As part of my Professional Diploma in Architecture course at the Centre for Alternative Technology we have to write an essay and practical every other month which aids our design projects and allows us to explore material, design and architectural topics in greater detail.
I’ve had great fun exploring the uses of hempcrete in construction and wrote my first essay and practical on the differences between hempcrete and the more generally used straw bale construction to try and decide which would be better to use on my current design project which is to provide affordable rural housing to the village of Ceinws in Mid Wales.
I know many of you are dying to read it so without further ado, here are my essay and practical, please enjoy reading them. I’ve taken out all the references and bibliography to reduce the blog size, but if you have any questions please let me know and I’ll send you a copy of the full text with references and bibliography attached.
Hempcrete Vs Straw Bale
The aim of this comparison is to aid my investigation into which external wall construction would be more suited to my current design project within the Ceinws Camp site, in the village of Ceinws in Powys. The project focuses on retrofitting and reusing the existing timber frame structures which are currently standing atop a concrete slab foundation and externally clad with corrugated sheeting with no known insulation in the external fabric of the buildings and finished internally with gypsum plasterboard. The aim is to upgrade these existing buildings to provide better insulation, natural ventilation and a healthier indoor environment using natural building materials. This investigation will provide direction for the correct and most appropriate solution to this project.
The two materials I intend to compare and contrast are hempcrete and straw bale construction; two easy to use building materials that could be sourced locally and built with volunteer labour, reducing both material and labour costs. I will compare them on their affordability, buildability, thermal and structural qualities and carbon sequestration ability.
Hemp was widely cultivated in the UK until the early decades of the twentieth century when the growing and possession of hemp was outlawed due to the increasing use of the plant as a recreational drug – cannabis. The industrial hemp plant that is legally permitted in the UK has been developed to contain very low levels of THC (tetrahydrocannabinol) which is the principle psychoactive constituent of the hemp plant and requires a license to grow commercially. Hemp as a material has had many uses for centuries of civilisation the world over. Hemp is a fast growing annual plant that reaches heights of 1.5m – 4m in just under six months. Most parts of the plant are usable from the ‘bast’ fibres in the woody stem of the plant, the ‘shiv’ in the inner woody stem, to the seeds and can be used in its whole state as a source of biofuel. The ‘shiv’ hasn’t historically been used intensively, however this is the primary constituent part of the hemp plant that is used in the production of hempcrete and so this is beginning to change.
Straw bale building has emerged since the 1800s after baling machines were invented for industrial farming in North America. The wasted grains of Nebraskan natives were bundled up, bound and used as building blocks to create simple shelters that protected them from the elements in areas where timber and stone were not prevalent. The combination of war and popularity of concrete stamped out the method for many years after the 1940’s until the rediscovery and revival in the 1970’s by an array of environmental enthusiasts who began to develop and refine the techniques. The first straw building to be built in the UK was built in 1994 and has increased in popularity ever since leading to several hundred in existence in the UK today.
The Oxford Dictionary definition of ‘affordability’ is inexpensive/reasonably priced. Availability of materials, their locality and the cost of the raw materials must be considered; is either one more affordable than the other, coming from further afield, able to be sourced locally, or required to be processed before arrival on site? My practical with all sorts of figures and numbers explored the prices of hempcrete and straw bale and concluded that straw bale would be cheaper per cubic metre.
Comparing directly the price of straw bales and only the raw hemp shiv the prices are fairly similar but where hempcrete increases in costs is the addition of lime as a binder. Straw bale would also be the cheaper to source with fewer materials involved and would be more likely to be sourced from the nearest supplier. This has additional benefits such as cutting down on transportation, and supporting local businesses. Hempcrete is a little more difficult to source in terms of who is providing the raw hemp shiv and at what cost – where they are based, the delivery involved and that some companies prefer to sell you both their hemp shiv and hemp lime binder together as a package potentially limiting options and monopolising the market.
Both products can be purchased outside of the season they are cut as long as they are stored in a dry environment. Hemp shiv has to be processed slightly more than straw bale to remove dust and smaller particles of hemp which will render the hempcrete mix unsuitable for building. The dust and fines absorb most of the water depriving the binder or the water it needs to take its initial set. Based on my research I would hazard the opinion that hemp shiv would be a little less likely to suffer from any potential damage if exposed to moisture during storage, as water is ultimately part of the hempcrete mix, however straw bale would definitely need to be stored dry.
Another aspect of affordability in most projects is labour costs. I believe labour costs can be drastically reduced if using local community volunteer labour with the bare minimum of skilled workers required. This project is intended to be built with local volunteer labour where possible and so the buildability of these construction products is key to ensuring the project is built well and to a high standard. Based on this I will look at the buildability of the two products; which, if any, require specialist installation, how easily and quickly can volunteers be trained to install the products and what are the main hazards and health and safety issues deriving from the two installation methods?
Hempcrete is fairly simple to install but it requires a certain amount of knowledge, skill and pre-build training before work can begin. Building with hempcrete requires timber shuttering to act as formwork, a mixer to mix the shiv and binder together and suitable personal protective equipment for everyone involved. The shuttering doesn’t have to be to a high carpentry standard as it will be removed once the hempcrete has taken its initial set but it could contribute to the overall finish and requires being straight and plumb. Lintels and appropriate timber fixings should also be positioned within the right places in the walls before casting, to ensure fixtures and finishes have something to fix to. When mixing lime and hemp everyone must wear rubber gloves and it is advised that latex gloves are worn under the rubber gloves for additional protection. Eye masks and breathing masks are essential as lime can cause serious harm if inhaled and wearing the appropriate protection will help towards your skin, eyes and lungs coming into contact with lime. Lime reacts even more badly when combined with water so it is especially important that all workers and any additional persons on site are prepped and wearing the appropriate gear and know what to do if lime gets onto their skin. It is also always advisable to have an eye washing station on site in the unfortunate even that lime were to get in anyone’s eye. Building with hempcrete involves mixing the hempcrete to the correct ratio, 4:1:1 of hemp shiv: binder: water, with just the right amount of water so the mixture is not too dry or too soggy, and placing not pouring the mixture into the shuttering. When the hemp is placed in the formwork it requires light tamping just enough to get the mixture into all the nooks and crannies but not heavy tamping. It has been suggested that for every three volunteers, one skilled person is required to instruct, teach and oversee. This provides adequate training and passing on of knowledge and skills while involving skilled and unskilled workers on site. Once the hemp is initially cast it requires two to three weeks to fully set before finishes can be applied, and in this time the cast hempcrete requires to be protected from any additional moisture. In some cases dehumidifiers can be used to speed up the drying process but it’s best left to dry naturally. Once dry the external and internal finishes can be applied – usually a vapour permeable lime based plaster or on the outside timber cladding or facing brickwork could be used as long as the mortar was lime based to let the structure breathe.
Straw bale construction isn’t as complicated and certainly doesn’t need as much preparation time in both preparing the material and the site for the installation of the bales. There are skills and knowledge building with straw bales that are in the interest of the workers building to know and a short introductory course is always advised before any unskilled workers take part in the build, but it is also fairly intuitive and easy and can be learned and picked up on site while building. The straw bales are easy to use, transport and place within the wall construction. They require strapping down to the base and wall plate to keep them compressed. There are certain techniques for splitting straw bales where non-standard lengths are required but it is advised that the building be designed around these standard sizes. If there are any holes in the construction then these can be tightly packed with straw by hand. Once in place the external finished can be applied straight away, ideally a vapour permeable product usually a lime based plaster or render. This helps to stiffen the walls and provides additional fire protection. During construction it is essential that spare straw is swept up frequently to prevent any fires or trip hazards while working. It is advised that gloves be worn when handling the bales but eye goggles and breathing masks are not required. Straw won’t affect anyone with allergies as it doesn’t contain pollen.
Of the two options straw bale building on site would be the more buildable option in terms of ease of installation, expertise required and health and safety on site. Hempcrete requires more skilled workers, with previous experience, some carpentry and requires plenty of personal protective equipment because of the lime involved. It also requires to be left for a few weeks to dry out and work cannot progress on the walls while that process happens, whereas using straw bales the external finishes can be applied straight away.
The existing buildings on the project are currently not up to standards for thermal performance and require to be properly insulated to provide a more comfortable inhabitable space, passively ventilated and to rely far less on energy consumption for mechanical heating and cooling.
My practical looked into the thermal qualities of both hempcrete and straw bale wall construction in more detail and the results showed that both have lower u-values than the building regulations require. The Welsh building regulations state that walls must have a maximum u-value of 0.21W/m²K, for England it’s 0.30W/m²K and for Scotland its 0.21W/m²K. Hempcrete at a thickness of 400mm has a u-value of 0.15W/m²K and straw bales have a u-value of 0.13W/m²K.
Both materials perform well under an air tightness test with results under 2m³/hr/m² at 50 pascals, which is better than the Building Regulations maximum result of 102m³/hr/m².
The indoor air quality using hempcrete buildings is usually of a very high standard. The material build-up of hemp and lime binders act in a hygroscopic way absorbing and releasing moisture and keeping the humidity of the indoor environment within optimum levels for human health.
In addition to this hempcrete provides thermal mass to the building, from its moderate density of hemp shiv and lime binder which set hard. This thermal mass enables the storage of heat in the buildings fabric which is only released very slowly when the temperature of the nearby air cools, therefore allowing some use of natural ventilation to maintain air quality without heat escaping.
Straw bale buildings indoor air quality is also of a very high standard but has to be careful of the moisture build up within its straw core. Environmental conditions including oxygen, pH. temperature and relative humidity are vital to the durability of straw buildings. If the conditions are optimal for micro-organisms the growth of these will be enhanced. It is essential that the moisture content of the straw bales isn’t any higher than 25% and it is recommended that all potential cold bridging such as metal or timber be wrapped in a sack cloth to prevent condensation occurring and creating a weak spot within the straw bale wall.
The likelihood of the straw bale wall providing optimum conditions for micro-organisms to grow is not likely especially if the straw bales are kept dry before, during and after installation. However it is not a problem that presents itself in hempcrete construction because of the lime in the hemp lime binder. For peace of mind and quality of construction, hempcrete would be better for guaranteeing indoor air quality.
Despite there being an existing structural frame in the project, I will look at the structural qualities of hempcrete and straw bales to see if either performs well structurally which could aid in the overall structural strength of the building, help with additional loading and the potential for less structural framing being required within the construction altogether, reducing material costs and skilled labour time.
Hempcrete has no load bearing potential other than being able to hold its own weight. Hempcrete however has a natural flexibility and is prone to bending when placed under structural strain instead of many similar materials which shear or crack under pressure. This elasticity and natural flexural strength of hempcrete shows a high deformability under stress, a lack of fracturing and the ability to sustain significant changes in shape without breaking. These properties can be advantageous especially when used in areas prone to earthquake. Ceinws is not and most likely will not be prone to earthquakes, however these qualities can also aid in the prevention of racking which can assist in the bracing of the timber frame, load bearing structure and ultimately reduces the number of timber requirements by reducing the number and size of diagonal bracing, which also reduces the time, materials and labour spent on the timber frame.
Straw bales can carry typical load bearings of floors, roofs and winter snow usually in buildings of max two storeys. They compress under load but a densely packed bale reaches its maximum compression level and won’t compress any more than this. In construction of straw bale walls, two methods can be used to compress the wall; either when the final course of straw bales has been laid then the wall plate can be lowered and the whole wall compressed and pinned, or the penultimate course can be compressed and the final course placed in between the penultimate and the fixed wall plate and the bales then released expanding and holding them all firmly in place. By compressing the wall it provides structural stability and reduces the risk of later settlement. Rendering the straw bale wall increases the stiffness and the strength of the wall further.
In terms of structural qualities while the hempcrete does provide strength against racking and reduces the timber required in the timber frame, the straw bale construction doesn’t require a timber frame structure at all and is perfectly capable of bearing the load of two storeys. In the case of my design project as there is only one story, and long sections of horizontal lengths hempcrete might be preferable with its additional flexural quality of helping to diagonally brace the walls.
This project looks at how communities can play a part in reducing CO² emissions and therefore the ability to sequester carbon or carbon ‘sink’ anywhere within the building fabric is a major factor in any design decisions made during the selection of appropriate materials and methods of construction in the project.
Both hempcrete and straw bales have negative net carbon emissions locking away more CO² in the building that would be emitted during the production, processing and transportation of the material. This is because plants take up CO² in their lifetime and this isn’t released into the atmosphere when the plant is processed and used in these materials because they remain in their original states. Embodied Carbon (EC) is the figure used to demonstrate how much (kg) of carbon is embodied in each material per m³.
Hempcrete with a hemp lime binder has an Embodied Carbon figure of -35kgCO²/m³ based on figures from the ICE Database 2008. Straw bales have an even better Embodied Carbon figure of -135kcCO²/m³. The difference here would be down to the process of producing lime and natural cements in the hempcrete mixture. They require energy and large amounts of heat to produce therefore producing more CO² into the atmosphere and reducing the Embodied Carbon figures. If you were to take hemp as the main component and instead of using a lime or cement based binder you used clay as researched in Ruth Busbridge and Ranyl Rhydwen’s paper ‘An Investigation of the Thermal Properties of Hemp and Clay Monolithic Walls’ then the Embodied Carbon of that particular hemp composite would be -196kgCO²/m³. This figure for a hemp-clay material is far better than the EC of hempcrete and a significant improvement on the straw bale’s EC figures, leading to the possibility of further research into clay binders for hemp.
As a conclusion to my investigation I would be tempted to go with hempcrete as a final solution, for several of the following reasons. The hemp shiv makes the most of a part of the hemp plant that isn’t usually well used and therefore reduces waste, whereas straw has other uses in its whole state. It has the potential to teach new skills to the volunteer workers and could lead to employment, whereas straw bale building is easy to pick up and therefore not really needed to be taught or paid for. The materials come from a tried and tested factory, whereas the quality of straw bales and their binding is reliant on the farm they are sourced from. The thermal performance of hempcrete and the simplicity of the details in building, cold bridging, roof and wall junctions can be more efficient because hempcrete can be cast in the floor, walls and roof providing an uninterrupted envelope. The width of the walls can vary depending on the u-value you wish to achieve. I am more confident about the hempcrete’s ability to absorb and release water when it needs and it is less likely to degrade from water ingress than straw bale, reducing the possibility of structural risk, damage and micro organisms growing or rodents burrowing.