Hemp masonry

Hemp masonry (also known as hempcrete, or hemp-lime composite) is a composite material made of hemp hurd – the inner woody part of the industrial hemp plant’s stem – lime binder and water. Sand can be added to minimise the use of lime binder and to provide a little thermal mass.

Although hemp masonry has recently become popular in the construction of homes with low environmental impact and good thermal performance, it has been used as a building material for at least 300 years. Hemp has also been used for fabric, ropes, and food for thousands of years. The industrial hemp plant used for these applications contains virtually none of the drug THC that is associated with marijuana.

As a building material, hemp masonry offers ecological and economic benefits from beginning to end: for the farmer, the processor, the builder, and the building occupant. Hemp masonry provides very good insulation at the same time as being highly vapour permeable, which makes for very comfortable buildings with good indoor air quality.

Photo: Amber Creative

Buildability, availability and cost

Hempcrete is a relatively low-tech building system, even though the chemistry within it is extremely sophisticated. Building with in-situ hempcrete is slower and slightly more labour intensive than other construction methods. Panelised construction products are becoming available which dramatically reduce construction time.

The skills involved are more about care and control than anything else, and with short training courses anybody can upskill to the required level. However, specialist builders and tradespersons are available to subcontract the whole hempcrete component of the works.

The 2 specialist ingredients of hempcrete are hemp and lime-based binder. These can be obtained and delivered in all parts of Australia and New Zealand. Additional optional sand is generally available from all builder’s supply merchants. The final ingredient is water. Formwork is widely available as form ply, which can be fabricated on site by anyone competent in carpentry, although specialist contractors often supply their own.

The cost of in-situ hempcrete is usually similar to a well-insulated brick veneer wall, but as in any building, the actual price will vary from builder to builder. Sometimes wall building can be arranged to function as a training exercise, attended by people interested in learning how to do it. These activities are run and carefully supervised by experienced experts, often associated with the supply chain.

Appearance

In its raw form (‘off-form’), hempcrete has a moderate texture with a golden honey colour. It is considered by many people to be a naturally beautiful building material in that form. If left off-form, it needs sealing with a fully vapour-permeable sealer, which can be clear so as not to interfere with the natural colour.

Externally, and often internally, walls are rendered for robust rain and water protection. A range of render products is available. All products are based on lime so as to approximate the strength of the substrate; this prevents delamination and maintains vapour permeability. The colour and fineness of surface texture can be varied.

Photo: Carroll Graham and Nick Roberts

Photo: Carroll Graham and Nick Roberts

Structural capability

Hempcrete is a relatively low-strength material compared with concrete and other masonry products. Therefore, it should be formed in such a way as to make it ‘monolithic’ – solid and full thickness wherever possible. Thin ‘leaves’ should be avoided, as these are prone to cracking. This is particularly important with wall frames.

Walls typically use an embedded timber frame to carry the major loads of roofs and upper floors. This should respond to the requirements of the National Construction Code (NCC), typically therefore being designed to comply with Australian Standard AS1684 Residential timber framed construction. All vertical, lateral and racking loads should be carried by the frame and not the hempcrete. The hempcrete then performs the functions of insulation and vapour management, and (unless off-form) provides a substrate for the external rainskin and internal linings.

In Australia, it is common practice to use hempcrete for just the walls of a building, but a lighter form of the material is increasingly being used for roofing and subfloor insulation.

Durability and moisture resistance

Hemp masonry usually dries to touch in about 7 days, but the curing process takes 3 months. This happens via a pozzolanic reaction that, in simplistic terms, ‘pseudo-petrifies’ the mix by facilitating a reaction between the calcium carbonate in the lime and the silica in the hemp. Once the pozzolanic curing reaction has taken place, hemp masonry is inert and resists mould build-up.

Hemp masonry is extremely good at regulating internal humidity because it is vapour permeable and has active lime content. The internal humidity of a home is usually higher than the external humidity, because living in a home creates water vapour through breathing, showering, and cooking. Water vapour will try to move from the inside to the outside because of the higher vapour pressure inside the home, which can cause problems for conventional construction systems. Hemp masonry provides an open pathway for water vapour to exit the building, while still maintaining good airtightness and insulation.

Thermal mass and insulation

Typically, a 300mm thick hemp masonry wall will have an R value of between 3.2 and 3.5, depending upon sand content. This is a good value for almost all Australian climate zones, in both cold and hot climatic conditions.

Sound insulation

The thickness and mass of hempcrete walls provides good acoustic damping from one side of the wall to the other. If walls are left off-form (that is, without render), the absorbent nature of the surface also creates an acoustically ‘dead’ space within each room. This means that sound is absorbed at the wall surface rather than reflected back again. It is the opposite of a ‘live’ room that reflects sound and can be very ‘jangly’, especially if kitchen noises are present.

Fire and pest resistance

Hempcrete is inherently fire resistant. Testing with direct flame exposure showed no combustion with minor surface charring after exposure to direct flame for over 1 hour (refer to References and additional reading).

Termites cannot penetrate the hemp lime mix, but as with all construction systems, careful detailing at the base of walls is still required. Australian Standard AS3660 Termite management – New building work sets out the acceptable methods. Refer to Insulation for details of an integrated slab edge insulation system that maintains the insulation envelope around the perimeter of the whole building, while also complying with the requirements of AS3660.

Toxicity and breathability

Completed hempcrete has zero toxicity. However, care must be taken to control dust during construction, especially from dry lime during the mixing process. It is standard practice for the person doing the mixing, and those working nearby, to wear particulate filters (dust masks). Gloves should also be worn, as the alkaline nature of lime can cause skin to crack.

The finished material is one of the most breathable building materials. It is therefore highly effective at balancing internal and external humidity and reducing condensation risk, especially in otherwise airtight buildings. The presence of lime in the wall also contributes to good air quality, as its alkalinity naturally combats the growth of mould spores.

Environmental impacts

Hemp fibre crops can be grown without pesticides or herbicides, and – depending on soil fertility – with minimal synthetic fertilisers. It is possible to harvest more than one crop per year. Hemp can be grown as a dryland crop without irrigation where there is good subsoil moisture. 

The hemp plant has a well-developed and expansive root system. It can be planted with ‘no-till’ techniques, thus minimising soil disturbance and moisture loss, while increasing soil biological activity and greater soil nutrient mineralisation. In a no-till environment, fallen leaves and remaining roots may also result in soil carbon retention. The ability of the hemp plant to accumulate heavy metals such as zinc and cadmium make it a useful bioremediator when grown on contaminated sites.

The hemp plant grows tall and produces large amounts of leaf which translates to carbon retention in the stem when used as a building material.

After harvesting, the plant is usually laid down in rows on the ground in the paddock, to allow dew to begin the process of ‘retting’, where the long outer fibres (bast) begin to separate from the inner woody stem (hurd). The separation of bast fibres is completed in a processor, or a specialised machine called a decorticator. The fibres are then available for manufacture into fabrics, ropes, or panels (several car makers use this in their interior panels). The hurd is graded into various sizes specified by the end user. For buildings, this is normally in the range of 5mm to 10mm.

Local processing also adds an economic benefit to rural communities, and if the processing plant is portable or small scale, may enable the grower to value-add before it even leaves the farm gate.

At the end of a building’s life, a hemp masonry wall can be deconstructed in such a way as to separate the internal timber frame from the surrounding hempcrete. This enables the timber to be reused and the hemp to be reused, or broken up and composted.

Design and detailing

Loadbearing walls

Damp hemp lime mix is normally installed around a structural timber frame, constructed to Australian Standard AS1684 Residential timber framed construction. This means the hempcrete is not performing any structural function.

Openings

Lintels can be designed in the usual way, but special techniques are required to avoid cracking in the corners of larger openings, generally considered to be 2.4m or more. The simple solution is to omit the hemp masonry altogether over larger openings, and infill with highly insulated cladding.

If hemp masonry is desired over larger openings, then the lintel should be designed to be a lot stiffer than the minimum allowable in the code. This would mean L/500 rather than typical L/300 (one three-hundredth of the span). Because such a lintel is likely to be deeper and thicker, it is important to provide extra adhesion for the hemp masonry. Mushroom plates (or ‘PP washers’ such as those used to hold foam sheets to wall frames in Exterior Insulation Finishing Systems (EIFS) construction), are installed at 150mm centres on either side of the lintel. As the hemp mix is installed, care must be taken to ensure it surrounds all sides of the mushroom plates.

A vertical crack control joint at each end of the opening, also expressed through the render, will avoid unsightly cracking. Lintels should be sealed with linseed oil to minimise the timber drawing moisture from the hemp masonry mix, which would otherwise increase the risk of the hemp not curing properly.

Finishes

External walls must be rendered, unless they are so deeply protected from weather that they are effectively internal. Note that any render should be no stronger than the substrate to which it is applied – this is a universal rule of thumb to ensure that render does not delaminate over time.

There is a good range of render material available, from natural lime renders, some with tiny flecks of hemp for extra texture, to production renders specifically tailored for such low-strength substrates. Note that these renders, while rainproof, must be fully breathable. Acrylic renders may meet the adhesion strength requirements but will be very much less breathable. They may inhibit the wall’s ability to draw humidity from the interior of the building.

Photo: Carroll Graham and Nick Roberts

Internal walls in dry areas can be left unrendered to display the natural texture and colour of the hemp – this is called ‘off-form’. Consistent placement of the hurd mix is of critical importance when an off-form finish is intended.

Inclusions like natural timber or artwork can be installed in off-form or exposed hemp walls, if the pieces are flat along one edge and are prefinished. Oxide colours can also be added to the mix to create naturalistic striations in the layering. Layers can be installed with varied thickness to create wave-like patterns. Off-form hemp masonry must be sealed with semi-penetrating clear sealer that is vapour permeable.

Photo: Amber Creative

Internal wet areas can be treated as external walls, but the surface finish must be considered before the wall is constructed. Ceramic wall tiles are best fixed to a cementitious sheet, in which case the wall will need to be ‘blocked out’ before forming to allow fixing of the sheets. That is, blocks of timber are fixed to the frame and are the width of the hempcrete thickness, so that they are exposed when the formwork is removed, allowing sheeting to be fixed to them. Alternatively, a void can be battened off the blocks in the wall, and water services run within it behind the sheeting. Some renders, like the ‘Moroccan’ and ‘Venetian’ render styles can be used in showers if waterproofed with ‘black soap’. In any case, a waterproof membrane is required behind showers and at the base of the wall. The wall area behind showers is non-breathable, but being of small overall area this is of no consequence to the operation of the building as a whole.

Photo: James Isaacs, Belubula Hemp Homes

The build process

Mixing hempcrete

Hempcrete is most often installed ‘in situ’, meaning it is mixed on site and placed into formwork. Best practice is to use a horizontal pan mixer, usually of 120 to 350 litres capacity. The hurd, lime, and water are mixed in carefully measured ratios and in strict accordance with the supplier’s instructions. It is important that the components are kept dry so that their moisture content is controlled. The addition of water to hemp mixes must be very carefully controlled as the hemp hurds are quite absorbent and can reduce available water in the mix.

Sand can be added during the mixing process for walls that need a little extra thermal mass. It should be washed river sand. Sand fills parts of the interstitial spaces, and also increases the surfaces contacted by the lime in the mix, which enhances the strength of the hempcrete. The mixed hempcrete is stiff and different to work with than other common building materials (for example, concrete).

Photo: James Isaacs, Belubula Hemp Homes

Construction process

The damp hemp lime mix is normally installed around a structural timber frame. The roof structure and roofing are installed first, which provides a dry controlled environment to mix and install the hemp. Conduits and pipes need to be in place before the mix is added. Hemp can also be used in prefabricated timber framed panels, preformed blocks and unframed panels.

Temporary formwork is placed on either side of the wall frame, with ties to the frame to keep it located correctly. Usually formed in ‘lifts’ of 600mm, the mixed hemp composite is placed carefully into the formwork – to ensure no voids are left – and tamped lightly into place, not compressed hard (as in rammed earth). This maintains the myriad of interstitial spaces that are fundamental to providing insulation.

Left in the forms for 12 to 24 hours, hemp masonry usually dries to touch in about 7 days, but the curing process takes 3 months. Initially, fresh hemp masonry is easily dented, so care must be taken on site. Subsequent ‘lifts’ can be done the following day right to the top of the wall.

The final lift may be an odd height dimension, with formwork adjusted or cut to suit. It is important that the final hundred millimetres of mix is installed well, even if it is close to the roof structure and access is difficult. Thought needs to be put into the strategy; often the mix needs to be installed side-on.

Photo: James Isaacs, Belubula Hemp Homes

Frames

Wall frames need to be made with hemp installation in mind. Generally, this means less timber can be used because hemp mix surrounds the frame, making many studs superfluous, whereas cladding and linings require support at corners, etc. Noggings should be installed vertically or at 45° rather than flat, allowing the hemp to be installed more easily under them without voids. Large groups of studs under lintels should be avoided, as these can ‘split’ what should be a monolithic mass of hemp, increasing the risk of cracking. Where there is a lot of timber, coating it with linseed oil will minimise the timber drawing moisture from the hempcrete mix, which would otherwise cause premature drying and cracking.

It is important for the hemp mix to be as contiguous as possible in the whole wall – large blocks of timber tend to split the hemp into 2 halves. This is a problem often seen where wall frames are manufactured off site, in a ‘lazy’ way, where multiple studs are installed to support larger lintels, rather than the more labour-intensive (traditional) method of notching lintels in to supporting studs. Therefore, it is important that frame manufacturers understand the distinction between hemp and other construction systems. Bracing should be either stainless steel strapping, or galvanised strapping painted to prevent corrosive attack by unreacted lime. Wet area and other fixtures with high load fixing points require solid timber blocking to be pre-installed. When the formwork is stripped, these are visible but flush with the surface.

Source: Envirotecture

Source: Envirotecture

Using hempcrete for insulation

A half-strength hemp-lime mix, without sand, can be used to provide insulation on top of a suitable ceiling or in the subfloor. While hempcrete has a lower R value per unit of thickness than other materials (for example, closed cell foam board), it is a natural and breathable material that stores carbon. If the depth of ceiling structure and floor design permit, it can create an effective thermal and acoustic envelope.

In Europe, hempcrete has been used in the construction of tamped hemp floors, and as insulation under conventional concrete slabs, although this is not common in Australia.

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• Building Research Establishment (2003). Thermographic inspection of the masonry and hemp houses – Haverhill, Suffolk, Building Research Establishment, London. 
• Evrard A, De Herde A and Minet J (2006). Dynamical interactions between heat and mass flows in lime-hemp concrete, 3rd International Building Physics Conference, Montreal, August 2006.
• Glé P, Gourdon E and Arnaud L (2011). Characterization and modelling of the acoustical properties of hemp shiv and hemp concrete, Ecole Nationale des Travaux Publics de l’État, Membre de l’Université de Lyon. 
• Gross C, Walker P and Pritchett I (2016). Structural performance of timber stud walls confined by hemp-lime, BRE Centre for Innovative Construction Materials University of Bath, United Kingdom.
• NNFCC (2015). Renewable hempcrete house: energy efficiency monitoring
  programme, Report for DECC, NNFC, York.
• Clarke D (2018). Hemp, hemp masonry and hempcrete. Sanctuary magazine.
• Sutton A, Black D, Walker P (2011). Hemp lime: An introduction to low-impact building materials [PDF]. Building Research Establishment. 
• The Australian Hemp Masonry Company.
• The Australian Hemp Masonry Company (2017). Australian Hemp Masonry Company construction manual, Australian Hemp Masonry Company, New South Wales.
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• Woolley T (2006). Natural building: a guide to materials and techniques, The Crowood Press, Ramsbury. 
• Yates T and Ferguson A (2008). The use of lime-based mortars in newbuild. NHBC Foundation, Milton Keynes.

Learn more

• Read Passive heating and Passive cooling to find out what works well in winter and summer
• Explore Materials for ideas on methods and materials for new home construction
• See Designing your home for considerations when planning a new home

Authors

Principal author: Dick Clarke 2020