December 22,2018-
Building my first hemp concrete house – The advice I would have liked to know before
by NHA
Interview with Serge Bolduc, a Canadian house builder
By Simon Leclerc
Simon Leclerc: We are located in front of a beautiful house that you are building here in Quebec, Canada, using hemp concrete for the walls and the floor. You have 21 years of experience as a builder, and this is the first time you have used hemp formwork. What made you accept this new challenge?
Serge Bolduc: I’ve known hemp since I was young, but a few years ago I visited a sugar shack in Canada that was built with hemp concrete. When I entered the house, I felt like I was outside, as if the house was merged with its environment. I am a very sensitive person and I felt as if the whole house was organic, with very little transformed material. It was a great feeling.
When my client approached me with his hemp house project, I was very happy to accept this contract and to discover this matter closely that made me feel so good. But since I had never worked with hemp before, it was really a new challenge for me.
Simon Leclerc: The house I see behind is not typical. Even if the construction is not finished yet, what did you learn from this specific experience?
Serge Bolduc: Each project has its unique peculiarity, but this one was very special, because the house has high walls and a large fenestration. These specificities made the project more difficult to evaluate and plan. The time scheduling was an ongoing challenge.
I found in Canada a hemp expert with 13 years of experience (Gabriel Gauthier, Artcan) who agreed to supervise the construction of the walls and the floor and take responsibility for them. This person had experience in a project like mine. He knew the right recipe to use and how to build the hemp structure (mix of hemp shive, clay powder, lime and water). He oversaw the construction of the walls, which were compacted with the help of friends and family members of the client to reduce project costs.
Before starting the work, we evaluated the time of construction of the walls, but it took twice longer as expected. Because compacting walls is a demanding job, we have struggled to find friends and family members willing to commit long enough. So we continually trained new ones, which slowed down the construction considerably.
To build the walls, workers must stack and pound hemp concrete mixes into the formwork with a fork designed for this purpose. These people must be sturdy, but they do not need to be specifically qualified for the job. It takes at least half a day to learn how to do the job well, and it can be learned on the spot. The rotation of the people who came to help was therefore our main challenge.
If I redid this project, I would ask everyone to commit for at least two days in a row, and even more if possible.
Simon Leclerc: Were there other elements, along with the rotation of the people who came to help, that slowed down the project?
Serge Bolduc: Usually, when I make a floor, I place 2 or 3 inches of rigid insulation on the ground (Styrofoam sheets) and I pour a concrete floor over it. But in this case, my client wanted to have both feet on the ground, to feel a direct contact with the earth (he only wanted organic matter under his feet). So we replaced the rigid insulation with 8 inches of hemp concrete mix, the same recipe as for the walls.
Because we have a very low temperature during winter in Canada, we could not let the air circulate under the floor to facilitate drying, as is the case in a warm environment. So this floor alone took 7 months to dry, much more than for the walls.
If I started again, we would use geothermal ventilation (Canadian well) to accelerate drying. This is the conclusion my expert and I arrived at, as it would allow the floor to dry on both sides (top and bottom).
Simon Leclerc: How did your client manage the deadlines?
Serge Bolduc: My client was in no hurry to settle in his house, which greatly facilitated the construction process. I would not have been able to meet tight construction time requirements due to the large number of unknown elements associated with a first hemp construction. This flexibility over time allowed us to go through each step properly.
Simon Leclerc: Would the hemp concrete recipe change in a cold environment?
Serge Bolduc: No, it would not change. In all condition, the goal is to obtain a mixture of hemp concrete as dry as possible. The drier it is initially, the harder it will be to compact in the walls, but the faster it will dry.
At one point in our assembly, it became easier to compact the concrete mix in the formwork and our expert stopped the construction. This was because the recipe contained too much water. It had to be rebalanced before restarting production.
Simon Leclerc: With Canadian building regulations requiring all employees on a construction site to be licensed professionals, how are hemp workers fitting?
Serge Bolduc: Since no one in the construction sector is specifically qualified to supervise the building of hemp concrete walls, the hired expert will be tolerated on a construction site. It will be possible to demonstrate that he does not occupy the functions of skilled workers. For the others who will help him to compact the mixture in the formwork, they must be volunteers (friends, families).
All this remains an area not completely defined, but tolerated by the system in place. Since this work is very specific, my expert has never had a problem with the regulations.
Simon Leclerc: You will soon be adding a coating to the hemp walls. Is it different from any other wall?
Serge Bolduc: It’s the same. Once we reach a certain level of humidity (my expert manages the percentage), hemp walls are considered dry. We can then add the coating. For this specific project, we will add a coating of lime and sand of 3/16 inches thick. My client will soon choose the final color. Many shades are available because color pigments can be added if we do not find a sand color that we like.
Simon Leclerc: For the structure of the house, does hemp need specificity or do you build it like any other structure?
Serge Bolduc: We used 2 x 6 beams for the structure of the house, but instead of spacing them 16 inches apart, we spaced them 24 inches apart. Then my hemp expert added extensions on each side of the beams to a total thickness of 12 inches so we could get R-25 insulation, which is the standard here in Canada.
Simon Leclerc: So with a house that would be built in a warmer environment, is it conceivable that the thickness of hemp walls would be less?
Serge Bolduc: Yes, absolutely. The level of insulation requirements in such environment would certainly be lower. The Canadian standard is R-25, and hemp offers an insulation performance of R-2 per inch (so 12 inches thick required, plus coating).
Simon Leclerc: If you’d start the same project again, what would you do differently to facilitate construction?
Serge Bolduc: As I said, because we worked with the client’s friends and family who came to help compact the concrete mix, it was difficult to really plan the duration of the project. So if I did it again, I would expect twice the time for every step of hemp construction. This would ease the pressure on the overall calendar.
Simon Leclerc: Is there anything else you would do differently?
Serge Bolduc: I would mandate an inventive person to create a mechanical way to pound more easily the mixture of hemp concrete in the formwork walls. If such a tool existed, it would have greatly facilitated our work.
Simon Leclerc: In order to properly consider such project, what should a person planning to build with hemp concrete consider?
Serge Bolduc: First of all, this person must make sure that he/she has many friends and family members ready to commit to his/her project, in addition to the expert responsible for supervising the construction. These people have to be in good shape, because it’s endurance work, and they have to give their time for at least two days in a row (ideally more), not just for a few hours to come and see how things are going. Otherwise, it will slow down the entire team because it takes time to train people properly and make sure they are effective.
I also recommend planning twice as many people as those who confirm their presence. Again, because they have other commitments elsewhere, they may not be available the day you need them and you do not want to slow down the entire team.
Simon Leclerc: Knowing the challenges you had with this house, why would anyone choose hemp concrete today?
Serge Bolduc: Those who choose hemp for their home are looking for organic materials that give them the feeling of living outdoors. I have never felt this elsewhere, except in a log house. You only have organic matters between the living area and the outside.
Hemp is one of the most compatible products you can find for humans. You can extract the oil, proteins and make clothes with fiber. It requires very little water and grows like grass. In addition, no vermin are attracted to hemp because of the silica it contains (mice, ants, etc.).
Those who want to live in an organic environment will choose hemp. They will have the feeling of being merged with the landscape around their house, without synthetic material. Not possible to be more natural. It’s a great way to connect with nature.
SUMMARY and ADVICE
– Make sure to hire a hemp expert who has experience in a project like yours. Many challenges will be encountered along the way and being able to find solutions in small details will make a big difference at the end.
– Anticipate larger delays than expected.
– Expect more friends and family members than you need and who will have confirmed their presence.
– If you can afford it, hire some professional workers too, in addition to your expert and the friends and family members. If you pay for them, national standards require that they be licensed professionals.
– Do not have a tight schedule, as drying may take longer than expected.
