*A home in Ledbury, UK, has been renovated with a thermal hempcrete envelope. Photo courtesy of Graham Durrant*

By Graham Durrant

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

Hempcrete was originally developed in France nearly 40 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.

*The wall of a historic home in Tagdells, UK, has been insulated with hempcrete. Photo courtesy Graham Durrant*

The embodied carbon credentials of hempcrete are excellent. It is carbon negative, as for every cubic metre (1.3 cubic yards) of hempcrete installed into a building at least 110 kg (.12 tons) 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.

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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.

*A historic building Cornwall is insulated with hempcrete. Photo courtesy of Graham Durrant*

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.

Moisture in walls

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 mold 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.

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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 mold.

Insulation properties

The K value of a 13 inch hempcrete wall is 0.07 W/mK (R value 4.7) 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 (705 lbs) 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.

Spray applied vs. Hand Cast

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.