Hempcrete for conservation

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 encase 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
Heart drawing framed by hempcrete

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.

Clay lump revealed during renovation

We went back to solid clay and then sprayed hempcrete onto it.

Timber frame and hempcrete to replace clay lump

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

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

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

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 https://www.youtube.com/watch?v=_9QZSP6RLVY. 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.

Brick building with hempcrete installation

4 Comments on “Hempcrete for conservation

  1. Absolutely brilliant share, thank you.

    I refurbished a small city tenement in Thurles with hemp plaster, Hemp crete and rolls of hemp wool fiber. Which transformed my house into a comfortable, healthy, living space.

    As Kevin Mc Cloud of BBCs grand designs program said referring to hempcrete. “it is the only building material that should be used going forward”!

    • Yes, there are trials in the US and a nascent industry is developing. I guess the elements the US is lacking are decortication facilities to process the shiv, an easier route through building codes, some clients, and more experience of building with hempcrete to avoid the mistakes that have already been made in Europe. The International Hempcrete Builders Forum is a good way for you to find out who is doing what in the US. Looking at your website the buildings you are restoring would be ideal for hempcrete and I would be happy to talk you through the process.

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