Using Hemp in Construction

Hempcrete is a natural building material with excellent qualities. It is breathable and has exceptional thermal performance.

Hemp fill

Hemp for use in construction forms a relatively small, but growing, proportion of the output from hemp farming in the UK. The main ways in which hemp is used in construction are to make hempcrete and to provide fibres for quilt insulation.

Photo by Alex Sparrow

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Hempcrete is made from lime and hemp shivs (a waste product from hemp fiber growing); it can be used for walls, floors, and for roof insulation; it’s breathable, absorbing and emitting moisture to regulate internal humidity and avoid trapped moisture and mold growth; it provides excellent acoustic and thermal insulation and thermal mass; it’s lightweight and reduces construction costs; and it’s environmentally friendly. The Hempcrete Book (Green Books, 2014) by William Stanwix and Alex Sparrow provides a full explanation of construction techniques, highlighting potential pitfalls and how to avoid them, and includes a comprehensive resources section and examples of completed builds, with design notes.

Hemp for use in construction forms a relatively small, but growing, proportion of the output from hemp farming in the UK. The main ways in which hemp is used in construction are to make hempcrete and to provide fibres for quilt insulation.

‘Hempcrete’ is the popular term for a hemp–lime composite building material. It is created by wet-mixing the chopped woody stem of the hemp plant (hemp shiv) with a lime-based binder to create a material that can be cast into moulds. This forms a non-load-bearing, sustainable, ‘breathable’ (vapour permeable) and insulating material that can be used to form walls, floor slabs, ceilings and roof insulation, in both new build and restoration projects.

Hempcrete was developed in France in the mid-1980s, when people were experimenting to find an appropriate replacement for deteriorated wattle and daub in medieval timber-frame buildings. Across Europe, awareness was growing about the extensive damage that had been done to such buildings in the post-war period through ill-advised repairs using ordinary Portland cement. Using this material to replace the vapour-permeable earth-and-lime mortars and natural cements in historic buildings prevented the buildings’ fabric from ‘breathing’. This in turn led to the retention of moisture within the fabric, which damaged the timber frames.

A replacement was sought that would not only preserve the vapour-permeable nature of a building’s fabric, thereby keeping it in good health, but also provide insulation. It was discovered that the stem of the hemp plant, highly durable and comprised of strong cellulose (capable of going from wet to dry and vice versa almost indefinitely without degrading), was the ideal aggregate to add to lime mortars to achieve this effect. Thanks to the cell structure of the hemp stalk and the matrix structure created by the individual pieces of hemp inside the wall, together with the properties of the lime binder itself, a hempcrete wall has a good ability to absorb and release moisture. Also, since a great deal of air is trapped inside a hempcrete wall (both within the hemp itself and within the matrix of the hemp shiv in the cast material), it is a surprisingly good insulating material, and the density which the lime binder adds gives the finished material a good amount of thermal mass. Almost as soon as this technique was developed for the repair of historic buildings, people started experimenting with its use in sustainable new build – and found that it was equally suitable for this application.

Is building with hemp a new phenomenon? It hardly seems likely that human civilizations would have cultivated the plant for millennia for such a wide range of uses without using it in their buildings. It is unlikely, however, that physical evidence of any such use in ancient times would survive, since plant-based building materials will of course eventually decay, return­ing to the soil from whence they came. After all, that is one aspect of the very reason that we are interested in them today: a low-impact building material will allow us to house ourselves ‘lightly’, without leaving a legacy of adverse effects on the environ­ment behind us.

There is some evidence, however, that building with hemp did not start in the twentieth century and, further, that properly maintained hemp buildings can last for centuries. A historic hemp house in Miasa village, in the Nagano prefecture of Japan, now recognized as a Japanese national heritage site, was built in 1698 and survives in good condition to this day.

At the time of writing, hempcrete has been used in building for around 30 years – since its ‘invention’ in the 1980s. The use of hempcrete has gradually spread, first across Europe and more recently around the world, and the number of people using it, both in new build and in the repair of older buildings, continues to grow.

In the UK a great many buildings, both commercial and residential, have now been built with hempcrete. A notable upsurge in the commercial use of hempcrete came with the Renewable House Programme, funded by the UK government between 2007 and 2010. Under this scheme, a range of developers received varying levels of public funding to build social housing using natural renewable materials, resulting in the construction of around 200 homes. Of the twelve projects funded, seven used hempcrete as an insulation material.

Since the UK construction industry is notoriously slow to adopt new practices, and has been largely sceptical of the need for (or even the possibility of) using natural materials, State-funded and -driven programmes such as the Renewable House Programme are invaluable in facilitating investigation into issues relating to the large-scale adoption of natural materials within the construction sector. The programme certainly had its challenges, and many of the projects undertaken suffered to some extent from the effects of contractors being given novel materials to work with. However, the overall results were encouraging, and no problems were encountered to suggest that hempcrete, along with other natural materials, would not be suitable for adoption on an increasing scale within the mainstream construction industry.

There are also some other uses of the hemp plant in construction, primarily of the bast fibres in the manufacture of fibre quilt insulation materials, and both shiv and bast fibre as an addition to lime plasters, providing additional strength and some insulation to the plaster. While these materials are not the main focus of this book, they are occasionally referred to throughout, so a brief overview of them is included later in this excerpt. First, however, we look at the different ways in which hempcrete can be used.

Hemp shiv for building

Currently in the UK there are no agreed standards for the characteristics of hemp shiv for the construction industry, nor for its production or processing. In France, where the hemp building industry is more established, there are strict guidelines for hemp farmers that govern the quality and colour of hemp shiv to be used in construction, and it is hoped that, in time, similar standards for the nature and quality of the product can be agreed for the UK industry.

The processing of hemp shiv for use as a building aggregate (once all the leaves, seeds and bast fibres have been removed) involves breaking it up into small pieces and removing any remaining fibre and dust. Hemp shiv for building should be as dry and clean as possible, with a minimum of fines (small pieces of bast fibre) and dust present.

The length of the pieces should be between about 10mm and 25mm, but this is not absolutely critical: successful walls have been made with shiv that contains shorter pieces, but it is generally acknowledged that pieces of this length produce a good matrix structure within the wall, which is beneficial for its thermal performance (a material’s ‘success’ in conserving heat and power in a building) and vapour permeability (the degree to which a material allows water vapour through it). Walls are also regularly built with hemp shiv that contains a certain amount of fines, although the proportion must be low, otherwise the fines can soak up too much water and potentially affect the setting of the binder. The absence of dust from the shiv is far more important, since excessive dust can have an even more significant impact on the structural integrity of the wall – in extreme cases leading to collapse. This is because the dust soaks up a very high proportion of the water added at the mixing stage, causing the binder to fail. The only way to avoid this is to compensate by adding a lot more water, but this will significantly extend the drying time of the hempcrete. The presence of excessive amounts of dust in hemp shiv for building is to be avoided at all costs.

Hemp should be stored dry, although it comes in plastic-wrapped bales, so there is a certain tolerance of these being left outside on-site in the short term if suitable storage is not available. The shiv should go in the mixer as dry as possible, to avoid excess water entering the mix, but if some areas of a bale do get wet it will not affect the quality of the finished hempcrete. However, if the shiv has been subject to prolonged exposure to moisture and starts to show signs of rotting (the colour changing to black), all black areas should be scraped out and removed from the bale before adding it to the mix.

There is no need for any treatment of hemp shiv with fire retardants or preservatives as long as it is being used with a lime-based binder for hempcrete. Once cast as hempcrete, the lime in the binder acts to effectively inhibit insect attack and protect from dampness and fire.

Sources of hemp for building

Until recently, the majority of UK hemp farmers supplied their raw hemp under exclusive supply contract to a company called Hemp Technology Ltd (previously ‘Hemcore’), which invested in a large hemp-processing plant in Halesworth in Suffolk and supplied hemp products to a range of markets, including the construction industry. Hemp Technology went into administration on 28 October 2013. They appear to be seeking investors, but at the time of writing it seems unlikely that the company will be revived in its current form. 

The shiv that Hemp Technology Ltd processed for use in construction was sold on to the market exclusively by the parent company, Lime Techno­logy Ltd, as Tradical® HF and was marketed as part of the Tradical® Hemcrete® system together with Tradical® HB, the hempcrete binder made by industrial lime producer Lhoist, which Lime Technology has the exclusive rights to supply within the UK. To date it has not been possible to purchase Tradical® HF without the equivalent binder, or vice versa, in the UK, and so this has artificially restricted the supply of hemp shiv on to the UK market. However, this may soon change in the light of the current situation at Hemp Technology.

Those wishing to source UK-grown hemp from other suppliers have been restricted to a small number of independent farmers who had not signed exclusive supply contracts with Hemp Technology, and who process their own hemp.

There is nothing to stop builders making links with farmers and sourcing their own shiv for building, but if the farmer is not used to supplying to the construction industry it is important that the essential qualities of hemp shiv for construction are understood by both parties. There needs to be clear agreement in advance about the qualities of the product required (as described earlier) and the cost, including that of transportation to site. Some natural building suppliers supply hemp shiv from independent UK farmers, although this is far from commonplace at the time of writing.

In early 2013 the French company Chanvrière de l’Aube commenced talks with several natural building suppliers in the UK about supplying its hemp shiv through these outlets. Samples shown to the authors were encouraging in terms of the shiv having a very good standard of dryness, a very low dust and fines content, and a consistent colour. However, broad discussions around price suggested that it might be more expensive than shiv currently on the UK market.

It remains to be seen whether importing French hemp shiv into the UK is a commercially viable enterprise. Such imports do not help the develop­ment of the (less well-established) hemp-processing sector in the UK, and the use of foreign shiv would make hempcrete less sustainable, since locally grown UK hemp (with, therefore, lower embodied energy – the energy used in sourcing, manufacture and transport of a material) is available for UK builders to use. On the other hand, the presence on the market of French shiv, subject as it is to the more stringent standards imposed by the French authorities, may prove competitive to UK shiv producers in terms of quality, if not price, and may provide the required impetus for the UK industry to develop its own quality standards for construction hemp shiv.

Cast-in-situ hempcrete

Cast-in-situ hempcrete refers to mixing hempcrete on-site and casting it into moulds construc­ted from shuttering, or formwork, to form the walls, floor or roof in the exact position that they will remain within a building. The shuttering may be temporary or permanent.

Because hempcrete is a non-load-bearing material, it is always cast around a structural frame, which provides the main load-bearing element of the building. This is usually, but not always, built of timber. This applies whether it is being used in a new build or a restoration context. In new builds the usual method is to construct a simple studwork frame from softwood, and bury this within the centre of the hempcrete wall, but alterations can be made to the frame to accommodate different design details, both of the wall itself and of internal and external finishes.

Mixing the hemp shiv and binder together with water can be done with a variety of types of mechanical mixer, depending on the quantity needed, the speed at which it is required, the method of application and access to the site.

The freshly mixed hempcrete is either placed (rather than ‘poured’, since it isn’t a liquid consistency), or sprayed into the void created by the shuttering. It is left for a short time to take an initial set (i.e. set hard enough to bear its own weight), after which the shuttering, if it is temporary, is removed and the hempcrete is allowed to dry out gradually over the next few weeks, until it is dry enough for finishes to be applied.

Hand-placing

The hand-placing of cast-in-situ hempcrete refers to the use of manual labour to place the hempcrete into the void created by the shuttering, as well as to ferry it from the mixer to the place where it is needed. The placing process needs to be carried out carefully to ensure both the quality and certain desirable characteristics of the finished material. The manual transport of the hempcrete is done using large tubs or buckets, since it is a relatively lightweight material.

The hempcrete is cast in shuttering, usually temporary, around the structural frame, which is usually placed centrally within the wall. Hand-placing is the ‘standard’ method of building with hempcrete, although, since it is quite a labour-intensive process, mechanical delivery systems (spray-applying – see below) have been developed. These are particularly suitable for very large-scale commercial applications.

The placing of the hempcrete material by hand allows a high level of control over the quality of the finished product, although there is a need to carefully monitor consistency of workmanship if lots of people are involved in the placing. The low-tech, hands-on nature of hand-placed cast-in-situ hempcrete appeals to self-builders, whether individuals or groups, who have the time to devote their own labour to the build process in order to reduce costs.

The hand-placed cast-in-situ method of applying hempcrete is the main focus of this book.

Spray-applying

The method of spray-applying hempcrete is often used in France and has now arrived in the UK thanks to Myles and Louisa Yallop of The Limecrete Company. They have been a driving force in the increasing use of sustainable construction methods in the mainstream building industry, and have championed the use first of hand-placed and now of spray-applied hempcrete over recent years. The Limecrete Company’s spraying machines, the first of their kind to be used in the UK, have been in operation since early 2012.

As with all things, spraying hempcrete has its advantages and disadvantages. The picture is developing rapidly, with new ways of using the machines, and new equipment to enhance the method, constantly being developed. The spray-applied method for cast-in-situ hempcrete follows broadly the same technique as that for hand-placed hempcrete, but with fully mechanized delivery and placing of the hempcrete, and some minor alterations to the structural frame to accommodate the way the machine works. The hemp shiv is of a finer grade than that used for hand-placed hempcrete, as shiv containing pieces longer than 20mm was found to block the hose on the spraying machine.

The hemp and binder slurry are mixed together at the nozzle of the sprayer. Due to the force with which it is projected out of the nozzle, spray-applied hempcrete adheres firmly to the surface on to which it is sprayed. Usually this surface is a permanent internal shuttering board, from which the hempcrete is gradually built out to the desired wall thickness in several passes. There is no need for a shuttering board on the other face of the wall – all that is required is the skill to build up a flat wall.

With spray application the structural frame is usually positioned on the inside face of the walls, where the permanent shuttering boards can be attached to it. However, since the spraying machine cannot spray around corners, to make the most of this method of application an open, easily accessible frame is required.

For self-builders wanting to carry out the work themselves, sprayed hempcrete is not really an option. The expensive machinery is not something you can hire by the week, and in any case the skill required to operate it can only be developed over time. However, spray-applying has the potential to reduce the number of people needed on-site during the placement of hempcrete, especially on large-scale builds. Against this, of course, you have to factor in the capital outlay, and time and money spent cleaning, transporting, maintaining and repairing the machine.

For builds of up to around 100m (a very large house or large community building, for example) the costs of hand-placing and spray-applying are comparable, with any variation usually depending on the frame design and site-specific issues. As you start to move up to larger builds, spray-application really comes into its own.

Pre-cast hempcrete

As an alternative to in-situ casting, hempcrete can be pre-cast into either blocks or framed panels. This usually brings distinct advantages in terms of predictability of the build process, since in most types of pre-cast hempcrete construction the drying of the hempcrete, or at least most of it, is completed off-site, so any uncertainty regarding the time needed for this is eliminated. This is a particular advantage for the schedule of works in large-scale commercial builds, and times when the construction phase must take place during winter.

However, the downside of pre-casting is that it is often a more complicated way of using hempcrete, with a higher number of processes and materials involved. This in turn means that it may be a less sustainable construction method, although in truth, accurate comparison between cast-in-situ and pre-cast hempcrete is extremely difficult owing to variables such as the scale of the build, design details, the materials used and the distance between farm, processing plant, factory and site. The main pros and cons with pre-cast methods are outlined below.

Blocks 

Hempcrete blocks are usually laid by wetting on the surface and bedded using a thin mortar of hydraulic lime and sand. They are coursed in such a way that thermal bridging (also known as cold bridging – a ‘cold bridge’ is a break in an insulation layer that allows heat to bypass it)between the outer and inner wall surfaces along the mortar joints is minimized. The blocks cut easily with a hand saw, which is useful for fitting them closely around the structural frame, but to improve the speed of construction and to minimize wastage the frame should be designed around the block size, or vice versa.

A number of different companies have been producing hempcrete blocks commercially for several years, both in Europe and, to a lesser extent, in the UK. At first glance blocks might seem to be the obvious way of using hempcrete, especially considering the benefits of off-site drying. However, there is a fundamental inefficiency in casting blocks which are then laid in mortar, because you need a mixer and formwork (shuttering) to make the block in the factory, and then several subsequent processes must take place before it becomes part of the finished building. Casting the wall in situ is more cost-efficient.

The main arguments against blocks are as follows:

In order to cast blocks of sufficient structural integrity that they will stand up to handling during manufacture, warehousing and transportation, the density of the hempcrete mix must be increased (i.e. the proportion of binder must be higher) compared with cast-in-situ hempcrete. This reduces the insulation performance, although it increases the thermal mass.

The blocks need to be laid using a bedding mortar which, although only a thin layer, can potentially create cold bridges through the wall.

The compressive strength of hempcrete is not such that these blocks can be used structurally to support the load of the roof, as concrete blocks would be, so although strong enough to support their own weight, they still need to be laid around a structural frame.

These three factors, combined with the fact that blocks are a more expensive and (potentially) higher-embodied-energy option, mean that cast-in-situ will usually be the preferred method for building with hempcrete.

There have been attempts to cast ‘structural’ hempcrete blocks, with a higher compressive strength, capable of taking a level of compressive force, but these are not really practical, since the increased density of the hempcrete that is required to provide structural performance means that the insulation value is significantly reduced. Such ‘structural’ blocks may be most suitable for internal walls, where a higher density is desirable to provide increased thermal mass or better acoustic performance within the building.

Despite their drawbacks, there are many examples of hempcrete blocks being used within the UK to good effect. They are particularly suitable for large-scale builds, for reasons already described, and can be combined with other forms of insulation. A notable example of such a build is the Adnams brewery warehouse in Suffolk, which was built using 100,000 hempcrete blocks combined with 1,000m3 of cast-in-situ hempcrete. Blocks may also be suitable for very small builds or builds where there is limited access, when mixing on-site may be undesirable for cost or logistical reasons.

Panels

At the time of writing, Hemcrete Projects Ltd, part of the Lime Technology group, is the sole producer of pre-cast hempcrete panels in the UK. The panels, which comprise a timber framework, a built-in insulation layer and a ‘breathable’ vapour-control layer, are produced and dried off-site, then supplied and assembled on-site by Hemcrete Projects. In contrast to blocks, there is no need for mortar at the assembly stage, which means no waiting for this to set before finishes can be applied.

Two types of panels are produced, Hembuild® and Hemclad®. The insulation layer in both types is generally hemp-fibre quilt, which gives them a lower U-value (better insulation performance) than cast hempcrete for a given wall thickness. This is a development driven by market demand for lower U-values, rather than an indication of natural-fibre quilt’s superior overall thermal performance to hempcrete.

Hembuild® panels are structural, i.e. the timber-frame elements are designed such that when joined together on-site, the panels form the structure of the building, as well as the insulation and the airtight thermal ‘envelope’ (the surface that contains the building’s internal heated volume). These panels are suitable for one- to three-storey buildings and are probably most suited to large residential buildings, schools and commercial buildings.

Hemclad® panels have an identical make-up except that the timber elements are not designed to be structural. Instead, they are joined together to act as a cladding around a separate supporting structural frame, often made from glulam (glued laminated timber), steel or concrete. These panels are primarily suitable for commercial or industrial developments, owing to their cost and scale, but have also been used around oak frames in residential buildings. Both types of panel can be manufactured and supplied in bespoke sizes, and are supplied in a range of U-values, depending on the requirements of the client.

The panels are air-dried at the manufacturing plant, which involves blowing air through them to achieve faster drying, and during the winter this air may be heated. Taken together with the increased use of mechanized processes and transportation, there is some doubt that pre-cast panels can claim the same level of environmental credentials as cast-in-situ hempcrete, and they are arguably less sustainable than blocks because of the additional processed materials they include. However, Lime Technology’s literature confirms that the panel systems are carbon negative in terms of their embodied energy.

The predictable nature of pre-cast panels, together with their availability in bespoke sizes to fit the building design, is attractive, especially for large-scale commercial builds. While the complexity of panel systems makes them inherently more vulnerable to thermal bridging and poor airtightness compared with cast-in-situ hempcrete – perhaps even more so than with blocks – if the design of the panels is sufficiently robust, these issues can be overcome through detailing and the addition of extra materials into the wall build-up. The published values for thermal bridging and airtightness achieved by Hembuild® and Hemclad® are certainly a demonstration of this. Unfortunately, however, the solution of using expanding tapes between panels to achieve airtightness means the addition of a high-embodied-energy, synthetic material into the wall build-up.

Cast-in-situ versus pre-cast hempcrete

The main advantage of pre-cast panels and blocks is that the hempcrete is already dried before it arrives on-site. This means that plastering can begin almost as soon as the wall construction phase is complete, whereas cast-in-situ hempcrete walls take several weeks to dry sufficiently for finishes to be applied, and this is dependent on local conditions. During the winter months this drying process makes casting hempcrete in situ a practical impossibility. With cast-in-situ hempcrete, factors such as temperature, exposure, humidity and effective management of drying need to be considered, accounted for and managed within the schedule of the build as a whole. The reliability and consistency of prefabricated panels or blocks are of great benefit in large-scale commercial or industrial applications, where the predictability of how the material will ‘behave’ once applied on-site allows for reduced costs, and is much easier to fit into a complicated, and often financially critical, schedule of works.

The cast-in-situ method, on the other hand, has the advantage of reduced labour costs because the casting and on-site construction are collapsed into a single process. The process of construction is also simpler, and is achievable with minimal mechanization. Cast-in-situ hempcrete, if it is hand-placed, needs only a mixer and a chain of people to ferry buckets of hempcrete, rather than (in the case of large pre-cast panels) a production plant, machinery for drying, and heavy machinery on-site to hoist the panels into place.

The other main advantage of cast-in-situ hempcrete is its ability to form a continuous monolithic layer of insulating material within the fabric of the building, and to be cast into almost any shape required by the building design. This gives it a unique ability as an insulation material to minimize thermal bridging and provide exceptional airtightness.

Embodied energy

Given the appeal of hempcrete as an exceptionally low-impact building material, considerations about the embodied energy of the two methods may be pertinent to many people weighing up the pros and cons of each. Factors include the prefabrication process and transportation to the building site.

Transportation costs (both financial and environ­mental) may be lower for cast-in-situ hempcrete, since materials are brought straight to the site rather than first going to another location to be pre-mixed and cast. In theory the extra journeys implied in having an ‘extra’ stage (block or panel assembly at a factory) should equate to extra fuel in transportation, but in practice this will be dependent on, for example, the distances between farm, processing plant, factory, binder producer and building site, as well as the distance of contractor journeys to factory or site, and these all vary from job to job.

The reaction of the binder with water during casting adds some weight to the cast material, so for the same volume of finished product, the raw hemp and binder needed for cast-in-situ hempcrete are lighter to transport than pre-cast panels or blocks. Set against this, though, are the potential efficiencies of scale achievable in the model of full-load deliveries (of timber, hemp and binder) to a central production factory and then a single delivery of completed blocks or panels to site – depending on the scale of the build, the sources of materials and the distances involved. There is also the potential for greater efficiency in terms of minimizing waste at a large-scale production plant, for example by using offcut timber to heat the factory – although minimizing waste on the building site is also perfectly possible, depending on the awareness and motivation of the contractor.

The fact that energy (for fans and/or heating) is often used in the drying of pre-cast panels and blocks, and that additional machinery is required for on-site assembly of panels, means that cast-in-situ hempcrete may be expected to have a lower embodied energy than prefab­ricated blocks or panels in this respect. However, the unpredictable nature of the drying process for cast-in-situ material is a factor here: because the thermal efficiency of hempcrete (as with any other material) is much higher when it is dry, if finishes are applied too early, when the bulk of the hempcrete is still too wet, additional energy will be used in heating the building when it is first occupied, as the hempcrete continues to dry out. In other words, depending on the skills of the contractor, the energy saved by not pre-drying the panels in the factory might instead be expended on heating the finished cast-in-situ building until the hempcrete has completely dried. This is a difficult question to settle, as detailed comparisons between the energy consumption in the two processes are required, and the type of binder or finishes used may have an effect on drying times. Further research, with full lifecycle analysis (LCA – a technique used to assess the environmental impacts associated with all stages of a product’s life) of the processes involved, would be of great benefit.

Other uses of hemp in construction

Hempcrete is not the only material made from hemp in the construction industry. Hemp shiv is also used as a reinforcing and insulating addition to coarse lime plasters and renders, and the bast fibres, from the bark of the stem, are used in the same way to create fine finishing lime plasters. The bast fibres are also used to make natural quilt-type insulation.

A few years ago, hemp fibres were being used in the manufacture of a strong breathable hemp fibreboard for the construction industry, but at the time of writing no such board is available from natural building suppliers in the UK.

Lime–hemp plasters

Over recent years several natural and heritage building material suppliers in the UK have been supplying lime plasters that contain hemp shiv or bast fibres (see Resources). The main benefit of the hemp is to add a reinforcing structure to the plaster, in the same way that animal hair or wheat straw (and probably also hemp!) did when it was added to lime plasters historically. Tests have shown that the addition of hemp fibres to lime plaster increases its strength by more than 300 per cent.

The addition of hemp also gives the plaster an increased insulation value, as a result of the air trapped in the hemp, although with suggested coverings of 20-50mm (depending on the manufacturer, product and substrate), the thermal performance is a lot lower than in a hempcrete wall. Since both the hemp and the lime within the plaster are vapour-permeable materials, these plasters preserve the breathability of traditional solid-wall structures.

Lime–hemp plasters are perhaps most often used in order to add some insulation to solid masonry walls in traditional and heritage buildings, or in sustainable new buildings where a thicker plaster may be required, for example when plastering cob, rammed earth or straw-bale walls.

In addition to the added insulation value, lime plasters reinforced with hemp have the following characteristics compared with other lime plasters:

They are more robust and so are suitable for areas vulnerable to cracking and knocks.

They experience less shrinkage, and so need less tending after application and are more suitable for patching in and making repairs to existing plaster.

No meshing or hair addition is required if the plaster is applied in coats of at least 10mm.

They can be applied in a single coat of up to 25mm, depending on the product and the substrate.

It is not usually necessary for the substrate to be wetted down before the plaster is applied, since they hold more water than a standard lime plaster.

Because they hold more water, they also need less tending after application.

They take longer to dry out – anything from a week to a month or more, depending on the substrate, the thickness applied and the weather conditions.

If you have built a hempcrete wall, unless it is a very thin one, then you already have enough insul­ation without adding it in the plaster, so at Hemp-LimeConstruct we have limited experience of these plasters and have not used any of the proprietary products available. We have, however, mixed our own lime–hemp plasters, with both coarse and fine hemp, when working on heritage buildings or when dubbing out uneven masonry walls. The extra structure the hemp provides enables a thicker coat of plaster to be applied, and we have found it an effective solution, and the plasters to be a pleasure to work with.

In the products currently on the market, different grades of hemp fibre are used to create plasters described as either coarse, medium or fine. Quite a variety is available from the different manufacturers, although most lime–hemp plasters are supplied as a ready-mixed ‘wet’ product made with air lime. Womersley’s supply a plaster that is mixed from quick lime and hemp with pozzolanic additives to ensure that it sets throughout the depth of the plaster; perhaps because of this, the maximum depth of coat advised is higher than for some of the other lime–hemp plasters on the market.

Hemp-fibre quilt insulation

The natural, breathable quilt insulation made from hemp bast fibres provides a sustainable alternative to conventional glass wool or mineral wool insulation products, such as is used for loft insulation. Various companies in the UK (see Resources) currently produce quilt-type insulation materials made from natural fibres: usually sheep’s wool, wood, flax or hemp fibre. These offer a healthier and safer alternative to conventional synthetic quilt insulations, which are often made from irritant or toxic materials.

Natural-fibre insulations have an equivalent or better thermal performance than conventional quilt insulations, with a typical thermal conductivity of around 0.04W/mK and a typical U-value of 0.16 at a thickness of 250mm. (Thermal conductivity is a measure of a material’s ability to conduct heat. A material’s U-value describes the ease with which it allows heat to pass through it – this varies with its thickness.) There is also evidence that natural-fibre insulations remain effective over a longer period of time than synthetic products.

Hemp-fibre insulation, in particular, provides additional environmental benefits. Thanks to the levels of carbon dioxide taken up by the hemp when it was growing (faster than wood, as hemp creates a woody stem in just months), hemp-fibre insulation can, like hempcrete, lay claim to having negative net carbon emissions – locking away more carbon in the product than was used in its production and manufacture. The production processes, according to manufacturers’ literature, use significantly less energy than that for synthetic insulation products.

The strength of the hemp fibres gives the insulation quilt a natural durability that allows it to keep its structure well over time, compared with synthetic insulation quilts, which often sag as they age and can collapse into a thinner layer in the presence of moisture, resulting in reduced insulation performance. This self-supporting structure makes hemp fibre suitable for warm lofts (insulating between the rafters) and even, according to the manufacturers, for use in timber-frame walls.

The cell structure of the fibres, combined with the low density and open structure of the quilt, allows hemp-fibre insulation to attract and hold moisture from the surrounding atmosphere, releasing it again in response to a reduction in humidity. This characteristic makes it especially suitable for buildings whose fabric is designed to be permeable to moisture vapour, such as heritage properties and hempcrete buildings.

Hemp-fibre insulation can absorb up to 20 per cent of its weight in moisture without any reduction in thermal performance, in contrast to many synthetic quilt  insulations, which deteriorate significantly when they get wet.

To consolidate the hemp fibres into the quilt insulation, there is a need for an adhesive binder, which is heat-sealed into the insulation during manufacture. This is typically recycled polyester, which means that such hemp insulation cannot claim to be a completely natural product. It is worth noting, for those wishing to minimize the presence of synthetic materials in the fabric of their building, that different manufacturers of hemp-fibre insulation use significantly different amounts of this substance in their products.

Those who are building with hempcrete, but don’t wish to use it for the roof insulation, may find that hemp-fibre quilt insulation is perfect as a complementary material. It can also be used in a hempcrete building as a convenient insulating material between suspended timber floors and partition walls. At Hemp-LimeConstruct we also use the insulation quilt to help provide airtightness at junctions between hempcrete walls and building elements made from other materials.


Reprinted with permission from The Hempcrete Book by William Stanwix and Alex Sparrow and published by Green Books, 2014.