Let’s Talk about Hempcrete

1. WHAT IS HEMPCRETE?

Hempcrete, or “hemp-lime composite building material” is a mixture of lime, hemp hurd and water that will petrify into a bio-composite building material.

The material is usually either cast on site on shuttering around a structural frame, mechanically spray-applied or pre-cast into blocks or panels.

Hempcrete is a non-structural and non-load-bearing lightweight material that can be used to form solid thermal walls of framed buildings and provide an excellent and breathable insulation wall form.

The exceptional eco-credentials of this natural and sustainable building material make using Tradical® Hempcrete for construction the obvious choice if you want to reduce your energy bills, your carbon footprint and the overall impact of your building on the environment.

2. WHERE CAN HEMPCRETE BE USED?

Hempcrete is commonly used to create walls, ceilings and roof insulation for new builds.

In contrast to conventional insulation materials (which tend to be installed in a cavity within the wall or added to the wall as an extra layer in the build-up), hempcrete combines the wall and the insulation into one material, with the only other material used being the structural frames.

Wet finishes such as internal lime or clay plaster and 2 coats of lime renders externally are the only other additions to a hempcrete wall. If desired, cladding such as timber, stone or brick can be used in place of wet finishes.

In walls and roofs, hempcrete is used in combination with other materials to provide an insulating layer, which can form all or part of the insulation layer, depending on the building design.

In the restoration or retrofit of traditional and historical buildings, hempcrete is often used to replace or repair infill panels in timber frame buildings, or to add insulation to solid walls in older buildings. It is the ideal material to use in the restoration of traditional buildings, due to its ability when wet to mould to unevenly shaped walls in old buildings and support itself without sagging.

3. PROS AND CONS OF HEMPCRETE

Pros

Hempcrete is a breathable and vapour-permeable material that is hygroscopic, meaning it regulates humidity in the air by absorbing water vapour and releasing it again when humidity levels drop. This is important for both the health of the building occupants and to keep the fabric of the building in good condition.

Hempcrete works in harmony with the traditional materials in older buildings by allowing the building to “breathe”, meaning that water vapour can pass in and out of the wall rather than being retained within it, which would cause dampness and damage to the fabric of the building.

Tradical® Hempcrete is naturally fire and pest resistant, meaning there is no need for potentially toxic chemicals to be added to it. Combined with its vapour-permeability and hygroscopic nature, hempcrete buildings truly are healthy living environments.

Hempcrete hemp + lime

Cons

As Tradical® Hempcrete is a relatively new material and is quite different from most conventional building materials, it can be tricky to work with unless certain key concepts and techniques are understood. Unfortunately, due to its rapid acceptance as a building material across Europe, there are examples where a lack of understanding of the materials and construction techniques have resulted in problems with the build.

This is especially common on large projects such as housing estates, where inexperienced contractors have assumed and expected Tradical® Hempcrete to behave in the same way as conventional building materials. However, such problems are not usually long term or irreversible. With a little basic knowledge and some hands-on experience, Tradical® Hempcrete can be a straightforward and rewarding material to work with.

It is important that tradespeople with knowledge of the building code and building practices undergo a specifically designed training course in the installation of Tradical® Hempcrete, to become a trained and certified Tradical® Hempcrete installer.

Architects who are not used to working with Tradical® Hempcrete may not find it easy to detail buildings correctly when using these materials for the first time. This can usually be solved by the architect learning how to detail Tradical® Hempcrete by reading on the topic, visiting existing buildings or attending a training course.

4. WHY IS HEMPCRETE GOOD FOR THE ENVIRONMENT?

Tradical® Hempcrete is a carbon-neutral material, meaning just as much carbon is taken out of the atmosphere from the growth of the hemp plant than is emitted from the production and application of materials on site.

Various estimates for carbon sequestered have been put forward and of course this varies depending on the exact type and source of materials, as well as the application technique. However, a leading UK manufacturer of hempcrete materials has suggested a figure of 165kg net CO2 sequestered per m3 for shuttered and hand-placed hempcrete, and 110kg CO2 sequestered per m3 for spray-applied hempcrete.

The reason the hemp plant absorbs so much carbon dioxide is because it is a tall, fast-growing plant which needs to create a hard-woody stem to support itself at its full height. It grows up to 4.5 metres in around 4 months in the UK climate. The strong woody cellulose of the stem is the part that is chopped up and used as a building material. In the past, the woody cellulose of the stem was considered a waste product of the oil, seed and fibre of the hemp plant, for which it was traditionally cultivated for. Today, the woody cellulose of the stem is a by-product meaning the whole hemp plant can be and is utilised!

As the lime binder used in Tradical® Hempcrete provides protection to the timber structural frame, faster-growing softwoods can be used for the frame, further increasing the sustainability of the building.

Hemp can be grown and processed all over the world in a range of soil types, as long as the climate is not too arid. This means hemp can be considered as a “globally local crop”, cutting down on the energy used to transport hemp hurd for building.

The hemp plant is naturally pest-resistant and is a weed-suppressant, eliminating the need for chemical fertilisers and insecticides. It requires very little fertiliser and is deep rooting, returning nutrients to the soil and conditioning the ground for the next crop. This makes it an ideal break crop for farmers in the UK to use between sowings of other crops such as winter wheat.

Hempcrete binders are made from building lime, which can be either pure lime or 70-80% lime. Building lime is produced by burning limestone, an abundant natural resource, in a kiln. Building lime is energy-saving as it is burnt at a much lower temperature of 900°C compared to 1200-1400°C for portland cement, which is produced by mixing building lime with other minerals. When finished and bagged, lime also has a much lower density than cement, reducing the energy required for transport. When used on site, lime will reabsorb some CO2 from the atmosphere as it sets and cures (the amount of CO2 absorbed varies with the type of lime used). The lime binders that OzHemp uses for Tradical® Hempcrete is from Tradical® in France. The hemp hurd that OzHemp uses for Tradical® Hempcrete is from Tradical® Chanvribat® in France, and has been specially selected, processed and certified as building grade quality hemp hurd.

Once constructed, Tradical® Hempcrete walls, ceilings and roofs have extraordinary thermal properties. These thermal properties have direct and long-term benefits to the environment, due to the massive reduction in energy that normally would be used for the heating and cooling of a building during its lifetime.

5. HOW DOES HEMPCRETE CREATE HEALTHY BUILDINGS?

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Hempcrete is completely natural and free from synthetic, fossil fuel-based materials. Because hempcrete is naturally fire-retardant and pest-resistant (nothing eats lime!) there is no need to add chemicals to ensure its performance in these areas (which is not usually the case with lightweight synthetic insulation materials).

Chemicals added to conventional insulation materials often contain Volatile Organic Compounds (VOCs), which have been implicated in the recent rapid rise in incidence of asthma and severe allergic reactions in western countries as synthetic materials “off-gas” toxic chemicals into our homes.

Hempcrete along with the lime and clay plasters that are applied to its surface are vapour-permeable and breathable, meaning that they allow water vapour to pass through. Hempcrete has hygroscopic properties due to the porous nature of both the hemp hurd and lime binder and the arrangement of particles in the hempcrete material itself. This means hempcrete absorbs moisture during times of high relative humidity in the air and releases it again when the relative humidity drops. The ability to passively regulate the humidity in an internal space means when there is excess moisture in the air (for example when cooking or from occupants breathing), condensation does not get a chance to form on the surface of the wall. This in turn discourages dampness and the formation of mould spores, which can be harmful to human health.

Regulating humidity in an indoor environment to a healthy level of between 40-60% relative humidity has been shown to inhibit the spread of viral and bacterial infections, allergic reactions, asthma and other respiratory conditions and also reduce the occurrence of mites, mould and funghi.

The ability for hempcrete to passively regulate the indoor air quality reduces the need for mechanical ventilation systems, further reducing the energy required to maintain a hempcrete building.

6. WHAT THERMAL PERFORMANCE CAN I EXPECT FROM HEMPCRETE?

The thermal properties of hempcrete are extraordinary. Hempcrete is insulative because of the pockets of air that is trapped within the material, both in the spaces between particles of hemp hurd and in the microscopic pores of the hemp hurd itself.

The typical thermal conductivity of hempcrete is typically 0.06 to 0.07 W/mK. U-values for hempcrete vary depending on its thickness, type of binder used, application techniques and the skill of the contractor. However, a typical u-value for a 350mm thick hempcrete wall is 0.17 W/m2K.

Hempcrete also provides thermal mass due to the density of the lime binder once it has set. This allows hempcrete the ability to store heat within the material itself, like a heater storage. Other lightweight insulation materials only store heat within the air that is trapped inside the material. The ability for hempcrete to store heat in this way as well as provide insulation has two important advantages.

Firstly, building with hempcrete allows for natural ventilation within a building. Modern buildings are highly insulated and rely on airtightness alone to trap the air within the lightweight insulation layer. Windows are often closed to preserve heat, with trickle vents and mechanical heat recovery systems used to maintain the indoor air quality. Hempcrete stores heat (from the sun or from internal heating) in the thermal mass of its walls and it will be released slowly as the building cools down. This means in a hempcrete home you can open the window if it gets a bit stuffy, knowing that all your expensive heat is not going to flow straight out.

Secondly, the slow rate at which a typical hempcrete wall stores and releases heat has a ‘buffering’ effect on natural changes in internal and external temperatures, for example temperature fluctuations in the night and day. This means that a constant internal temperature is maintained with very little heating or cooling. Hempcrete keeps your home cool in summer and warm in winter, with a significant reduction in energy bills.

The combination of thermal mass and insulation properties makes hempcrete unique. Compared to other natural materials, hempcrete can be thought of as halfway between straw bales (which has high insulation but very little thermal mass) and cob (which has high thermal mass but very little insulation). These properties work dynamically to maintain comfortable indoor temperatures with very little need for mechanical heating, cooling or ventilation.

7. HOW MUCH DOES IT COST TO BUILD WITH HEMPCRETE?

Building with hempcrete should cost no more than it does building with conventional materials, but it will not be significantly cheaper either.

As with any material, costs vary depending on the complexity of the design, the exact specifications and a number of other factors. Broadly speaking though, the cost of building with hempcrete should be similar to the cost of building with a conventional brick and block cavity construction, including high quality insulation in the cavity.

Keep in mind that this comparison is only looking at the construction costs. The significant savings over the lifetime of the building due to the thermal performance that hempcrete provides, as well as the improved health of the building’s occupants, should also be taken into consideration.

Please contact us if you would like a quote to build with Tradical® Hempcrete.

8. WHERE CAN I FIND A HEMPCRETE BUILDER?

Our company is committed to providing the highest quality products and greatest accessibility to training and support to the builders and tradespeople within Australia.

We can recommend certified Tradical® Hempcrete builders/installers to assist you with your build.

Please contact us to discuss your individual requirements.

For more information on detailing hempcrete buildings, see: The Hempcrete Book: Designing and building with hemp-lime by William Stanwix and Alex Sparrow, published by Green Books in September 2014.
Available from https://www.amazon.com.au/.

‘Let’s Talk About Hempcrete’ has been provided and referenced courtesy of https://www.ukhempcrete.com/.

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