Hempcrete has a vast potential for environmental sustainability, especially when compared to the problematic Plain Concrete. It’s time for a closer look.
By Ellen Rubin
I’m enamored with the idea of living in a house that makes healthy, sustainable, and economical living possible. Using hemp as a building material creates a non-toxic environment with stable humidity and temperature control that is not only more comfortable but will help save money on heating and cooling, and is made from a renewable and recyclable material. In this case, the adage, “everything old is new again” can be applied to just about everything hemp. In our never-ending quest for the next best thing, the great can get overlooked and this is what happened when it came to building with hemp.
Hemp hits the trifecta of sustainable living: good for the individual, the industry, and the environment. To me, living or working in a building that inhibits mold, maintains a steady temperature and humidity, has great acoustic qualities, and deters pests sounds pretty great. Builders should be excited about a product that is less toxic, lighter weight, yet durable. Everyone should be happy about a product this good for the environment because it not only has a smaller carbon footprint, but is, in fact, a carbon sink, and doesn’t require huge amounts of water, and no pesticides, herbicides, or insecticides to grow.
Whether it’s growing, processing, constructing, or dwelling, hemp has a lot to offer. We’ll examine each of these qualities in greater detail in this article, go through the advantages and disadvantages and look at how a building material from the past is making a comeback. Hempcrete may just be the key to creating clean, healthy, and durable buildings of the future.
What is Hempcrete?
Hempcrete, sometimes called cellulose aggregate concrete (CAG), lime-hemp concrete (LHC), or hemp concrete, has just three components: hemp hurd or shiv (the woody waste product of a hemp stalk), lime to act as a binder, and water. When mixed together in the right proportions, a concrete-like mixture called hempcrete is formed. Hempcrete construction can be achieved three ways: ingredients can be mixed on-site and used as an infill between studs, pre-made panels, or more commonly, pre-made blocks that can be used to enclose a new construction, an addition, or renovation of an existing building. (See below for a greater description of each application.)
Hemp hurd is considered a waste product of industrial hemp agriculture. The fibrous threads used for textile construction are stripped from the woody inner part of the stalk in a process called retting. The inner part comprises 70% of the plant and can be broken up into pieces and used for either animal bedding, paper production, hemp plastics, or in this case, hempcrete.
The ideal hurd length for hempcrete is 10-25mm pieces. This size allows for good matrix formation to ensure sturdy construction. If pieces are too large, there won’t be enough binder coverage to create a strong bond. Smaller pieces, called fines, will soak up too much water and affect the setting of the lime binder. If hemp dust is present, even more water is soaked up during mixing and the structural integrity will be severely impacted. Therefore, it’s important that hurd be of the right size and dust-free. Very experienced builders can adjust the hemp/water/lime ratio to compensate for hurd size, but it’s simple enough to purchase quality product with the right size hurd.
The other main ingredient in hempcrete is lime, specifically hydraulic lime or hydrated lime. Hemp and lime combined create a sum even greater than the parts. Lime contains silica or alumina and, when exposed to water, acts as a binder for the hurd pieces as the silica in the lime binds with the silica in hemp to form a strong material. Because of lime’s high pH level of 12, it has antimicrobial and antifungal properties that increase these innate hemp properties to help keep hempcrete resistant to mold. Lime is slightly caustic, so the use of protective gear, like dust masks and gloves is required.
Environmental Advantages to Growing Hemp
Hempcrete is environmentally sustainable in the growing phase and continues to be sustainable even after it’s a finished product.
Growing hemp has many, many advantages for both the farmer and the environment. First, it’s easy to grow and thrives throughout much of the world, as long as it’s not too close to the equator. The leading growers are Canada, China, and France so there’s likely a fairly local source. Transportation costs, both monetarily and in emissions, shouldn’t be too excessive. Since the US legalized industrial hemp growing in 2018, and is now the 4th largest grower, it’s becoming even more accessible.
Farming hemp is profitable because more hemp can be grown per acre than any other crop; between 1.3 and 1.5 million plants can be grown per acre, or 30-35 plants per square foot, if you are growing it for fiber and hurd use. (Plants grown for seed and medicinal purposes need to be planted further apart.) At harvest, this translates to almost 9 tons of product per acre. Building a 1250-foot house requires the harvest of only 2.5 acres of land. Hemp can be grown with little to no irrigation, depending on location, and because of its character and the fact that plants are grown so close together, no fertilizers, pesticides, insecticides, or herbicides are needed. Hemp fields also provide a great environment for wildlife.
Hemp grows very quickly. It will reach 3 feet and be ready for harvest in around 3 months. Two crops can easily be harvested per year as opposed to the 20-25 years that it takes for a forest to mature to a harvestable stage. Hemp also captures carbon twice as effectively as trees. Hemp plants have very deep roots, up to 9 feet deep, that help prevent soil erosion. The roots, as well as the leaves, absorb toxic metals from the soil and air; it was planted around Chernobyl to absorb cesium and strontium. The stems and leaves that fall during a growth cycle help replenish nutrients in the soil, eliminating the need for artificial fertilizers. No chemical usage means no toxic runoff to pollute our waterways or groundwater.
To summarize, hemp grows more densely, faster, and without chemical additives to leave soil healthier than before it was planted making it an exceptional rotation crop for farmers. Finally, growing hemp captures more CO2 than trees. For each ton planted, 1.52 tons of carbon is removed from the air. This is twice the amount captured by forests.
Environmental Advantages of Using Hempcrete to Build
There are numerous advantages to building with hempcrete, both for the builder and the owners. The most significant are listed below:
Carbon Dioxide Sequestering
Among the most important environmental advantages of building with hemp is that it continues to sequester CO2 even after it’s harvested and used to create hempcrete. It’s a carbon negative building material since it captures carbon dioxide when it’s mixed with lime rather than releasing it. The CO2 is kept within the walls of the home for as long as it stays standing. 716.6 pounds of CO2 is stored in 1 ton of dried hemp or 6.88 pounds per cubic foot. If you build with hemp blocks (see below for building options) you would sequester 220-265 pounds or 100-130 kg of CO2 per 32 building blocks. Usually, building products emit CO2 rather than sequestering it. Overall, a hemp building would have a 40% lower carbon footprint than a traditional brick building.
Insulative Effectiveness
Much of our energy usage goes into heating and cooling our homes and businesses. The more effective the insulation, the less we need to rely on gas or electric means to keep them comfortable. Depending on the type of build you use (blocks vs infill) it’s possible to create a completely encased home of hempcrete that’s air-tight. If the infill method is used, especially with the studs encased in hempcrete, there are no thermal bridges through which drafts can infiltrate.
Hempcrete is a very, very effective insulator. The hemp hurd has microscopic spaces within each piece that allows it to trap and retain heat. Hempcrete absorbs the heat during the heat of the day and releases is as temperatures drop at night, maintaining a stable indoor temperature. This is referred to as its thermal mass.
The R-value of hempcrete is 2.4 – 4.8 per inch, an R-25 rating, compared to 0.1 – 0.2 per inch for concrete for a 12” deep wall. Other types of insulation such as cotton or fiberglass batting are drastically less effective. Most people (75% in the US) rely on fiberglass insulation and 90% of homes are under-insulated. Fiberglass insulation has the additional disadvantage of off-gassing and the degradation of fiberglass fibers affects indoor air quality. If you want to improve a home’s insulation without major renovations or rebuilding, consider hemp batting for a cleaner, more efficient option. (Some sources are listed below.)
Humidity
While an air-tight home might imply a haven for humidity and mold, this couldn’t be further from the truth for hempcrete. Similarly to thermoregulating capabilities and temperature, hemp is hygroscopic. Therefore, when it’s humid, hempcrete will absorb excess humidity and release it when it is dryer so mold and mildew can’t grow and thrive. Because of the large internal surface structure of hemp fibers, 1314 pounds of water vapor can be stored in 35.3 cubic feet of hempcrete. It can maintain a relative humidity of 93% while still retaining the ability to absorb moisture without degrading. All of this leads to an exceptional indoor environment because the house can breathe. Contrast this to the common PE vapor barrier used with most concrete which is analogous to wrapping your house in a giant plastic bag, trapping gases and moisture inside so mold and mildew can thrive.
People who have lived in hemp houses report fewer allergy, respiratory, and skin symptoms and an appreciation of cleaner air. An even humidity level also saves on HVAC costs, especially in summer weather when high humidity makes it feel warmer. Structurally, when you encase the timber frame in hempcrete rather than fiberglass with a vapor barrier, you prevent the frame from rotting because the hempcrete will absorb any excess moisture before it can affect the timbers.
Cleaner Air Quality
In addition to resisting mold by controlling humidity, hempcrete doesn’t off-gas or release any toxic substances like many conventional building materials. All the components of hempcrete are antimicrobial and anti-fungal, and hempcrete releases no Volatile Organic Compounds (VOCs). Things like polyurethane foam, treated timbers, and plastic sheeting all contain toxic chemicals, including formaldehyde, that off-gas and release potentially dangerous chemicals.
Acoustic Advantage
Because hempcrete is porous, it’s a very effective sound insulator. This quality is variable depending on how much hempcrete is compressed or the amount of lime binder is adjusted. For instance, dense hempcrete is less sound insulative than the compression used in common infill techniques. The average width of hempcrete infill and blocks, up to 12 inches, also adds to the sound deadening effect. Hempcrete was successfully used to build Marrickville Hemp House in Australia specifically to contend with a nearby airport. This sound deadening quality is so effective that there has been some experimentation with using hempcrete as a road surface to reduce traffic noise.
Structural Integrity
We’ve already talked a bit about the fact that hempcrete, on its own, doesn’t have the compressive strength to support a building – it needs some sort of additional support structure. However, using hempcrete increases the structural integrity of framed buildings. Using hempcrete around conventional 2×6 wall framing supports 3-4 times the load that a standard stud wall would. The hempcrete provides additional support against weak axis bending or buckling. Hempcrete also helps a building resist lateral movements so diagonal braces and horizontal noggins can be reduced or even eliminated.
Hempcrete’s low density resists cracking and the combination of hempcrete bolstered wood framing should be able to withstand hurricanes and tornadoes, much like concrete structures. Its properties also make it the perfect medium for building in earthquake zones. It will flex, rather than crack. A house in Miasa village in the Nagano prefecture of Japan, built in 1698, is still standing and in good condition.
As long as there is sufficient framing, hemp buildings can be built as much as 5 – 10 stories tall using hemp blocks or formed panels. It’s not uncommon to expect hemp buildings to last more than 100 years.
Fire & Pest Resistance
Hempcrete buildings resistant/repel all types of pests: rats, mice, insects, and termites, without the need for chemical additives such as formaldehyde. It’s also extremely fire resistant. To earn a two-hour fire rating a blow torch was directed at a hempcrete block for 2 hours. Not only didn’t it burn, but in spite of the 1,000°F flame, the opposite side of the hempcrete only reached a temperature of 120°F. The materials and lime mortar plaster, plus the dense mass, allows hempcrete to pass very high fire safety ratings in Europe. Hempcrete construction gives inhabitants sufficient time to evacuate because it hinders the spread of fire, burns only locally, and doesn’t create smoke so the chances of smoke inhalation injuries are greatly reduced. In additional to saving lives, great safety ratings also means lower insurance costs.
Recyclability
If you’ve ever visited a building site you’ve seen all the scraps and waste that ends up scattered around the area. There are piles of gravel, pieces of fiberglass batting, torn sheets of plastic sheathing, and chunks of unused concrete that hang around decades after the building is complete. I still find chunks of concrete when I’m gardening. Pretty much all of this is eliminated when you build with hempcrete.
No sheathing or fiberglass insulation is necessary, and only small amounts of concrete is used for the foundation or footings. Instead, the leftover bits of hempcrete can be broken up and spread around to become degradable mulch. The same principle stands for hemp structures that have reached the end of their useful life – even though that may be a century or more after it’s built. That hempcrete can simply be broken up and used as mulch or reused as fertilizer.
Ease of Use at Building Sites
While hempcrete is strong and resilient, it’s lightweight. A hempcrete block weighs about 40 pounds, about 12-15% of what a comparable sized block of concrete weighs. If the builder is using blocks, these can easily be carried by a person, and if they are using infill, it can be moved from a portable mixer to the wall using buckets.
Disadvantages of Building with Hempcrete
So far, everything about hempcrete sounds pretty perfect, but there are a few negative factors to be aware of. Even though building with hemp has been around for at least 1,500 years, it isn’t a common product yet. Therefore, it’s harder to find experts who can help you design and construct your building, harder to source product, or struggle with building codes that don’t accommodate hemp structures. (There is a list of resources for product, design, and consultancy sources at the end of the article, as well as some notable hempcrete buildings.)
France was one of the first countries to embrace building with hemp in the 1990’s so they have more expertise and have already codified specifications. Hempcrete buildings are more common in Europe and Canada than many other parts of the world. Even though the US legalized agricultural hemp growing until 2018, there is no standardized code for engineers and builders to follow, and many localities don’t allow building with hemp yet.
Because of its scarcity, and the lack of expertise, building with hemp may require higher up-front costs. This can be balanced by long-term energy and insurance savings due to its fire safety, pest resistance, and ability to withstand natural disasters. It’s possible that the materials cost will decrease as supply, expertise, and interest in hempcrete increases.
As we’ve already discussed, hempcrete doesn’t have the compression strength of concrete, so it’s necessary to build a foundation or footing using concrete. Compared to building with some other material such as siding, or even brick, you will lose some interior square footage using hempcrete because the walls are up to 12” thick. You can balance this with not needing sheetrock on the interior or anything more than a thin layer of hemp/lime plaster on the exterior to weatherproof the building. Lime or silicate paints are acceptable as a finish, but using other traditional paints is analogous to surrounding your breathable surface with plastic.
Finally, depending on whether you use hemp blocks or the infill technique, and where you are building, drying time of wet hempcrete may become a factor. Hempcrete dries very slowly in cold or damp conditions and may require more than the standard 4-6 weeks to fully cure, delaying construction progress. Choosing to use hemp panels or blocks negates this issue because they are dried and cured prior to transport to the building site.
Choosing Hemp Blocks vs. Infill Building
Purchasing pre-made hempcrete blocks is a viable choice that eliminates uncertainty about drying times and lack of local expertise in properly mixing hempcrete. Plus, they are just easy to use and add predictability to the building schedule. There are several ways in which the blocks can be attached to each other. Some companies build-in lego type shapes so they can be stacked. If you choose straight rectangular shapes, a thin mortar of hydraulic lime and sand can be used to “glue” them together. Using blocks that are made and dried off-site under controlled conditions ensures uniformity, expert construction, and sufficient curing so what is shipped to the building site is ready to use.
Blocks are easy to cut-to-shape with just a hand saw so they can fit closely around a structural frame. They are also a good option for large-scale builds; 100,000 blocks were used at the Adnams Brewery warehouse in Suffolk, England. In addition to new buildings, they can be used for renovations, additions, or as added insulation on existing structures.
The disadvantages of blocks vs. infill are that block hempcrete may be slightly denser, or compressed, to ensure they hold up to shipping, and it’s possible that there may be some thermal gaps where the blocks must be stacked around the framing structure.
Another option is pre-made panels. They are still easy to transport and can be quickly put together on site. Having panels eliminates the need to fit individual blocks around the framing. Panels aren’t as common as blocks or infill structures. The final option is to spray hempcrete on a base in layers until the desired depth is reached. This requires more expertise than any other method.
The infill method uses hempcrete to completely encase the framing. This is the most insulative way of using hempcrete. Boards are erected on each side of the frame, leaving a void to be filled with hempcrete that’s mixed on site and carted in buckets. The hempcrete is tamped down, being careful not to compress it too much. The boards are removed after 24 hours, or as soon as the hempcrete sets enough to hold its shape. Another 4-6 weeks is necessary for it to completely cure – more if it’s wet or colder.
The disadvantages of using this process are that a certain amount of expertise is necessary, especially to achieve optimal, consistent results, and weather dependency creates a certain amount of uncertainty in the building scheduling. The advantages are that you can customize the compaction level if you need greater acoustic sound deadening. It also eliminates any possible thermal bridges.
All of these methods can include a coating of hemp plaster to seal exteriors against the weather and gives the exterior a stucco appearance.
Comparing Hempcrete to Concrete
There are a number of similarities between building with concrete and hempcrete: they both require thicker walls, and can withstand fire and extreme weather conditions. However, they aren’t interchangeable. Environmentally, hempcrete is far and away the better material. There are two instances when hempcrete can’t be a replacement for concrete:
- Hempcrete only has 5% of the compressive strength of concrete. This means that it requires metal or timber load-bearing support framing for the roof. Hempcrete is used to fill in around the framing.
- Hempcrete can’t be used in areas that are constantly wet, such as foundations and footings.
Otherwise, every aspect of hempcrete far overshadows concrete, especially when it comes to sustainability.
The amount of CO2 required to manufacture and cure hempcrete lime is a fraction of what’s emitted with Portland Cement. Fossil fuels are needed to heat limestone, shells, chalk, shale, and/or clay to a very high temperature until it forms stone which is ground up to create Portland Cement. For every ton of cement produced, over 900 kg of CO2 is released. Then the cement is mixed with sand and gravel, both non-renewable resources that are being mined faster than they can be replaced naturally through erosion. Between 5% and 8% of the world’s carbon pollution is attributed to concrete production.
To make hempcrete, you just need lime, water, and hemp hurd. The three can be mixed on-site with a portable mixer. Building with hemp would dramatically reduce the 4.4 billion tons of concrete produced each year. While hempcrete can’t replace everything concrete, it certainly has a prominent place in building sustainable, clean buildings.
Final Thoughts…
Hempcrete has so much to offer architects, builders, farmers, homeowners, and the environment. It’s user friendly from seed to recycling. Farmers can grow a profitable crop without excessive irrigation, or any pesticide, insecticide, or herbicide chemicals. Every part of the plant is useful for one of the 25,000 possible products made with hemp. The deep root system enriches the soil and prevents erosion. Hemp sequesters a great deal of CO2 from growth through harvesting, and even once it’s been used as hempcrete.
Hempcrete requires only hemp hurds, limestone, and water to form a building material that’s fire proof, resistant to pests like termites, rodents, and ants. It has exceptional thermal regulation because it absorbs heat during the day which is released when temperatures cool. It absorbs moisture when it’s humid, and releases it when it’s drier to create a stable humidity level and preventing mold and mildew from growing. These properties produce a clean, comfortable living environment, especially for those who have allergies, because there is no off-gassing of any type of toxic fumes.
Hempcrete can be mixed on-site for buildings using the infill system, or pre-made blocks or panels can be purchased and shipped to the job site. Although hempcrete is extremely durable, it’s also completely recyclable and biodegradable. Any waste from a site can simply be spread as mulch to enrich the soil.
It’s hard to argue against a building material with so few drawbacks – and those that exist are easily overcome – and so many amazing benefits. As more and more builders and architects become familiar with the material and its possibilities, hopefully more hemp homes and commercial buildings will become available.
Examples of Hempcrete Building
If you are interested in seeing what hempcrete can do, below are some projects, with links, that have been undertaken around the world. While many are in Canada, France other parts of Europe, building with hemp has been gaining acceptance everywhere.
Canadian examples:
Harmless Home, built in 2015, was the first of its kind on Vancouver Island using hempcrete blocks. YouTube Video, Facebook Page, Article by the home’s pioneering architect
EcoLock has built a 5-floor, 112,000 square-foot storage facility in Kelowna, British Columbia. Article addressing sustainability benefits.
United States:
Push House was the first hempcrete home in the United States. It was built by the former mayor of Asheville, North Carolina. The 3,400 square foot home was built in 2010.
The Mdewakanton Band of the Dakota, Lower Sioux Nation in Minnesota is embracing hemp manufacturing to house their people and as a source of income. They are creating an integrated vertical operation where they will grow the hemp, manufacture hempcrete blocks, and build the housing. As a sovereign nation they are not limited by building codes that are still in place in many other parts of the state and country, making it easier for them to build. The Gitxsan First Nation on the Sik-E-Dakh reserve in British Columbia, Canada, and several other Minnesota Sioux bands are exploring similar integrated projects.
Europe & UK:
The Voice of Urban Nature, by Overtreders W, used pink dyed hempcrete panels within demountable timber modules for a garden pavilion in Almere, Netherlands.
Flat House, by Practice Architecture is made from large prefabricated hempcrete panels. It was erected in only 2 days in Cambridgeshire, England. The panels have exposed hempcrete in the interiors, while the exterior is clad in hemp fiber tiles.
Pierre Chevet is a wellness center in France, designed by Lemoal Lemoal architecture. It was built using interlocking hempcrete blocks that don’t require mortar or adhesives. The inside walls were treated with hemp plaster while the exterior is clad in white cement-fiber panels
Rue Marx-Dormoy is experimental public housing in Paris. It was designed by Barraulk Pressacco. Buildings use hempcrete sprayed in layers onto gypsum-fiber panels and fixed to a timber framework.
Tigín Tiny Homes, in Ireland, builds oversized caravan tiny homes made of corrugated hemp panels and cork. The hemp fibers are coated with sugar-based resin from agricultural waste. They work with Margent Farm, a hemp grower in Cambridgeshire, also the source of hemp for the Flat House.
Clay Fields is an affordable housing complex of 26 homes in Elmswell, Suffolk, England. It was designed by Riches Hawley Mikhail architects using sprayed hempcrete. This was the first UK build to use this technique in 2008. The homes also feature Isonat insulation, a combination of hemp and linen.
Lina Bellavicova designed and built her hempcrete home in the Czech Republic. It has wood stud framing with hempcrete walls inside and a green roof to help it blend into the surroundings. All materials were locally sourced.
Other Areas of the World:
Solar energy and hemp construction combined in a cooperative build with international experts from the Solar Decathlon to design and build an off-the-grid home, Sunimplant, in Morocco.
The first hempcrete building in Israel was built in Ein Hod, north of Tel Aviv in 2017. It was designed by the Tav Group and serves as an artists’ village. The walls are covered with a thick layer of earth-based plaster. Videos of Ein Hod Hemp Home
Marrickville Hemp House in Australia was designed with the specific goal of showing how acoustic requirements to block out aircraft noise can be met, while still maintaining thermal integrity. Hempcrete was used in both the walls and ceiling. This project was a renovation that added space and improved air quality within the house.
Companies that work with Hempcrete Products
American Hemp LLC is a source of hempcrete for North and South Carolina
American Lime Technology sells Tradical hempcrete, pre-cast wall systems, lime mortars and plasters.
Americhanvre in Allentown, PA focuses on the fire resistance, insulation capabilities, and overall carbon impact of hempcrete. They have spray applications, cast hemp, and supply hurd and lime binder. Their spray system is called Ereasy Spray System
Chanvriere de l’Aube is a French company that sells low dust, consistently colored, hemp hurd.
Dun Agro Hemp Group is a Dutch company but also has offices in Indiana. They sell everything hemp from seeds, to insulation wool, and hemp wood.
HempBlock sells in Australia, Hawaii, Canada, and South Africa. The blocks come in various sizes, some include an interlocking system.
Hempbuild is a based in Ireland and is Europe’s largest distributor of Hempcrete.
Hempcrete Projects Ltd (part of Lime Technology group) is a UK company that produces pre-cast hempcrete panels, blocks, and supplies. They offer webinars, consultancy, design, and training services.
Hempitecture is located in Jerome, Idaho. They have a 33,000 sq ft. factory and also sell HempWool (thermal batting insulation).
HempStone offers construction, advisory, research, education, and supplies.
Hemp Tech Global is a Canadian company that can help design, build, and provide logistics or personnel for homes. They can help source hurd from international suppliers.
ISOHemp is a Belgian company that sells blocks for both new builds or renovation projects. They manufacture over 1 million blocks per year.
Just BioFiber supplied the materials for the Harmless Home. They sell interlocking hemp blocks.
Limecrete Company is a UK company that sells natural hydraulic lime and aggregate that can be used as an alternative to concrete. They’re also involved with spray hempcrete.
Other Information Sources
HempBuild Magazine – resources, upcoming events, articles & online community.
Hemp Building Institute offers educational programs and is a source of research.
Hempcrete Book by William Stanwix and Alex Sparrow
Tiny Hemp Houses a Colorado company that sponsors tiny hemp houses, education programs, and consulting.
World Green Building Council works with businesses, organizations, and governments to deliver on the
Paris Agreement and UN Goals for Sustainable Development.