How is Hempcrete Made?

How is Hempcrete Made? What’s It Made Of?

It takes only 3 materials to create hempcrete: hemp hurds (or “shives”), lime binder, and water.

Hemp hurds are the woody inner parts of the hemp stalk. They are first broken into fragments and separated from the fiber by way of breaking and scutching (separating impurities from raw materials by using hand or a machine called scutcher).

The lime which is used in the mixture acts as a binder. Hemp hurd contains a high silica content that helps it to bind well with lime. There are many benefits of using lime in the mixture. These include:

  1. It is breathable. Lime has high porosity and high water vapor permeability.
  2. It has low thermal conductivity.
  3. It is a long lasting material and can bind hemp hurd for years.
  4. It is easily recyclable and is resistant to cracking (the cracks heal automatically).

Hemp and Lime Construction

A sketched diagram that shows the layers of making hempcrete with lime, and how the material is laid within a house or building foundation

Hempcrete has a λ value (conductivity which the rate of the passage of heat through the material) of ~0.076 – 0.11 W/(m.K) when mixed with a mild to moderate hydraulic lime consisting of ~85% hydrated lime and ~15% clay or pozzolans. For floor slabs, a more hydraulic lime is used and sand is added to improve strength and water resistance λ value ~ 0.13 W/(m.K). For the ceiling insulation and the internally insulating stone wall coating, a pure hydrated lime is used and a higher percentage of hemp to provide a more insulating and lighter mixture λ ~0.08 W/(m.K). Reference


How To Mix Hempcrete

The mixing of hemp lime mostly depends on the size of the building. It is mixed in a large forced action pan mixer. If you have to mix small amounts, you can do so in a conventional drum cement mixture.

It is very important to know the exact proportions and mixing techniques. The proportions being used vary with the type of binder that you are using to bind the materials. You can use the ratio of 4:1:1, meaning: 4 buckets of hemp hurds, 1 bucket of water and 1 bucket of lime. (You can also ask your supplier).


The Steps

Here are the steps you can follow to mix the materials.

  1. Add water into the mixer.
  2. Add lime binder into the mixer.
  3. Let the water and lime mix to a slurry
  4. Add hemp hurds to the mixer and allow it to mix properly
  5. Tilt the mixer to get an even mix.
  6. As soon as the hemp hurds are mixed, stop the mixture.
  7. Rotate the mixture and use your hands to clean the mix and if not mixed properly rotate it for few more seconds.

In order to test the mix, form a ball of mixture in your hand and squeeze it. If the ball breaks in two portions, it is perfectly mixed. Empty the mixer and use your hands to scrape off the last bit of hempcrete. Your mixture is now ready to be used for construction.

How To Make Hempcrete Blocks

A hempcrete block angled to show its shape and size and how similar it looks to a concrete block

One of the most popular products available in the market are hempcrete blocks. These blocks are manufactured by pressing, aging and packing hemp lime with the help of machinery. They are are solid but light, durable and possess incredible insulation properties.

The high silica content present in hemp shives (approximately 70%) makes it easy for the raw materials of these blocks to bind together.

The production of hempcrete blocks is divided into 3 basic steps:

1. Mixing:

In this step the raw materials (hemp hurds, water and lime) are taken in proportionate quantity and mixed together.

2. Molding:

The hemp lime mixture obtained in the first step is poured in a mold of the desired width in a special press.

3. Open-air curing

The fragile blocks are taken for open-air drying which gives strength and hardness to the blocks. Usually, it takes around 6 to 10 weeks for the blocks to become ready to be used.

Hemp blocks can be used in a variety of ways, notably insulation (walls, roofs and floors) and as plasters for renovation.

A table of materials that includes hempcrete and compares it to other building materials based on it's carbon footprint