What is Hempcrete? Everything You Need to Know

Here’s an interesting statistic: the manufacture of concrete produces up to 8 percent of the world’s carbon dioxide (2.8 billion tons) every year. At this alarming rate, there’s an urgent need for a more sustainable building material.

But is there an eco-friendly alternative to concrete with the same (or more) tensile strength and durability?

Yes, and it comes from hemp!

One of the oldest and most sustainable building materials – hempcrete – is making a comeback. Now that several states have legalized the recreational and industrial use of cannabis, a big untapped market is emerging for hemp as a building material.

But what is the meaning of hempcrete? Why is it the best alternative to concrete for a greener future? Here’s everything you need to know.

What is Hempcrete?

Also known as hemp concrete or hemp lime, it’s a biocomposite made from a mix of the woody core of the hemp plant, lime binder, and water. The hemp core (also hemp hurd or shiv) has a high silica content that allows it to strongly bind with the lime.

As a result, you get a lightweight, cementitious material with good insulation properties that weighs six to eight times less than concrete.

What is the Story Behind Hempcrete?

The use of hemp as a building material can be traced back to 2000 years ago when it was used by the ancient Gauls to build bridges and homes. Hemp has also been a popular building material in Japan, China, India, and more recently in Europe. In fact, there’s a historic hemp house in Nagano, Japan that was built in 1698 and it’s still in good condition. Only a strong, flexible compound can withstand the elements for such a long period.

Hempcrete, as we know it, was developed in the late 1980s in France. It was first used to boost the thermal performance of medieval timber frame buildings. Europe has never prohibited hemp cultivation, and over the years, they have taken advantage of this renewable, eco-friendly building material.

In the U.S., the use of hempcrete has been limited, especially after hemp was banned in the 1930s. However, it’s slowly gaining traction – the legalization of hemp cultivation in 2018 has led to an increase in hemp products (including hempcrete) in the market.

What is the R-Value of Hempcrete?

In building and construction, the R-value is the measure of how a two-dimensional barrier, such as a wall or a window, resists the conductive flow of heat. Hempcrete has an R-value of 2-3.5 per wall inch, giving it desirable thermal and structural properties for wall, roof, or slab insulation.

hempcrete R-value

What Part of the Hemp Plant is Used for Hempcrete?

The woody inner core of the hemp plant is used as one of the raw materials for hempcrete. These woody parts are broken into small fragments and separated from the fiber to create hemp hurd (or hemp shiv).

What is Hempcrete Used For?

Hempcrete is applied in various ways, including:

Plastering

hempcrete plastering

Hempcrete can be sprayed on the exterior and interior surface of your building, making it suitable for patching in or making repairs to cracks in a wall.

Form Packing

Hempcrete is commonly packed around wooden framing to provide adequate strength to the structure. It’s packed densely toward the surfaces and loosely at the center to maximize its insulating potential. If you want to, you can clad the exterior.

Hempcrete Blocks or Bricks

hempcrete blocks

Hempcrete blocks can be molded into various block designs to suit your specific construction project. Unlike concrete blocks, hemp blocks are lightweight and can be cut easily for quick and seamless construction.

Insulation

Unlike conventional insulation, which is added as an extra layer to the wall or installed within it, hempcrete doubles as the wall and insulation with other materials acting as the main structural frame.

For roofs and floors, hempcrete is usually combined with other materials to provide insulation. It can form a part or all of the insulation layer depending on the design of your building.

Hempcrete Infill Panels

When retrofitting or restoring old or historic buildings, hemp concrete is often used to improve insulation to walls or to repair infill panels in timber frame houses. It’s recommended for historic buildings because of its vapor-permeability and ability to mold into unevenly shaped spaces on the wall without sagging.

What States in the U.S. are Building with Hempcrete?

Industrial hemp was legalized in the 2018 Farm Bill, meaning that there aren’t any restrictions to build with this sustainable energy-efficient material in all states.

What Kind of Lime is Used to Make Hempcrete?

You can use both hydrated and hydraulic lime for the manufacture of hempcrete. The lime binder is flexible, allowing movement of a hempcrete construction to prevent cracks in your masonry.

What is the Melting Point of Hempcrete?

Hempcrete is fire-resistant, meaning it doesn’t melt. Rather, it decomposes because it’s made of several ingredients. For hempcrete to heat and decompose instantly, it needs very high temperatures of up to 900 degrees.

The fire resistance of hempcrete makes it a suitable choice for homes that are at risk of fires such as homes within a wildfire zone.

How to Make Hempcrete

The raw materials of hempcrete include hemp hurd, lime binder, and water. Generally, the manufacture of hempcrete involves three simple steps: mixing, molding, and curing.

How to Mix Hempcrete

hempcrete mixing

With the raw materials at hand, you need to get your ratios right as it’ll determine the density of the final product. Start by mixing the lime binder with 80 percent of the water into a concrete mixer or pan. Let the mixture sit for some minutes until it starts milting. Next, gradually add the hemp shiv and remaining water to create a pasty mixture.

How to Make Hempcrete Blocks

In this step, the mixture is put in a special press to mold the hempcrete into blocks. Unlike concrete, you can mold hempcrete into any shape you want.

How to Strengthen Hempcrete

This involves drying out the hempcrete blocks in the open air to harden and strengthen them. Depending on the level of hardness you’re looking for, the weather conditions, and the contractor’s skills, the curing process can range anywhere from six to ten weeks.

How Much Hemp Hurd, Lime, and Water is Needed to Make Hempcrete?

While there is no specific ratio to follow, you can create a solid biocomposite with one part water, one part lime binder, and four parts hemp hurd. The mixture should not be too squishy to hold form and not too hard that makes it impossible to cast.

How to Use a Hempcrete Mixer

To make hempcrete, you’ll need a large pan mixer of about 180 gallons. You can also do this in the conventional cement mixer, but it’ll be a little bit difficult to create a smooth, even mix. When using a hempcrete mixer, use the appropriate proportions of binder, hemp hurd, and water according to the manufacturer’s instructions.

To ensure a consistent mixture with the binder evenly distributed, mix the raw materials in the least time possible (a few minutes will do). If you mix it for a longer period, it will start to form small balls, which makes it unsuitable for building purposes.

To check whether your mixture is ready, take a handful of hempcrete and squeeze it in your hands to form a ball. Then, press your finger into it – the ball should easily break into several pieces. If it’s too squishy, it’s too wet; and if it crumbles, it’s too dry.

How to Cast a Hempcrete Wall

hempcrete wall

Simply fill the fresh hempcrete and spread the mixture with your gloved hands to ensure it evenly distributes around the void, especially on the frame timbers and hard-to-reach corners.

Next, using the palm of your hands, firmly pat down the top layer. Don’t overcompress the hempcrete as it may reduce the air bubbles trapped inside it, compromising the insulation capability of the wall. Additionally, overcompressing will require you to use more hempcrete which is uneconomical and extends the drying period.

While you can use a tamping stick, using gloved hands will allow you to feel the compression required. The tamping sticks will, however, come in handy when you need help you access hard-to-reach areas and sections where the hempcrete may require added compression for structural integrity.

Ensure you place the hempcrete equally all around the shuttering until it’s full. If need be, install more shuttering to make the next lift, while ensuring you don’t damage the one below.

Once the casting is firmly intact, you can remove the shuttering board from the top. If you need to pause the construction for a day with a half-full shuttering, ensure you taper the mixes down in a slope to allow you to start the next day with hempcrete on top of what’s already there. Leaving a vertical day joint will form weak spots in the structure.

When casting across doors and windows, cast the lintel at one go and extend at least 12 inches on each side of the wall to boost the structural integrity of the timber spanning section. For small timber spanning like roof battens, cast the hempcrete into the lintel on either side of the frame for adequate support as the hempcrete dries. If you have a longer spanning, more timber is required to provide the support.

At the bottom of the wall openings such as windows, ensure you cast the hempcrete at about an inch higher than the desired level. You’ll have to cut it back with a hand saw when you’re finished to achieve a seamless, solid structure.

How to Install Electricity on Hempcrete Walls

Your electrical system has to be installed inside the hempcrete walls before they’re poured. This will eliminate the need for bolting boxes and conduit to the surfaces of the walls.

Carefully follow the measurements on your blueprints to safely secure your electrical boxes and conduit in the hempcrete form. This way, you won’t have to worry about pulling the wires or passing inspections later.

To install electricity into the hempcrete walls, carefully mark the locations and paths of your electrical boxes and conduits according to your blueprints. An electrical technician can help you ensure the elements are within the building codes standards of your area.

Next, run the electrical conduit in the hempcrete form. Ensure you leave at least 12 inches of the external conduit to allow you to easily join it to the rest of the electrical supply system later. Place the electrical boxes (for outlets, junctions, switches, or control panels) on the interior of the hempcrete wall and nail them in position.

Once the hempcrete cures and forms, get a professional to pull the wires through the electrical conduits as well as install the outlets, switches, junctions, and other electrical fixtures in the electrical boxes.

Finally, call for inspection to ensure your electrical system is up to standards.

How to Roof with Hempcrete

You can use hempcrete to insulate flat concrete roofs. The application is quick and easy thanks to hempcrete’s light weight. To roof with hempcrete, a screed is poured on the site and then a sealing layer is placed on top for protection and to keep the screed intact.

This on-site solution will keep your building from overheating during hot seasons and guarantees maximum durability of your flat roof.

How to Insulate the Floor with Hempcrete

hempcrete floor

Using hempcrete on your floor insulates the slab and offers adequate support to the whole house. The hempcrete should be dry, allowing the floor finish to be applied quickly and effectively. Even better, this type of insulation can be combined with many screed types like clay, lime, and cement.

With hempcrete insulated floors, you get quick installation, excellent compressive strength, and high technical flexibility like inserting ducts in the screed or installing underfloor heating.

How Much Does Hempcrete Cost?

A hempcrete house costs about $100-$200 per square feet, including everything from plumbing to accessories. However, the amount will largely depend on how accessible hempcrete is in your area. With the legalization of hemp cultivation, the cost of building a hempcrete home is projected to drop.

hempcrete cost

How Light is Hempcrete?

Hempcrete weighs six to eight times less than standard concrete. This makes hemp concrete relatively easy to transport and use, reducing construction costs and minimizing the risks of injury.

Moreover, when used during the remodeling of older buildings, hempcrete doesn’t overload the structures due to its low density.

How Strong is Hempcrete?

Generally, hempcrete has a density ranging from 40-55 lbs./cubic meter. With higher binder content, the density can be amped up to 2,200 lbs./cubic meter, boosting the mechanical strength of hempcrete.

hempcrete density

How Does Hempcrete Hold Up in the Winter?

Since hempcrete can absorb water from the atmosphere, it can effectively regulate the humidity levels in your home or business.

The air pockets inside the material pull in the moisture from the air when humidity levels are high and release it when the air is too dry. This also saves on your HVAC costs and fosters greater health for the occupants of your building.

How Long Does It Take for Hempcrete to Decompose?

Hemp concrete is a biocomposite, making it fully recyclable. It takes 3-6 months to completely decompose. So, when structures built out of hempcrete reach the end of their useful lives, they can be broken down and used as mulch on bushes and trees to save on water and stop weeds. Over time, the hempcrete will break down into lime and hemp components, which act as beneficial fertilizers.

hempcrete biocomposite

On the other hand, other building materials like concrete, aluminum, and steel usually end up in landfills, which is detrimental to the environment.

How is Hempcrete Flame Resistant?

hempcrete non-flammable

Mixing hemp with lime binder creates a non-flammable material, providing fire resistance for up to two hours. It can, therefore, pass all building fire code inspections in your area.

How Long Does Hempcrete Last?

As mentioned earlier, there are hemp mortar houses that date back to medieval times. While your hempcrete house may not wind up to last that long, you’ll have some peace of mind knowing that your home can last for 1,500 years or more.

Why is Hempcrete Better Than Concrete?

Hempcrete comes with a lot of advantages over other building materials like concrete. Here are some of the benefits you should consider before starting your next construction project:

1. Low Carbon Footprint

During the growth cycle, hemp (just like all plants) absorbs carbon dioxide from the atmosphere, reducing the greenhouse effect. But what makes the hemp plant stand out is its ability to absorb carbon even after being harvested and used as a building material.

2. Non-Toxic and Non-Hazardous

Fiberglass is an excellent insulation material that’s widely used in many buildings today. However, research shows that fiberglass may contain a reasonable amount of human carcinogens and regular exposure may lead to cancer.

Conversely, when you use the all-natural hemp concrete, there are no such worries.

3. Excellent Insulation

Whether you’re using hempcrete in block form or as a mold, it’ll absorb heat during hot days and release it into the building when the sun goes down.

While it may not be as effective as other synthetic insulation materials, it’s more affordable, eco-friendly, and lasts longer.

4. Thermal Mass

Compared to other masonry construction materials, hempcrete does a better job of storing heat, eliminating the need for an aggressive HVAC system. This will lead to reduced energy costs, and thus, lesser greenhouse gases in the atmosphere.

5. Mold Resistant

Excessive mold growth can not only affect the aesthetics of your building but also compromise its structural integrity. If not effectively removed, mold growth can cause constant repairs, which are quite costly in the long run.

Although it’s absorbent of water, hemp concrete is highly resistant to mold and won’t play host to dangerous bacteria thanks to its highly alkaline lime component. This makes hempcrete a good alternative to wood, especially if you live in wet areas.

6. Natural Pest Resistant

Besides repelling mold, hemp is also an excellent pest deterrent due to its high pH value. This ability to repel rapacious rodents, cockroaches, and termites eliminates the need to treat your hempcrete with hazardous chemicals such as formaldehyde.

hempcrete high pH value

7. Sound Insulation

Hempcrete is relatively porous. When used on walls and interior partitioning, it traps sound waves, dampening them. This gives you a tranquil indoor space, especially if you live in noisy neighborhoods.

8. Natural Disaster Resistance

While hempcrete doesn’t exhibit the load-bearing capacities of high-level concrete, it has an impressive ability to bolster wood framing, strengthening it by up to four times. This will keep your building intact even after strong hurricanes.

Additionally, hempcrete is a better alternative to concrete in earthquake-prone areas because of its superior flexibility. Instead of cracking and breaking, a hempcrete building will twist and bend accordingly without breaking.

9. Available as Plaster and as Block

With the legalization of hemp cultivation in some states, you can now expect hempcrete in various other forms, including traditional blocks and plaster.

10. Can Create an Air-Tight Seal Around Your Home

With a house sealed against the elements, indoor air quality will improve, you’ll enjoy less draft, your HVAC system will run more efficiently, you’ll reduce your energy bills, and so much more. Hempcrete can form a lightweight, dense, and formidable air-tight seal for your home – unlike concrete or mortar.

Why is Hempcrete Sustainable?

hempcrete sustainability

Hemp (the main raw material of hempcrete) has a lot of environmentally friendly benefits. First, it’s a hardy plant, meaning it doesn’t require any pesticides or herbicides to grow. This eliminates the risk of harmful run-offs, and it’s especially important for farms that often have to deal with contaminated groundwater.

Second, you can harvest your hemp fields in just five to six months after planting. This means you can double your annual yields and dedicate less land to cultivation.

Why Isn’t Everyone Using Hempcrete?

Due to legalization issues over the last few decades, the use of hempcrete is still a new phenomenon to many people. There isn’t a lot of technical know-how on building with hemp because the material is not common. You may find it difficult to find a builder who can accommodate your desire for your dream hempcrete home.

The good news is that more and more construction companies are learning to build with hempcrete to capitalize on the growing hempcrete market.

Why is Hempcrete Not Readily Available?

Hempcrete may add some upfront costs to your construction project largely because you may have to look further for a hemp supplier.

Fortunately, the future is bright for the industry in America. After the legalization of hemp cultivation, more farmers are ditching other cash crops to grow hemp as it doesn’t require a lot of maintenance. This will translate to more hemp products in the market, including hempcrete.

Why Build with Hempcrete?

For a better future, the construction industry is aiming at reducing pollution and depletion of natural resources. With its eco-friendly production and application, hempcrete is in the spotlight as the future building block of the construction industry.

Even with the hempcrete disadvantages, building with hempcrete is still a viable choice – and it’s quickly gaining traction. It’ll not only help cut down on carbon footprint but also promote a healthier and more affordable life.

hempcrete