Insulation: MATERIALS ENCYcLOPEDIA
Applications for this system
Infill insulation in frame walls
Hemp hurd or shiv
Setting agents (cement, hydraulic lime or other pozzolans) if required/desired
Ratings Chart for Hempcrete insulation
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- INDOOR AIR QUALITY
- FUTURE DEVELOPMENT
The hurd or shiv of the hemp plant is the woody core left over once the fiber has been stripped away. This hurd is lightly coated with hydrated (and sometimes hydraulic) lime and water and the resulting mix is placed into formwork on the wall. Slip forms can be moved up the wall as hempcrete is added. The material is stable in the wall immediately after tamping. Purpose-built machinery has also been developed to spray hempcrete into wall cavities, but this method is not yet common in North America.
The finished insulation is left exposed on both sides of the wall in order to dry. The hempcrete provides a dense, flat surface and makes an ideal substrate for plasters. Sheet barriers and more conventional siding and wall covering can also be used.
Environmental Impact Rating
Harvesting — Low to Moderate
Hemp tends to be a low-impact crop, as it typically requires little, if any, pesticide or herbicide. It does often require large inputs of fertilizer when grown commercially, but yields of hemp straw per acre tend to be much higher than with other grain straw.
Lime and hydraulic binders are mechanically extracted from quarries and can have low to high impacts on habitat and ground and surface water contamination and flow.
Manufacturing — Moderate to High
Hemp stalks require mechanical cutting and shredding, with relatively low impacts.
The production of lime is very energy intensive, requiring burning of limestone at high temperatures. Unlike portland cement, lime recombines with CO2 from the atmosphere while curing, leaving a lower carbon footprint than other binders.
Transportation — Moderate to High
Sample building uses 9,075 kg of hempcrete:
13.6 MJ per km by 15 ton truck
8.5 MJ per km by 35 ton truck
1.45 MJ per km by ocean freighter
Hemp is not widely grown in North America and may have to travel long distances. It is a lightweight material, offsetting some transportation effects. Lime is a heavy material and accrues impacts proportional to distance traveled. Premixed hempcrete products are currently imported from Europe.
Installation — Negligible
Waste: Low to moderate
Biodegradable/Compostable — Unmodified hemp.
Landfill — Lime and other admixture bags, hempcrete offcuts.
Chart of Embodied energy & carbon
R 0.9-2.5 per inch
The density of the mixture and the degree of compaction in the cavity will result in a wide potential range of R-values for hempcrete. Laboratory tests of premixed hempcrete products have given results in the range indicated, but it is possible for those values to be lower or higher in a given installation due to variable tamping density. Claims of extremely high R-values for hempcrete are not based on lab testing protocols but on notions of “mass enhanced R-value,” which attempts to blend thermal resistance and thermal mass qualities. While it is true that in situ thermal performance can be different from measured R-value, the same is true of any insulation/wall system. Thermal resistance cannot be improved by thermal mass.
Material costs: moderate to high
Chopped hemp is not widely available in North America, and specialty processing leads to high costs. Commercial hempcrete mixtures are available from Europe, but shipping costs and limited distribution raise costs.
Labour Input: High to Very High
There are two stages to the labour input for hempcrete. Mixing requires the thorough blending of water and lime (plus any admixtures) with hemp. Conventional mixing machinery is not intended for the lightweight, dry consistency of hempcrete, and finding an adequate mixing strategy is key to keeping labor input as low as possible. The material can be mixed by hand using hoe, shovel or rake, but this is slow, hard work. Pan mixers are well suited to this kind of material, but can be difficult to source.
Packing hempcrete into wall cavities is typically done manually. The material is lightweight and easy to pour into place by bucket or shovel. It is then tamped to give it cohesion. The majority of the labour is involved with making and moving the formwork used to contain the hempcrete.
Hempcrete can be installed using purpose-made sprayers if the project is in a region serviced by an equipped installer.
Skill level required for homeowners: moderate
Newcomers with some practice and careful formulation and control of tamping procedures can do the mixing and placing of hempcrete. Formwork requires basic carpentry skills. Spray applications will require a hired professional.
Sourcing & availability: moderate to Difficult
Hempcrete is available commercially in some regions as a do-it-yourself mix or it can be professionally installed. Commercial brands of hempcrete are often imported into North America from Europe and must be sourced outside of conventional building supply stores. The raw ingredients to make hempcrete are also available in some regions. Laws restricting the growing of industrial hemp in the US limit availability of local production, but Canadian sources are legal and can be imported into the US.
Hydrated lime is available at most masonry supply and building supply stores. Hydraulic lime, if desired, is typically imported from Europe and will require specialty sourcing.
The lime in combination with the high-silica hemp hurd is a very durable material. The lime mineralizes the hemp over time, making it much less susceptible to moisture damage and mold growth than other plant-fiber insulation. Protected by cladding or plasters on both sides of the wall, hempcrete can be expected to last hundreds of years.
Commercial varieties of hempcrete have undergone sufficient lab testing to gain code approval in the UK and Europe. These test results can be used in North America to show code equivalency, but at this time no codes recognize hempcrete as an accepted solution.
Indoor air quality: High
Lime and mineralized hemp have excellent moisture handling capabilities and the high pH of the mixture is antifungal; both qualities lend themselves to high indoor air quality. There are no off-gassing glues or binders in most hempcrete mixes, though commercial blends should be vetted prior to ordering.
Resources for further research
Allin, Steve. Building with Hemp. Kenmare, Ireland: Seed, 2012. Print.
Bevan, Rachel, and Tom Woolley. Hemp Lime Construction: A Guide to Building with Hemp Lime Composites. London: IHS BRE, 2008. Print.
The commercialization of hempcrete in Europe is due in many respects to European support for industrial hemp production. It is legal to grow hemp in Canada, but the hemp industry has been slow getting established and does not currently support production of commercial hempcrete. The growing of industrial hemp is still illegal in most of the US, so development of hempcrete is dependent on lawmakers changing current restrictions.
If European hempcrete products prove popular, greater distribution of them in North America will likely follow. This will increase access and awareness and may drive regulatory changes that will encourage regional production.
Tips for successful hempcrete insulation
1. Hempcrete can be difficult to mix with typical on-site mixing equipment. The dry nature of the mix combined with the small, lightweight hurd does not work well in conventional cement mixers or mortar mixers. Mixers that actively stir rather than tumble will work faster and with fewer problems.
2. Work out accurate systems for measuring quantities of hemp, lime and water. An overly wet mixture compacts a lot when tamped, lowering insulation values and increasing drying times.
3. Allow sufficient time in the building schedule for drying. This process can take up to one week per inch (2.5 cm) of thickness, depending on weather conditions. This can require scheduling adjustments for builders unused to such long drying times. The addition of a portion of portland cement, hydraulic lime or other hydraulic binder can reduce drying times but will also increase the density of the finished insulation.
4. As the insulation value of the material depends on the degree of compaction that occurs when the hempcrete is tamped into the forms, ensure consistent installation procedures throughout the process, especially when larger crews are involved.
5. Use hex-head screws to secure formwork. Forms are moved frequently and typical screw heads get covered in lime and can be hard to locate and use.