Hempcrete is a bio-composite building material made from three ingredients: hemp hurds (the woody inner core of the hemp stalk, also called shiv), a lime-based binder, and water. When combined and allowed to cure, these components form a lightweight, breathable solid that provides insulation, thermal mass and moisture regulation in a single material layer.
It is not concrete. The name is misleading, and it trips up almost everyone who encounters hempcrete for the first time. Concrete is a structural material – it carries load, forms foundations, holds buildings up. Hempcrete does none of those things. It has a compressive strength of roughly 0.5 to 3.5 MPa, compared to 20 to 40 MPa for standard concrete. In hempcrete construction, a separate structural frame — almost always timber — supports the building. The hempcrete is cast or placed around that frame as infill, providing the building envelope: insulation, airtightness, moisture management and a substrate for plaster finishes.
Understanding this distinction matters, because it shapes every decision about where and how hempcrete is used. It is a wall material, not a structural one. And within that role, it performs exceptionally well.
How Hempcrete is Made
The raw material is industrial hemp (Cannabis sativa), grown as a fibre crop. After harvest, the outer bast fibres are stripped for use in textiles and composites. The remaining woody core – the hurd – is chopped into irregular chips roughly 10 to 25mm in length. This is the aggregate that gives hempcrete its bulk and its insulating properties.
The binder is typically a formulated lime – often a blend of hydrated lime and a hydraulic component such as natural cement or pozzolanic additives. The lime serves two purposes: it binds the hemp hurds together into a stable matrix, and it creates an alkaline environment that protects the organic material from biological degradation. Over time, the lime continues to absorb CO₂ from the atmosphere through a process called carbonation, gradually hardening and effectively turning the wall into a slow-acting carbon sink.
The mix is simple in principle: hemp hurds, binder and water combined in a forced-action mixer or pan mixer until the hurds are evenly coated. The ratio varies depending on application – a wall mix uses less binder and more hurd for better insulation, while a floor or roof mix may increase the binder proportion for greater density and compressive resistance.
How Hempcrete is Constructed
There are two primary construction methods for hempcrete: cast-in-situ (mixed and placed on site) and prefabricated blocks (manufactured off-site and laid like masonry). Each has distinct advantages, and the choice between them shapes the project timeline, cost structure and skill requirements.
Cast-in-situ
This is the original method and remains the most common approach for residential hempcrete construction. The process works as follows: a timber structural frame is erected first, then temporary formwork – typically plywood or timber boards – is fixed to one side of the frame. The wet hempcrete mix is placed into the formwork cavity by hand or using a sprayer, tamped lightly to remove voids without over-compacting, and left to begin setting. After roughly 24 hours, the formwork is removed and repositioned higher up the wall for the next lift.
The critical variable is drying time. A freshly cast hempcrete wall contains a significant amount of moisture that needs to evaporate before the wall can be finished with plaster. Depending on wall thickness, ambient temperature, humidity and airflow, this drying period can range from four to eight weeks – sometimes longer in cool or humid climates. The wall will feel cool and damp to the touch until it has fully cured.
This is the trade-off. Cast-in-situ hempcrete produces a monolithic wall – a continuous, seamless mass of material that wraps around every timber member, fills every cavity, and eliminates cold bridges within the wall plane. The thermal and hygroscopic performance of a well-cast monolithic wall is difficult to beat. But the project needs a schedule that can accommodate weeks of drying, and the work itself requires training. Getting the mix ratio right, tamping to the correct density (too loose and the wall lacks integrity; too compacted and the insulation value drops), and managing the curing environment all demand experience.
For anyone considering cast-in-situ hempcrete, specialist training is strongly recommended. Organisations such as the School of Natural Building in the UK and practitioners across Europe and North America offer hands-on courses that cover mixing, placing, formwork design and finishing.
Sprayed hempcrete
A variation on cast-in-situ, sprayed hempcrete uses a pump and hose to project the wet mix against single-sided formwork or directly onto a substrate. This method is faster than hand-packing, particularly for larger wall areas and commercial-scale projects. The equipment required – typically a peristaltic or rotor-stator pump with a wide-bore hose – represents a significant investment, which makes spraying more common among professional hempcrete contractors than owner-builders. The technique requires experience to achieve consistent density across the wall face.
Traditional concrete production releases significant amounts of CO2 due to the energy-intensive manufacturing process of cement.
Thermal Performance
Hempcrete’s thermal conductivity typically falls between 0.06 and 0.09 W/mK, depending on density and mix ratio. For context, standard concrete sits around 1.0 to 1.8 W/mK – meaning concrete conducts heat roughly 15 to 20 times more readily. A 300mm hempcrete wall achieves a U-value of approximately 0.17 to 0.26 W/m²K, which meets or exceeds building regulation requirements in most European climates.
But the numbers alone understate the material’s real-world performance. Hempcrete is hygrothermal – it manages both heat and moisture dynamically. The porous structure of the hemp hurds absorbs and releases water vapour from the indoor environment, buffering humidity swings without mechanical ventilation. Simultaneously, the combination of insulation (from trapped air within the hurd particles) and thermal mass (from the lime binder matrix) means the wall absorbs heat during warm periods and releases it slowly as temperatures drop.
The practical result is that hempcrete buildings tend to feel more comfortable than their R-value alone would predict. Temperature swings are moderated, humidity stays within a comfortable range, and the reliance on mechanical heating and cooling is significantly reduced. In-situ monitoring studies have consistently shown hempcrete walls outperforming their modelled thermal predictions – a rare quality in building materials, where real-world performance almost always falls short of laboratory values.
Carbon and Environmental Performance
This is where hempcrete is genuinely exceptional, and where the data is unambiguous.
Hemp is one of the fastest-growing crops available for construction use. A single hectare of industrial hemp can sequester approximately 9 to 15 tonnes of CO₂ during its growing cycle, which takes around 100 to 120 days from sowing to harvest. The carbon absorbed by the plant is locked into the hemp hurds and remains stored in the wall for the life of the building.
The lime binder does produce CO₂ during manufacture – calcining limestone requires significant heat energy. However, the lime reabsorbs CO₂ over time through carbonation, partially offsetting its manufacturing emissions. The net result, confirmed by multiple peer-reviewed lifecycle assessments, is that a hempcrete wall stores more carbon than was emitted in its production. It is one of very few building materials that can credibly claim to be carbon-negative.
By contrast, cement manufacturing alone accounts for approximately 8% of global CO₂ emissions. Producing one cubic metre of standard concrete releases roughly 100 to 300 kg of CO₂. The comparison is not close.
At end of life, hempcrete can be broken down and composted, returned to agricultural land as a soil amendment, or crushed and reused as aggregate in new hempcrete mixes. It does not contribute to the construction and demolition waste streams that make conventional building one of the most resource-intensive industries on earth.
Where Hempcrete Works — And Where it Doesn't
Hempcrete is well suited to wall infill, retrofit insulation, and building envelope applications in residential and low-rise commercial construction. It performs strongly across a wide range of climates – its hygrothermal properties are particularly valuable in temperate maritime climates with variable humidity, but it also works well in hot-dry and cold continental conditions where its thermal mass helps moderate temperature extremes.
There are real limitations, though. Hempcrete is not suitable for foundations, ground-contact applications, or any context where the material will be in sustained contact with standing water. The lime binder provides good resistance to occasional moisture, but prolonged saturation will compromise the material. A hempcrete wall needs to sit on a plinth or base course that lifts it clear of ground-level splashing – typically at least 150 to 225mm above finished ground level.
It’s also not load-bearing, although there are suppliers working on this, such as HemPanel. Every hempcrete building requires a separate structural system. For most residential projects, this means a timber frame, though steel frames and even masonry piers have been used in some applications. The structural frame must be designed before the hempcrete detailing begins, and the two systems need to be coordinated carefully.
Hempcrete walls are thicker than conventional insulated wall assemblies. A typical external hempcrete wall is 300 to 400mm thick – sometimes more for passive-level performance. On tight urban sites or where floor area is constrained, this additional wall depth can be a significant design consideration.
And hempcrete remains a specialist material in most markets. Experienced hempcrete contractors are not yet widely available, material supply chains are still developing in many regions, and building code approval can require additional documentation or engineering input compared to conventional construction. These barriers are lowering as the industry matures – but they are real factors for anyone planning a hempcrete project today.
Cost is the other question that comes up early in any hempcrete project. The short answer: materials cost more upfront than concrete, but the total cost of a finished wall assembly – and the long-term operational savings – tell a different story. For a full breakdown of how hempcrete and concrete compare on cost, thermal performance and carbon, see our detailed comparison.
Prefabricated Hempcrete Blocks: An Alternative to Cast-in-Situ
For much of hempcrete’s modern history, construction has meant mixing on site – combining hemp hurds, lime binder and water, packing the wet mix into temporary formwork around a timber frame, and then waiting. The drying time alone can stretch to six or eight weeks depending on wall thickness, climate and airflow. It works, but it demands patience, skilled hands and a project schedule with room to breathe.
Prefabricated hempcrete blocks change the equation. Manufactured in factory-controlled conditions and delivered to the site pre-cured and dry, these blocks are laid in a thin bed of lime-based adhesive mortar – not unlike conventional masonry. Walls can be finished within days of installation rather than months, and the skill threshold drops considerably. A mason, a competent builder or even an experienced owner-builder can lay hempcrete blocks using familiar techniques.
The trade-off, however, is worth understanding clearly. A cast-in-situ hempcrete wall is monolithic – a single continuous mass that fills every gap, wraps around every timber member, and eliminates cold bridges within the wall plane. A block wall introduces mortar joints, and those joints, however thin, can create minor thermal bridging if not carefully detailed. The wall is also less forgiving of irregular framing, since blocks come in fixed dimensions and need to be cut to fit around obstacles. For retrofit projects on older buildings with uneven structures, cast-in-situ remains the more adaptable method.
What's Available
The most established manufacturer at an industrial scale is IsoHemp, based in Belgium, whose factory in Fernelmont produces up to five million hemp blocks per year. IsoHemp blocks are available in thicknesses ranging from 75mm (for interior partitions and acoustic separation) to 360mm (for external wall envelopes with high thermal performance). Their range includes standard blocks, perforated P-blocks for load-bearing applications within their Hempro system, and U-blocks designed for lintels and ring beams – effectively a full masonry vocabulary translated into hemp-lime.
In the United States, suppliers such as Hemp and Block produce hempcrete blocks in standard masonry dimensions, including 8×8×16 inch and 12×8×16 inch formats. Their range includes notched blocks designed to interlock with conventional stud framing – a practical detail for retrofits, where the blocks slot between existing timber studs and leave a flush surface on one side with continuous hempcrete mass on the other.
In Australia, RespiraBuilt is importing hempcrete blocks that are readily available for use.
These are not the only suppliers operating in this space, and availability is expanding. But the market remains young. In many regions, hempcrete blocks still need to be imported, and supply chains are not yet mature enough to compete on lead time or cost with conventional masonry products. This is changing as production scales up, but anyone specifying blocks for a project should factor in procurement timelines early.
Where blocks make the most sense
Prefabricated blocks tend to suit commercial and larger-scale residential projects where schedule predictability matters. When a build cannot accommodate weeks of drying time – or when construction is taking place in winter, when cast-in-situ hempcrete dries too slowly to be practical – blocks remove the most significant variable from the programme.
They also open hempcrete up to builders who would not otherwise use it. Cast-in-situ hempcrete requires specific training in mixing ratios, tamping technique and moisture management during curing. Blocks reduce the process to something closer to conventional blockwork, which means more contractors can take on hempcrete projects without specialist subcontractors.
For smaller residential projects, particularly self-builds and owner-builder renovations, the choice between blocks and cast-in-situ often comes down to preference and circumstance. Some builders value the monolithic continuity and tactile process of working with wet hempcrete. Others prefer the speed and predictability of blocks. Neither approach is inherently superior – they achieve the same thermal, hygroscopic and carbon-sequestering performance through different means.
Performance considerations
A hempcrete block wall delivers the same fundamental benefits as a cast-in-situ wall: low thermal conductivity (typically 0.06 to 0.09 W/mK), hygroscopic moisture buffering, carbon sequestration and excellent indoor air quality. The blocks are non-load-bearing in standard applications and still require a separate structural frame.
The key performance difference lies in consistency. Factory production controls the density, binder ratio and curing conditions of every block to a degree that on-site mixing cannot reliably match. This means more predictable thermal performance across the wall – fewer localised variations in density that can affect insulation values. For projects pursuing energy performance certification or where thermal modelling needs to be accurate, this consistency can be a meaningful advantage.
The main risk is at the joints. Standard installation uses a 3mm layer of lime adhesive mortar between blocks, which has higher thermal conductivity than the hempcrete itself. In a well-detailed wall, this is a minor effect. But if joints are thicker than specified or if blocks are laid unevenly, thermal bridging at the joints can reduce the overall performance of the wall. Proper installation technique matters – and manufacturer installation guides, such as those published by IsoHemp, should be followed closely.
Hempcrete blocks can be cut with a standard hand saw or appropriate power tool to fit around openings, services and irregular framing. This workability is one of the material’s practical strengths – it behaves more like a soft masonry product than a rigid one, and adapts to most detailing requirements without specialist equipment.
