Building with Hemp

About hempcrete

Hempcrete is a mixture of hemp hurds (the chopped up stalks of the plant) and a lime and water slurry. Hempcrete is not load bearing, so needs to be used with a wooden frame. The hempcrete is cast using a form, creating a monolithic envelope around the wooden frame.

There is evidence that the Romans used a mixture of hemp and lime in some construction. Modern hempcrete for building is thought to have been reintroduced by Charles Rasetti in 1986 in France. Hemp is a popular construction material in Europe for both homes and commercial buildings. Australia has more than 260 hemp buildings, including a few commercial builds. There are at least six hemp homes in Aotearoa New Zealand.

Hempcrete whare don't need to be clad, reducing the need for more building materials, and because it has been made with both hemp and lime, it cures or hardens over time, whilst continuing to sequester even more carbon from the atmosphere into the walls. The walls are sealed with a lime plaster render, which allows the walls to breathe.

Hempcrete has a natural hygroscopic performance capacity where it can absorb the moisture from inside the building and will release moisture back into the interior space when internal humidity has dropped.

Hempcrete has both a high thermal resistance rating, or R-Value and strong thermal mass performance, meaning that heating or cooling of hempcrete home is minimal. It replaces the need for insulation - the walls are the insulation.

A drawback of hempcrete is that the footing needs to be protected from the ground to prevent rising damp. The Australian Hemp Masonry company suggests using concrete slabs on ground, strip footings, or piers which can be timber, concrete, steel or masonry and bearers.

Other forms of hempcrete have been developed including prefabricated walls which are assembled on-site, hempcrete blocks and 3D printing.

A 2019 Australian study fire tested hempcrete and found no damage to the material.

Check out the inspirational story of these Tuhoe wahine developing hemp production for housing in their rohe.

Hempcrete is a carbon neutral or negative building material

• Can achieve a negative carbon footprint through the sequestering of CO₂ from the atmosphere during growth

• Hemp sequesters significant amounts of carbon from the atmosphere

• One cubic metre of concrete creates around 150kg of CO₂

• One cubic metre of hempcrete sequesters around 300kg of CO₂

What it means for whānau

• You can help grow and build your own whare! This would help reduce the costs of the build

• Cabins up to 30m² without toilets and kitchen facilities do not need council consent, those that do will require council consent. Te Puni Kokiri funding would not be available for building cabins - we want to explore ways whānau may be able to rent-to-buy cabins (to cover the cost of the build)

• Te Puni Kokiri have a focus on small scale papakāinga housing (3-10 whare per papakāinga)

• Whānau will need to be involved in lots of hui and kōrero with all owners of the whenua throughout the process. Download a copy of the Te Puni Kokiri guide here.

Learn more about hemp construction in this issue of the New Zealand Hemp Industries magazine "iHemp" from May 2021