and the rock itself. Estimates compiled from industry groups and the think tank Chatham House put cement production at up to about 8% of the world's human-made CO2 emissions. When Bloomberg reported on hemp construction in 2019, it put the share at roughly 7%.
The second problem is sand. Concrete needs huge quantities of it, and not just any sand: the rounded grains of desert dunes are too smooth, so builders rely on angular sand from rivers, lakes and coasts. An IMF article by Bruce Edwards, citing the UN Environment Programme, noted that sand and gravel are the highest-volume raw materials humans use after water, with around 40 billion tonnes extracted every year. Cement production alone consumed almost 26 billion tonnes of sand in 2012, up from 11 billion in 1994. That demand has fuelled warnings of a global sand shortage.
What hempcrete is made of
Hempcrete combines three ingredients:
- Hemp shiv (or hurd): the woody inner core of the industrial hemp stalk, chopped into small pieces. Industrial hemp is a non-intoxicating variety of Cannabis sativa grown for fibre and seed.
- A lime binder: usually hydrated lime or natural hydraulic lime, sometimes with a small amount of cement or pozzolan to speed up setting.
- Water, which activates the binder.
The mix can be packed into temporary formwork around a timber frame, sprayed into place, or cast into blocks and prefabricated panels that go up much like masonry. Hydrated lime hardens slowly by absorbing carbon dioxide from the air, a process called carbonation, so walls typically need two weeks to a month to gain useful strength and longer to dry fully. Too much water slows drying and can stop carbonation altogether.
Despite the nickname, hempcrete is not a type of concrete. It contains no gravel, and it behaves much more like a rigid insulation board than like a structural slab.
Where hemp works, and where it does not
The single most important fact about hempcrete is its strength. Typical mixes reach a compressive strength of only about 0.3 megapascals, a tiny fraction of ordinary structural concrete. For that reason it cannot be used for load-bearing walls or foundations. It is used as infill: the frame, usually timber, carries the weight, and the hempcrete fills the spaces between the studs to insulate and enclose the building.
Within that role it has real strengths:
- Insulation: published values work out to roughly R-1.7 to R-3 per inch, depending on density and mix.
- Moisture control: it is very porous and vapour-permeable, absorbing humidity when the air is damp and releasing it when it is dry, which helps prevent condensation.
- Fire and pests: the lime binder makes it hard to ignite; one UK supplier reports a one-hour fire-resistance rating for its system. The high silica and alkalinity of the mix also help it resist mould and rot.
- Weight: it can be as little as a sixth of the density of conventional concrete.
The limits are just as real. Exterior walls need a protective lime render, the base of the wall must be kept clear of rising damp, and performance varies a lot with the hemp, binder, proportions and how firmly the mix is compacted. Claims that hemp walls are "better than concrete" at fire, temperature and ventilation only make sense when you compare a specific tested wall system with another for the same job.
A short Bloomberg video from 2019 showed one Canadian producer at work:
This Canadian company is using hemp to create a cement they call "hempcrete" pic.twitter.com/Ff6SjOj5Xs
— Bloomberg TicToc (@tictoc) August 31, 2019
Who is building with hemp
France is where modern hempcrete took off. Builders there have used hemp and lime since the early 1990s, often to restore old stone and lime buildings, and the material remains more common there than almost anywhere else. Bloomberg's 2019 report quoted Quentin Pichon, founder of a French architecture firm that specialises in hemp, who said the material was becoming increasingly common and had even been used in a seven-storey municipal building; the location was not given, so that example is hard to confirm. Stories about hemp in Roman or medieval mortar circulate widely, but they are difficult to verify.
In Canada, where hemp construction has grown especially in Ontario and Quebec, Bloomberg profiled Mac Radford of JustBioFiber Structural Solutions. Radford said demand for his interlocking hemp blocks was outrunning supply, that the company was profitable, and that after a planned expansion backed by a reported $28 million investment it could make enough blocks for about 2,000 homes a year. Those figures are the company's own projections.
In the Netherlands, the farming company Dun Agro showed a prefabricated hemp house in late 2018, as The Mind Unleashed reported at the time. Its panels were made from hemp fibre, water and a binder pressed into moulds and left to dry for about three months before assembly.
In the United States, hemp building was held back for decades because growing hemp was effectively illegal until the 2018 Farm Bill. Building with it still usually requires approval from local building officials, and in 2022 the Hemp Building Foundation submitted a proposal to add hemp-lime construction to the International Residential Code.
The climate claims, checked
Hemp grows quickly and absorbs carbon dioxide as it grows, and that carbon stays stored in the stalk once it is built into a wall. The lime binder also reabsorbs some CO2 as it carbonates over the life of the building. Life-cycle studies summarised on Wikipedia's hempcrete page estimate that carbonation recovers roughly 18 to 38 percent of the emissions released in making the binder, and some assessments find a net-negative carbon balance for hempcrete walls.
Two caveats matter. First, producing lime still releases CO2, so the binder is the largest part of hempcrete's footprint, and mixes that add cement score worse. Second, transport counts: hemp shipped long distances loses much of its advantage. The honest summary is that hempcrete can be a low-carbon, even carbon-storing, way to insulate and enclose a building, while it does nothing to replace the concrete in foundations, columns and slabs.
The bigger picture: sand, emissions and what comes next
Because builders need angular sand, much of it is dredged from riverbeds and coasts. Heavy extraction can erode banks, lower water tables, damage fish habitat and destabilise bridges. In some countries the trade is controlled by organised gangs; the IMF article describes "sand mafias" in India whose conflicts have reportedly cost hundreds of lives. Any material that reduces the amount of concrete in a building eases that pressure a little.
Realistically, hemp is one tool among several. Lower-carbon cements, recycled aggregates, mass timber, rammed earth and straw are all part of the same search. Hempcrete's natural niche is low- and mid-rise homes and renovations, where its insulation and moisture control are valuable and a timber frame can do the structural work.
If you are considering it for a project, look for products with published test data for fire, thermal and moisture performance, check what your local building code allows, and work with an architect or builder who has used it before. Used in the right place, hemp can help cut a building's footprint; asked to do concrete's structural job, it simply cannot.
Sources: Wikipedia: Hempcrete; Wikipedia: Environmental impact of concrete; Bruce Edwards, The Insatiable Demand for Sand, IMF Finance & Development, December 2015; Bloomberg, 30 August 2019 (original report on JustBioFiber and Quentin Pichon). Embedded video: Bloomberg TicToc on X.
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