The House that Hemp Built – Construction: Hempcrete

  Shale TInkler     2014-01-07   View all blog articles

From the Archive: 01-12-2010

Once the frame of the building was assembled, the roof sealed and waterproofed, it was time for the next exciting eco-tech part: the casting of the hempcrete walls.

The basics behind this are blending hemp stalk, which is high in cellulose, with a lime based binder and water. The bond takes place over time between the carbon in the hemp and the calcium in the lime which forms calcium carbonate, which essentially the same as shell, coral or limestone.

It also produces a breathable, organic, insulating, light and durable wall.

We used a light concrete horizontal mixer to make sure the hemp stalk was completely coated with the lime mix.

The builders who were working with it were so used to cement that they could not believe the mix ratio we gave them was right, and added more water to get a more cementitious mix, as they thought the mix was too light.

Luckily we were on site to put their concerns to rest and it was just the first layer of one wall that was mixed too wet and is taking longer to dry.

The building industry, especially in South Africa, is one that is very standardised around brick and mortar building and anyone trying to introduce new concepts often gets treated with suspicion.
These builders have really had their eyes opened with this building, and once the trusted us and went with the prescribed mix of 100 litres of hemp to 25kg of binder and 27 litres of water, they could quickly see the consistency we were going for, and understand the benefits of having a light and fluffy mix with lots of small air pockets within the wall to help create the awesome insulation properties.

The wall is created around the wood frame by using shutter board which is connected to each side of the frame. We went for a 300mm thick wall.

The hempcrete is cast between the boards in layers of around 40-50cm at a time. It is lightly tamped down, but not too much as you want to maintain some of the air bubbles. Care has to be taken to ensure it gets into all the gaps.

We are now nearly at the top of the ground floor walls, with the bottom already drying out nicely.

Once the whole wall is complete and given a chance to dry, the next stage would be sealing the hempcrete with a hempscreed layer, which uses sand and a higher percentage of lime binder.

It is amazing to see how a wall can be created so easily with material that grows so easily, especially seeing as all the carbon that the hemp absorbed while it grew, is now getting locked into the wall, and replacing all the extra carbon that a brick and cement wall would have produced.

Not only that, as it is drying, the hempcrete continues to absorb carbon as the bond between the hemp and lime takes place.

Nature has the solutions…