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Posted on February 3, 2021 by

A few thoughts on using hempcrete in the conservation and renovation of solid wall buildings.

Hempcrete was originally developed in France nearly forty years ago to repair traditional timber-framed buildings where the clay daub had perished. The concept was to use a natural material and petrify it in a lime binder to create an insulative replacement for the daub. Trials were carried out with a variety of carbon-based materials and hemp shiv was found to be the most robust and best insulating plant material when mixed with lime. The hempcrete we use today is a refined version of this concept and it is spray applied or hand cast into shuttering to build new or insulate existing walls.

Hempcrete fill between timber

The embodied carbon credentials of hempcrete are excellent. It is carbon negative, as for every cubic metre of hempcrete installed into a building at least 110 kg of carbon are locked up for the lifetime of the building. The material is non-toxic and forms an ideal substrate for lime plasters.

Although hempcrete is regularly used in new-builds in the UK, there are several advantages to using hempcrete to repair or renovate older buildings too. Hempcrete is equally advantageous in stone, brick, clay or timber-framed buildings and its versatility on a variety of original substrates means that it can blend in easily to create an insulative, breathable covering on existing walls, or replace the original infill entirely when it is considered beyond repair. The insulation that hempcrete offers is exceptionally good and clients who have lived in cold hard-to-treat buildings before and after hempcrete has been installed are always amazed by the difference the hempcrete makes to their comfort.

Historic farmhouse renovated with hempcrete

Hemp is an extremely durable plant material and the individual particle shapes randomly entangled create a resistant interlocking matrix. This matrix and the stickiness of the lime binder combine to make a solid but yielding wall, ideal for the repair of old buildings.

Older buildings are rarely straight or flat and hempcrete can blend seamlessly to any wall shape that is needed, leaving the building with whatever look is required to maintain its historical importance. These large alcoves in a former chapel were replicated in hempcrete 180mm further forward than the original alcoves to provide adequate insulation.

Hempcrete sprayed onto chapel alcoves 215w, 732w, 768w, 458w" sizes="(max-width: 858px) 100vw, 858px" />

This heart drawing in the same building was left over from graffiti from an earlier era. The clients were keen to keep it, so we masked it off and then hempcreted around it to make it an interesting feature.

Traditional sheet materials used to repair older buildings are unable to blend to the contours of the building so easily, incurring higher labour costs and producing significant waste. Many of these sheet materials are not breathable and as the wall is then made up of separate layers of insulation and a carrier board for the plaster (Plasterboard, Celotex), interstitial condensation becomes a likely consequence.

Hempcrete panels in medieval timber frame

Interstitial condensation occurs when warmer moist air from inside the building makes its way through the wall, cooling as it goes. Warm air is capable of carrying more moisture than cool air. As the air travels towards the outside of the wall, it cools and the water is forced to condense out, leaving unseen water within the wall. Interstitial condensation is particularly damaging to buildings as it goes on largely unnoticed within the wall, creating mould and breaking down the structure of the wall from the inside. Multi-layered walls with non-breathable layers are most prone to this damaging source of moisture, and insulative, breathable monolithic walls like hempcrete are largely immune to it. Hempcrete avoids these layering issues as it is an insulation and a substrate for plastering all in one, and due to its breathability any water that could potentially collect in the wall is easily drawn away to the outside. The monolithic quality of hempcrete also means that there are no cavities in the walls for mice or draughts to run around, saving on energy bills and reducing rodent problems.

It is perhaps important to remember that it is water trapped in the wrong place that leads to the destruction and decay of most buildings so managing moisture is crucial. Hempcrete cannot trap moisture as it is extremely absorbent and moisture gradients operate to draw moisture from the original fabric of the building and away to the air. As hempcrete is more absorbent than timber it protects timber frames indefinitely by drawing moisture away, rather than trapping moisture. Polyurethane-based insulations have no ability to absorb moisture and so when interstitial condensation occurs in a wall the water is drawn onto the timber, creating a moist environment for decay to occur. The lime base in the hempcrete makes the material alkaline and naturally anti-bacterial making it difficult for decay or mould to take a hold. The breathability of the material leads to lower internal relative humidity which in turn limits decay. With good detailing thermal bridging is reduced with hempcrete and cold spots are largely eliminated which in turn reduces the potential for mould.

These three photographs show a clay lump building that was badly damaged by a cement render and lost the first 50mm of its surface.

Timber frame and hempcrete to replace clay lump 300w, 768w, 640w" sizes="(max-width: 1024px) 100vw, 1024px" />

The clay lump on the near end of the building was beyond repair and so was replaced by a timber frame and hempcrete.

Renovation of clay lump with hempcrete 300w, 768w, 640w" sizes="(max-width: 1024px) 100vw, 1024px" />

This building clearly illustrates how versatile hempcrete can be, amalgamating different building elements into one insulative, protective wall.

The K value of hempcrete is 0.07 W/mK which makes it a very effective insulator, and unlike almost all other lightweight building materials it behaves like a thermally massive material. As a relatively lightweight walling material, at 320kg per cubic metre, hempcrete can sit very easily on the minimal footings frequently found in very old buildings without needing additional foundations or underpinning. Conversely, hempcrete behaves like a thermally massive material, heating up quickly and cooling down slowly, enabling it to even out diurnal temperature variations. Allowing the very fabric of the building to warm in this way helps the building to stay at a more constant temperature and reduces condensation risks. This has obvious advantages for the preservation of old buildings and the comfort of the occupants. It is worth noting that most other insulation products lack any thermal mass and so the only thing that warms up in a building is the air. This is a particular problem in the restoration of old buildings as they are difficult to get airtight, and draughts simply push away the warm air to outside. The air tightness and thermal mass of hempcrete remove this problem.

Hempcrete sprayed onto flint barn wall

Spraying hempcrete in the renovation of old buildings is generally more cost effective than hand-casting as it reduces labour costs by avoiding the need for shuttering and the difficult task of getting hempcrete behind shutters at the tops of the wall. Spraying is also more versatile, allowing a wider range of thicknesses to be applied. It gives the ability to create contoured walls to replicate the original shape façade of the wall, adding thickness or removing it easily until the required shape is achieved.

Spraying hempcrete also provides much better adhesion to the substrate than hand-casting and this can be particularly useful in renovation. Where a stone or masonry building is being renovated the thickness of the hempcrete is a crucial factor as to whether the hempcrete is relying on adhesion to the substrate or is a self-supporting hempcrete wall. For example, if a 60mm coating of hempcrete is applied to an existing wall the hempcrete layer will rely largely on adhesion to the wall for its strength. If the hempcrete is more than 120mm thick the adhesion to the wall becomes less important as the hempcrete would then be a solid enough wall in its own right. Theoretically, if the original wall were removed the 60mm of hempcrete could not be self-supporting but the 120mm could be. This means that a relatively thin covering of hempcrete needs to be secured to the original wall to remain intact, and adhesion is the best way. Mechanical fixings can be used with hand-cast hempcrete to achieve this, but it is more complicated and costly.

When hempcrete is hand-cast it is premixed with little or no free binder to stick it to the wall. When hempcrete is sprayed the hemp shiv and wet binder are delivered separately and mixed at the wall surface, allowing free binder to adhere to the wall and the hemp. This free binder is particularly useful when spraying infill to old timber-framed buildings as the adhesion to the timber is greater, avoiding any tendency for the hempcrete to shrink when drying. This prevents the formation of gaps where the timber frame and the insulation meet.

It is a common notion in the conservation of old buildings that any material addition to the fabric of the building should be able to be removed should the need arise. It is difficult to conceive of a reason why hempcrete would need to be removed from an old building, considering the protection it affords, but its removal is perfectly possible if desired. Hempcrete can be cut with a saw, scratched away, drilled or ground off when necessary, making it a material that can be easily joined to or modified, if necessary, in the future.

When spraying, the thickness of the hempcrete can be controlled to suit the intended shape and depth of the wall build-up. This is particularly useful where shape and contour are required to maintain the aesthetic of the original building. The friable nature of exposed daub on damaged buildings can make repairs difficult. Spray-applied hempcrete bonds well with these surfaces with no resulting shrinkage.

Spraying hempcrete onto wattle and daub 300w, 768w, 640w, 1200w" sizes="(max-width: 1024px) 100vw, 1024px" />

The inner daub and internal plasterwork on this wattle and daub wall were able to be preserved even though the outside layer was beyond repair.

Spray hempcrete on wattle and daub building 169w, 575w, 360w" sizes="(max-width: 650px) 100vw, 650px" />

Some additional wattles were added to improve the structure, then it was sprayed with hempcrete. It is difficult to think of another cost effective, insulative solution to this problem that could have restored this building as appropriately.

Relatively new and ecologically sane building materials have a reputation for being expensive. I am pleased to say that on renovation of old buildings, particularly listed buildings, we are not necessarily the expensive option. The speed and simplicity of the hempcrete spray technique enables us to very cost competitive and our attention to detail has left us with nothing but happy customers.

Hempcrete has a one hour fire rating (BS EN 1365-1:1999) and can withstand a blowtorch with only minimal charring. When I give talks about hempcrete people are always amazed to see a blow torch put on the surface of a hempcrete block with only minimal charring occurring and almost no heat transferred to the back of the block. In a French test a large blow torch was put on a 350mm hempcrete wall and after two hours the temperature on the far side had only risen by 2 degrees I have known of one period property that was renovated with hempcrete and subsequently had a fire. The hempcrete remained entirely intact except for where the firemen hacked into it with an axe, convinced that the hempcrete would be smouldering inside. There was no such smouldering and the house was repaired, leaving all the original hempcrete intact.

Hempcrete protects buildings from temperature swings, condensation, fire and water damage when detailed correctly, ensuring these buildings can provide comfort to occupants for many years to come and without compromising the history of the building.

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Comment by Gordon Soderberg on February 8, 2021 at 7:47am


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.

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