Hempcrete garden wall in the food forest

In September, Jayeson Hendyrsan from Hempcrete Natural Building Ltd, based in Bowen Island, gave a hempcrete workshop at the college.

Hempcrete is a form of natural building that uses a combination of hemp fibre, hemp hurd, hydrated lime, and optionally, sand and cement.  The amounts of those ingredients will change depending on application.  The hemp hurd provide the structure while the lime glues everything together.

Hempcrete wall in the food forest

A quick note about hemp vs marijuana.  Both hemp and marijuana are in the same genus, Cannabis.  There are three different species in this genus: C. sativa, C. indica and C. ruderalis.  There are numerous subspecies and varieties of Cannabis, many which have been bred by humans.  Some varieties that have been bred to produce high amounts of psychoactive components for medicinal, spiritual or recreational drug use.  There are also many varieties that have been bred for fibre, seeds and oil.  These varieties are called hemp (or industrial hemp).  They are used to produce hemp seed (for people or animal feed), hemp oil, wax, resin, rope, canvas, cloth, pulp, paper, and fuel.  Hemp plants typically have very low quantities of THC, and only approved varieties are permitted for growing.  Hemp is a multi-functional plant and has many uses and applications-  see the hemp wikipedia article for more info.

The foundation was 1 ft deep and filled with gravel.  Cement blocks were placed on top of the foundation.

When you process hemp stalks, you get two materials: hemp hurd and hemp fibre.  Hemp hurd comes the inner core of the hemp stalks.  It looks like fine woodchips when it has been processed. Hemp fibre is more stringy and come from the outer 'bark'  of the hemp stalk.  Both can be used in building.

Lime usually comes from limestone.  Limestone is typically put in a kiln, to created quick lime (calcium oxide, CaO).  Then water is added to make slaked lime or hydrated lime (calcium hydroxide, Ca(OH)2). Hydrated lime is used as mortar in hempcrete and sets, or dries, by absorbing CO2 from the air.  Lime has been used for centuries as mortar in buildings.  It is preferred to Portland cement in some applications because it is more elastic, it doesn't trap moisture (it breathes), stone and brick bond better, it strengthens over time and some say it is more environmentally friendly because the lime reabsorbs CO2 while drying.

For an interesting look at historic lime kilns, check out the first episode of BBC Edwardian Farm, where they make quicklime to spread on their field to raise the pH (starts around minute 43).

Materials for the project

Hempcrete has many advantages over conventional buildings:

• reduced use of petrochemical products, 
• insects/rat resistant, 
• fireproof, water and earthquake resistant
• breathable
• provides thermal mass
• quiet
• low maintenance, and long lasting

Some other considerations for hempcrete:

Hempcrete is not load bearing so you still need some kind of timber frame, but not as many supports as a typical timber framed house.   You also need to build forms for hempcrete. Hempcrete is probably as labour intensive as other natural buildings.  You need a cement mixer, as well as protective gear (because the lime is highly alkaline and will burn your skin).  Hempcrete does provide thermal mass, so it helps to regulate the temperature in your house.  Usually a material that has high thermal mass will absorb heat and radiate it slowly, but this same material will be bad insulator.  If you live in an environment that has extreme temperature changes with cold winters and hot summers, you might choose a different method of natural building- maybe strawbale, or add insulation to the outside of your hempcrete walls.  Another issue is that because of huge amounts of regulatory control of the hemp industry, it is difficult to source hemp fibre.  This may hopefully change in the future.  Right now most of the hemp sources come from Manitoba.  Contact Jayeson (hempcrete.ca) to source hemp fibre and hurd in Canada. Hempcrete was historically used in Europe, and is being 'rediscovered' there as a form of natural building. For more info about hempcrete, look in your library for Steve Allin's 'Building with Hempcrete'

Half the wall form.  Rebar secured into the cinder blocks with cement.

For the hempcrete workshop we built an outdoor garden wall in the food forest.  Our food forest was planted this year, and there is very little for shade at the moment except some cottonwood trees, and a weeping willow.  I thought that a kids play area would be perfect under the willow tree because with the long branches, it already feels self contained and in its own world. The willow tree also provides shelter from the sun, which is important on hot summer days in Creston. I thought a wall would help delineate areas for play and areas where it would be best to stay on the paths to avoid trampling young plants. We also made the wall curvy and inserted holes, bottles and designs to make the wall more whimsical, to become part of the play.  

Installing the other half of the form

Hemp hurd, fibre and other materials

A platform was built so we could pour the mixture of hemp, lime, sand, cement and water directly into the forms after mixing.  We used more sand and cement in this mixture than in a regular house wall because our garden wall is outside in the elements.

The hempcrete mixture was poured into the form (note the rebar supports) and was tamped.

Hempcrete mixture at the top of the form

Masks and gloves to protect from the lime mixture

The hempcrete wall forms were taken off 4 days later. The wall was still not completely dry, but it held its form.

Nine days after building.  Notice the holes and designs to add a playful aspect to the wall.

We will add a plaster in the spring, so stay tuned...