Everything to Know About Hemp Concrete

By carefully selecting the materials used in our homes and apartment units, we can make a difference and revert to building practices that combine both comfort and sustainability. Hemp concrete, which is plant-based concrete, is just one of the ways to do so. Check out the following to learn more about it.

All About Hemp Concrete

Hemp concrete, also known as hempcrete, is an agricultural concrete used in bioclimatic architecture. In fact, in Canada, hemp grows naturally and is cultivated by grain farmers. So, once hemp is harvested, the grains (which are very nutritious) can then be sold and the straw can be used to produce hemp hurd. 

In other words, it’s actually the hemp hurd, the inside of the stalk, that’s used to make hemp concrete.

Composition

Hemp concrete is a simple product to make. First, shavings from hemp stalks are used instead of the original granular mineral that is found in concrete recipes, which will then be used to make the load-bearing walls of a structure. Amongst the following three types of mineral binders, one is used:

• Lime;

• Cement;

• Pozzolana.

Next, all ingredients are mixed with water and crushed terracotta, to which adjuvants can be added. But, be careful! Hemp concrete has nothing to do with Roman cement. What makes Roman cement such a strong material lies in the volcanic rocks used at the time, combined with seawater. However, nowadays, volcanic rocks are scarcely available, thereby making industrial amounts of such concrete impossible.  

Advantages of Hemp Concrete

Hemp concrete is a favourable material: 

• It doesn’t rely on quarries, which aren’t limitless;

• It’s organic. It’s used in its totality, from grain to stalk;

• Requires very little gray energy (90 kWh/m3);

• Plant growth absorbs CO2;

• Good thermal resistance and soundproofing qualities;

• Its porous (thus it breathes);

• It’s insulating;

• Its durability is equal to that of cellular concrete and bricks;

• It breathes, thereby contributing to air quality;

• Resistant to humidity.

Would you like to learn more about thermal insulation? Check out our article on the subject matter. (French only)

Hemp Concrete & Insulation

Thermal Insulation

The high thermal and acoustic performance of hemp concrete is based on its structure. Hempcrete has a complex microstructure due to its composition, but also because of all its derivatives, such as hemp wools, which we'll delve into down below.

Texture aside, hemp concrete has a porous quality that also plays a part in the material’s thermal insulating properties. On average, hemp concrete is 75% and wool is 99%. 

A material’s insulation is measured according to W/m.K, which is Watts per metre-Kelvin. From this, a coefficient is determined, known as the lambda coefficient. The lower this coefficient is, the more insulating the material will be. Here is how hemp concrete performs compared to other building materials:

Thus, we can say that when it comes to insulation, hemp concrete is a fine option for mounting walls. It allows for a home to be kept at 21°C without major energy consumption, both in summer and winter.

Want to know more about eco-friendly materials? Check out our article 10 Green Home Renovation Materials.

Soundproofing

Hemp concrete’s soundproofing capabilities are based on the same structural reasons that make this material a very good thermal insulator: its porosity.

The soundproofing quality of a particular building material can be assessed by means of the sound reduction index and the sound absorption coefficient. Without going into great detail on the physics of acoustics, one can summarize the quality of sound insulation by its performance.

Due to the porous nature of hemp concrete, sound waves are absorbed into the material through visco-inertial and thermal effects. As mentioned, hemp concrete has a porosity of 75%. By way of comparison, metals have a porosity of less than 10%.

How Does it Resist Humidity?

I think we all know how grain or straw is dried and how entire crops are lost when moisture-damaged. Consequently, the issue at hand is the resistance of hemp concrete to humidity, especially in areas with a climate like Quebec's.

Hemp concrete's resistance to humidity is based on the type of binder used in its conception. Hydraulic lime has the distinctive advantage of containing silicates and aluminas, which harden when in contact with water. As a result, hemp concrete is a water-resistant material, which makes it even more durable.

What makes hemp concrete a good material to use in Canada is its positive response to freeze-thaw weathering. This resistance is directly linked to the binder’s quality and the natural porosity of hemp concrete. 

Cost of Hemp Concrete

When it comes to the price of hempcrete, it’s a rather layered question since it’s entirely dependent on the construction project. For small-scale projects, such as renovating a select room (i.e. basement), hemp concrete slabs go for $2 to $3/sq. ft. For projects for which the amount of hemp concrete needs to be doubled, the cost, including labour is between $10 to $20/sq. ft. 

On the other hand, bigger projects cost roughly $150/sq. ft. Therefore, to benefit from a fair evaluation regarding a project’s cost, it’s best to request a quote from an industry expert. 

Hempcrete Installation Methods

Architectural hemp concrete has four separate uses in the construction industry. It can be used:

• To fill in a wooden structure (wall);

• To insulate floors;

• To insulate roofing;

• To plaster a facade.

From these four uses, we can distinguish between wall building and insulation. Thus, assembling hempcrete walls rests on two different techniques: the formwork (hemp formwork) and the blocks. On top of that, there are two insulation methods: hemp plaster and hemp wool.

Cast-In-Place Hempcrete

Cast-in-place hempcrete forms a single, seamless piece, thereby reinforcing thermal resistance. In fact, the whole of the home’s walls will be poured in one piece. By putting in place a wood frame, hempcrete will then be spread out evenly and compacted atop it.  

Next, a two-month drying period is required. Then, the lime finish can be applied. This technique guarantees optimal insulation since it avoids thermal bridges.

Hempcrete Block

These are prefabricated hemp concrete blocks. This method, which requires lime pointing between each block, is less expensive than using hempcrete formwork. It requires the following:

• Less labour;

• Less drying time;

• Blocks are lightweight, since hempcrete is super porous, they’re easy to lay.

Hemplaster

Hemplaster is ideal when renovating an old building; this material also gives the building a certain appeal. It’s actually a pretty versatile plaster that allows for the following:

• Soundproofing;

• Insulation;

• Decorative finish;

• A three-in-one solution that's definitely worth trying.

Hemp Wool

Hemp wool hasn't yet been commercialized in Quebec, as it's a product that isn't well-suited to its harsh winters. However, it could replace synthetic wool, which isn't more efficient during the harsh Canadian winters. Consequently, it could be a credible alternative to synthetic wool, even if it isn't an ideal solution.

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Why do most projects require an expert?

Hemp concrete construction projects are usually carried out by an expert because hempcrete is elasto-plastic. In other words, its ability to withstand compression requires that it be placed with a load-bearing structure.

A wooden structure is most often preferred. Freedom is the key here as hemp concrete can be combined with a wooden frame, solid wood, or even a double wooden formwork.

Moreover, not all hemp concrete is the same. One has to consider the amount of binder used by the manufacturer, as well as the material's compaction strength and duration during production, as it will affect the characteristics of the hempcrete.

This is one of the reasons why having an expert involved in most projects where hemp concrete is used is a must. Since it’s a bioclimatic material through and through, an expert will know how to evaluate the type of hemp concrete that'll offer the best results depending on the environment and the constraints involved.

Hemp Concrete: Is it More Eco-Friendly?

Whether a product is environmentally friendly or not depends on its life cycle.

What’s Gray Energy?

Gray energy determines the total amount of energy a product requires throughout its lifecycle. Meaning, from its inception to its extraction or production, to its recycling or destruction. 

Within the construction industry, its unit of measurement is kWh/m3, or megajoule (MJ). Note that a cubic metre corresponds to 35.31 ft3. Hemp concrete is undeniably advantageous in terms of gray energy. Simply compare it to other materials and see for yourself:

The Gray Energy of Hemp Concrete

As previously mentioned, hempcrete's gray energy is unrivalled by other building materials.

The question that comes to mind now is: Why does it have such low gray energy? The main reason is that hemp grows in most regions of the world, including Canada, making it a prime candidate to build a bioclimatic house in Quebec.

The second reason is that hemp grows at a rapid rate. Harvesting the fibre can be done within 60 to 90 days of growth, while the grain can be picked within 110 to 150 days of being planted. Therefore, it's one of the most renewable building materials in the world. But that's not all, its ability to absorb CO2 makes hemp a truly eco-friendly material.

The third reason why hemp concrete is an eco-friendly material is: During growth, hemp plants absorb CO2 from the atmosphere. When harvested, each ton of hemp will have absorbed 1.62 tons of CO2. However, its role doesn't end there, since a house built with hemp has the capacity to absorb more than 20 tons of CO2 alone.

The fourth reason why hemp is environmentally friendly is because of the binders. Lime, cement, and pozzolana don't contain any glue or resin which could be a health hazard for residents. Also, the fire retardant treatment commonly applied to hemp concrete walls is composed of a salt-based solution, which not only reduces the risk of fire outbreaks but also protects walls from rodent attacks.

The last reason rests on the fact that hemp concrete limits the exploitation of mineral aggregates. Global reserves, if not depleted, are becoming steadily scarcer and more expensive to harness.

Are you interested in green renovations? Check out our Eco-Friendly and Green Renovation Guide to learn more about the subject matter.