The long version

Why we should build with hempcrete

Over the last few years there has been enormous growth in the hemp industry worldwide and the myriad of uses for hemp seem endless. The majority of these uses relate to the fibre, flower or seed. What is left once these have been taken is the broken woody core of the hemp plant. This hemp ‘shiv’, as it is known, is an inescapable by-product of the hemp industry and currently has quite limited options for use or disposal. It can be used for animal bedding, burnt as a fuel source, or used as the main constituent of a natural insulative building material called hempcrete.

There is little denying that climate change is the greatest threat to our future and absorbing carbon out of the atmosphere and keeping it here on the ground is a crucial element of any decision we take. As the hemp industry grows more and more shiv is being produced and this theoretical ‘waste product’ of the hemp industry has the potential to reduce carbon emissions significantly if used wisely. The disadvantage of using shiv as a fuel is that it immediately releases the carbon originally sequestered by the plant. Animal bedding, once used, begins to break down and again releases carbon back into the atmosphere within months. So, whilst these are legitimate uses for hemp shiv, sequestering carbon for any length of time can only be done by making hempcrete and putting it into buildings.

Hempcrete is a mixture of hemp shiv, water and a lime-based binder. Most commonly, these three ingredients are mixed together and placed into shuttering to form a solid wall. What complicates any further explanation of hempcrete as a building material is its versatility. It can be made into blocks or panels, it can be spray-applied, it can be used in roofs and, under certain circumstances, even floors. Hempcrete can be used in a variety of thicknesses from 50 to 450mm, in new-build housing or in renovation. When spray-applied it is particularly useful in building renovation to insulate existing walls. Once dry the hempcrete is generally plastered or rendered with a lime-based plaster or render mix, although other options are possible including cladding. Hempcrete works well with stone, brick, clay, adobe, cob, wattle and daub and timber.

Hempcrete in its modern form was developed in France around 40 years ago as a replacement for wattle and daub panels in timber-framed buildings. The first hempcrete house in the UK was built in 2002 in Haverhill, Suffolk, alongside standard building industry houses. Below are some comments taken from a report from the Building Research Establishment in 2002:

“The SAP ratings and U value calculations suggest that the Hemp Houses should be using significantly more energy than the brick houses and this is demonstrably not the case.”

“In both cases it will be clear that the theoretical performance of the hemp houses does not match their performance in real life in that they are using significantly less energy than predicted.”

“Looking at the data collected using thermal and humidity loggers we find that the temperatures maintained in the hemp houses have been consistently one or two degrees higher than in the brick houses for the same amount of heat input.”

It must also be added that, judging by the limited understanding of hempcrete at the time, much more efficient walls are being built with hempcrete today. With hempcrete locking up carbon down here on the ground and creating opportunities to live more comfortably whilst lowering energy bills, it is a truly viable, carbon-negative building material with enormous potential.

One cubic metre of hempcrete will sequester approximately 110kg of carbon from the atmosphere. An average sized house would be built using around 50 cubic metres of hempcrete for the walls, meaning that an average hempcrete house would lock up 5.5 tonnes of carbon for the lifetime of the building. This compares to a more standard new house of a similar size, in which the walls would be likely to emit 48 tonnes of carbon into the atmosphere (M. Berners-Lee 2010). This is a saving of 53.5 tonnes of carbon for every house built. It is clear to see that the carbon balance is not even close and, notwithstanding any slight variation in calculation methodology, hempcrete houses can significantly reduce the embodied carbon of buildings.

Hempcrete’s ability to save carbon does not stop there. Hempcrete is an insulative walling material and it has two crucial attributes in its favour to reduce carbon emissions and save on energy bills. The qualities of moisture management and thermal mass, which are largely overlooked by building regulations, mean that hempcrete buildings require less energy to keep occupants warm and comfortable. The post-occupancy survey of the Cheshire Oaks Marks and Spencer store built in 2012 found that:

“The holistic approach taken to designing the store’s sustainable features has resulted in it being 42% more energy efficient, with 40% lower carbon emission, per square foot than a benchmark M&S store. The use of highly innovative building materials such as hempcrete and its exceptional air tightness has resulted in the store using 60% less heating fuel than predicted.”

K. Jugdoyal & B. Croxford 2013

A 60% reduction in heating fuel for a building is significant when we consider that, as a general rule, post-occupancy evaluations of buildings tend to show buildings consuming more energy than predicted by their U value calculation. It is very likely that the two important attributes of hempcrete are at play here alongside its simple ability to insulate. The ability of hempcrete to manage moisture and its provision of a useful amount of thermal mass make it an ideal building material.

The human body is particularly sensitive to moisture in the air and keeping relative humidity in the comfort zone allows comfort at lower temperatures. Conversely, if you think of the last time you struggled to get warm in a damp house even though the heating was switched on and the temperature read over 20 degrees you will understand the importance of humidity on human comfort. The breathable qualities of hempcrete allow the walls to moderate moisture in the air, absorbing and desorbing moisture spikes, keeping relative humidity in rooms within the comfort zone. This in turn affords comfort at lower temperatures and setting the thermostat lower has obvious advantages for fuel bills and the planet.

Hempcrete is an unusual insulator in that it allows some heat energy to enter the wall. This is an important attribute as it enables heat to be stored in the wall, levelling out fluctuations in temperature. Most modern insulation materials are placed on the inside of exterior walls meaning that the only thing in the room that is warm is the air. This is a problem because any open window or door or draught will cause the room temperature to drop quickly as the heat has not entered the fabric of the building. For a relatively light-weight building material, hempcrete displays the quality of thermal mass and stabilizes internal temperatures using the same principle as a night storage heater: accumulating heat and slowly releasing it as required.

In addition to being carbon negative and energy efficient hempcrete is non-toxic, rodent-proof, antibacterial and fire-resistant. The quality of fire resistance is of particular significance as recent events have shown just how hazardous and flammable many modern man-made insulation materials can be. There are numerous examples of fire testing hempcrete on the internet where blowtorches are held within a few millimetres of the material for significant periods of time, resulting in only light charring.

Being monolithic hempcrete walls have no cavities or voids leaving nowhere for rodents and draughts to run wild. Eaves details on most houses are a real weakness from the point of view of thermal performance, draughts and rodents. Hempcrete can fill the voids and gaps around the eaves without causing the rafter ends and wall plate to rot. This is because it is breathable and will not trap moisture. Hempcrete is antibacterial and more absorbent than timber so it draws the moisture away from the wood, preserving the timber indefinitely. This is particularly important for the conservation of old timber-framed buildings and allows timber window frames a very long life.

Hempcrete is a durable material when detailed correctly and there are several archaeological sites where hemp and lime have been found still intact after several hundred years. One particularly interesting site is in the Ellora Caves in India where hemp and lime dating back to the sixth century AD have recently been discovered. Archaeologist Dr M Singh concluded that many of the above attributes of hemp and lime were known to the inhabitants and that they were using it to help regulate humidity in the caves. There is at least one contractor in Western France using it today under similar circumstances to achieve the same goal.

Hempcrete is an adequate sound insulator and there is a growing body of anecdotal evidence that it can absorb reasonable quantities of electromagnetic interference.  Although further research is required this could make hempcrete a desirable building material for those concerned about EMI and health.

The hempcrete industry in the UK has moved a long way since 2002. We could now count the buildings made of or renovated with hempcrete in the thousands and as a contractor I am pleased to say that I have nothing but happy customers behind me. Where post-occupancy surveys have been carried out, hempcrete has proven to perform extremely well.

As with any new technology there has been a sharp learning curve and not all projects have gone to plan. The first company in the UK to develop the hempcrete industry supplied hemp and binder and some training, but that training was largely inadequate, leading to problems with some early builds. The material is much better understood today, and it is now clear that the key to successful buildings is to control the installation process. The building industry is a conservative force and government seems to be doing very little to prevent the building industry from pursuing a ‘business as usual’ approach. The sheer volume of materials with high embodied energy delivered to building sites every day is a testament to how little has really changed in the construction industry. Concrete, glass, steel, bricks and toxic petroleum-based insulations, omnipresent on building sites, could give the impression that climate change has not been taken seriously by the construction industry. With buildings being responsible for at least 45% of our carbon emissions there is a crucial need for quality housing that locks up carbon and reduces energy consumption. Hempcrete has the potential to meet that need, generating a greener economy in the process.

Berners-Lee, Mike. How Bad Are Bananas?: The Carbon Footprint of Everything. London: Profile Books, 2010.

Building Research Establishment. Hemp Houses Project BRE Client report number 209-717. Watford: BRE, 2002. Jugdoyal, Keeran & Croxford, Ben. Marks and Spencer Cheshire Oaks – Building Performance Evaluation First Year Performance Summary 25 September 2013. London: UCL, 2013.