Overview of monolithic biocomposites / hempcrete
The reasons to BUILD WITH hempcrete:
- High Insulation Value
- Rock-like Wall Strength
- Long-lasting, Durable Material
- Fast Construction Times
- Simple and Economical
- Mould and Pest Resistant
- Easy to Change or Repair
The reasons to LIVE IN hempcrete:
- Non-toxic and Natural
- Thermally Comfortable
- Acoustically Absorbent
- Atmospheric CO2 Removal/Storage
- Heating and Cooling Cost Savings
- Good Thermal Mass
- Built-in Humidity Management
The reasons to SUPPORT hemp:
- Quick Growth and Highly Sustainable
- No Fertilizer, Weed Killer, Pesticides, Fungicides
- Low Water Requirements
- Multiple-use Plant Crop
- High Carbon Sequestration
- Soil CO2 Rejuvenation in Crop Rotation
Hempcrete is a biocomposite made from Hemp Hurd + Lime (or binder) + Water
Unfortunately it is not a simple formula, as there are many different mix ratios and combinations, depending on who is mixing it and where it is being used.
Usually companies add “pozzolans” to the lime binder, which are earth minerals that react chemically with calcium hydroxide in the presence of water to form compounds with cementitious properties. These can make the lime harder or dry faster, depending on what is needed for the area in which it is being mixed.
Also, there is no standard hemp hurd size and it may include fibers of hemp or flax. The mature industrial hemp stalk must be “de-corticated” to remove the inner woody core of the hemp stalk from the outer strong fibers, and depending on the equipment the hurd can come out in a variety of sizes and size ratios.
One large company, Lhoist, has come out with a special mix of lime binder called Tradical®, which when mixed with their hemp shiv makes what they label as “Hemcrete”. A few other companies are experimenting with, or have found their own, proprietary lime binder and hemp hurd mixes to create hempcrete with specific performance characteristics.
It is because of this wide variation of mixes that it can be difficult to give an exact set of performance specifications for hempcrete, but here are a general range of values measured for hempcrete:
|Compressive Strength: 0.5 – 3.5 MPa||(low for structural components)|
|Thermal Conductivity: 0.05 – 0.20 W/mK||(generally low)|
|Thermal Capacity: 1000 – 1560 J/kgK||(high)|
|CO2 Sequestration: 108 kg/m3||(high)|
|Acoustical Absorption Factor: 0.5 – 0.9||(quite high)|
|Moisture Diffusion Coefficient: ~ 1.10-9 m2/s||(very good)|
|Water Vapour Diffusion Resistance Factor: ~ 4.85||(high)|
|Water Absorption Coefficient (capillary): 0.0736 – 0.15 kg/m2/s||(high)|
In general, there are 5 different methods used to create monolithic biocomposite (ie. hempcrete/hemp-lime) buildings:
1. Monolithic Cast Walls – In-situ, Wet-mixed Hempcrete Pouring
One of the most common methods for building hempcrete walls is a standard slip-form, which is similar to building concrete walls.
First, the structural framing is constructed for the building with all conduits and pipes for electrical, plumbing and other building mechanical installed in the walls. Then, either wooden or plastic forms are attached directly to, or spaced off, the structural framing to define the outer and inner face of the wall surface. One side of the wall can be formed completely from floor to ceiling, while the other side will be built up in sections as hempcrete is poured into the voids between the formwork.
Once the forms are in place, the hempcrete is mixed on-site and either poured or hand-placed in the forms to surround all the framing and mechanical fittings in the wall. It is important to try and fill all the voids in the wall, but if anything is missed, it can easily be patched up after the forms are removed. Also, there is an effort to tamp down the hempcrete at the surfaces of the wall for better surface integrity, yet leave the interior hempcrete as loose as possible for better insulating value.
The hempcrete will require some time to set up (roughly 24 hours) before the forms can be removed and placed higher on the wall for the next part of the pour. The hempcrete in general will need at least a month to cure and dry out to attain most of its strength, and then possibly more time, up to 3 months depending on exterior conditions, to dry completely.
2. Hempcrete Bricks and Blocks
There are a number of companies in Europe that have created different forms of hempcrete bricks and blocks, which are non-structural and used as wall infill/insulation. Once the framing for the building is done, then the bricks are either stacked and mortared together inside the framing, or abutting the framing to form the actual wall assembly. This method saves on the drying time that is needed for the slip-poured hempcrete wall, but it is less monolithic and cohesive than a wall that is poured all at once to fill all cracks and crevices.
There is currently only one structural hempcrete block in the world and it is being developed in Canada. Just BioFiber is based in Calgary and has patented an insulated hempcrete block that very much reminds one of Lego blocks.
These blocks have the structural elements integrated into the matrix of the hempcrete and the entire block is dried at the production facility. Once the blocks arrive on site, they are constructed similar to standard concrete masonry units with mortar to achieve a completely strong and solid wall system. The engineering that has gone into the design of this block is quite susbtantial, and it is reported that the walls could go as high as 800 feet, which makes them a very exciting new prospect. With this block wall system, the wall is essentially complete once built, saving huge amounts of time and labour. It is a completely well-insulated at 11″ thick, and only requires and exterior lime stucco and interior lime plaster finish to complete.
3. Complete Structural Wall Panels
Another method that has been used to construct hempcrete walls is the complete structurally insulated panel created in a factory. These SIPs are eseentially custom wall sections that are built with the hempcrete and structure combined, and shipped to site complete. Each panel is then assembled on site according to design drawings to quickly form a finished building envelope, above a pre-built foundation. These panellized wall systems have mainly been created in England. Some very handsome commercial buildings are built from them, and have proven to outperform buildings built from standard building materials.
4. Spraying Hempcrete
This method is quite similar to pouring hempcrete on site with the difference of industrial spray equipment being employed. Rather than mixing the hempcrete and filling the formwork from the bottom up, large amounts of hempcrete are mixed in a cement mixer and then an industrial sprayer is used to build out a hempcrete wall from interior formwork. It is said to be much faster and have a better insulating value than handpacked hempcrete, but the equipment is quite expensive and takes far more know-how. As of now, there doesn’t seem to be any companies emplying this method in North America, but I’m sure it will come as regulations on hemp cultivation are relaxed.
5. Modular Factory Panels with On-Site Biocomposite Infill
This new patented building technology of monolithic biocomposite construction is in its early stages of development by Calmura Natural Walls and shows much promise for expanding the use of hempcrete around the world. Calmura has tested prototypes at lab-scale and been able to create modular panels of biocomposite, either using hemp hurd or waste wood chips, which are easy to handle, cut accurately and install. These will serve as permanent shuttering panels in place of placing and removing temporary formwork. The factory-produced panels will carry a certified performance that can be specified by architects and engineers, and will speed up the on-site work of contractors, builders and wall finishers. The space between the panels can be filled with other insulation materials, but it is deemed that the best infill will be lightweight wet-mix biocomposite, or hempcrete, to create a solid, monolithic wall to help strengthen and support the structural wood framing of the building. Calmura has been developing this technology for a number of years and now working to bring it to pilot scale operations for further development and proof.