Hemp & Lime Part 1
A research diary — This episode gets into some details of how to use hemp in construction. Look out for the next episode for a look at the bigger picture of hemp as a material.
At Wally Farms, we are using a mixture of hemp and lime to construct our walls. We are interested in using hemp for its ability to sequester carbon and to demonstrate its viability as a building material. According to researchers at the University of Cambridge, hemp can capture atmospheric carbon twice as effectively as forests. It is also very healthy to live with and has great thermal properties.
Industrial hemp plants grow tall stalks with a woody core that store a lot of CO2 while the plant is growing. That CO2 is then stored in the building instead of being released into the atmosphere.
The core gets processed into small woody pieces called hurd and then is mixed with lime and water to create a mixture called hempcrete.
Even when considering all the machinery and energy involved in this process, the walls of our two micro homes are still a net “sequesterer” of carbon and make up for most of the emissions we are not able to avoid in other parts of the building.
The walls of one micro home are 13 inches thick and a total of 700 sf of hempcrete. Based on life cycle analysis for hempcrete the total amount of carbon stored is 6320 kgCO2 — the equivalent of 100 tree seedlings grown for 10 years.²
In addition to reducing emissions, there are a number of other reasons why it is a great building material.
Moisture
Hempcrete can move moisture through the wall when necessary. Rather than working with vapor barriers, hempcrete can absorb and release moisture into its cellular structure.
Mold-Resistant
The lime has a high pH level and is antimicrobial and antifungal. Hempcrete resists the development of mold. This allows for a healthier interior of the building and makes it unique among insulation materials.
Non-Toxic
Hemp can grow using far fewer pesticides than other crops, creating less environmental damage when growing. When installed in a building and fully dried it does not release any off gas into the interior.
Thermal Performance
Hempcrete’s thermal performance is good but not fantastic. That said, while its R-value of approximately 2 would be considered mediocre, it has some qualities that typical energy performance calculations focused on R-value do not account for. It creates a thermal mass, as heat transfer from interior to exterior or vice-a-versa is relatively slow. Thermal mass combined with passive moisture control, it shows that hempcrete can outperform materials with higher R-value when looking at the end user’s energy bill in most cases.
We used two construction methods at Wally Farms for hempcrete. We built an interior layer of prefabricated hemp blocks manufactured by Coexist.The blocks were made and cured off site and then installed similar to regular CMU blocks. (They also have the same size). The advantage of the blocks is that once built, we don’t have to wait for the wall to dry, which can take 6–8 weeks and can slow down the construction process.
Coexist is new — just as almost everything related to building with hemp in the US is new — and therefore still in experimentation and learning mode. We had blocks from two different mixes, neither were perfect. One mix crumbled too easily when trying to cut it. Being easy to cut is an advantage over CMU (Concrete Masonry Unit) blocks, but this can backfire when the lime binder doesn’t hold the hemp together. The blocks are also very light compared to CMU, so it is much easier to install. It requires the same skills as masonry without the heavy lifting. Coexist’s blocks are not load bearing.
Many who experiment with this material are interested in creating a load bearing modular system, similar to that of bricks or panels, in order to more easily use hempcrete in the construction process.
Cannabric, manufactured in Spain, is one such example. Just Biofiber, manufactured in Canada, is another example that uses a plastic core for structural stability.
We then added another layer of hempcrete between the studs using a sprayer. Spraying hempcrete against the layer of blocks or against formwork is messy. A lot of material falls to the ground, and can be picked up again. But compared to the blocks or typical cast-in-place installation, it is also really fast. This outer layer will have to dry for a few weeks before we can add a final layer of wood fiber insulation and then our exterior siding. How long exactly will depend on the weather.
Spraying was done by Americhanvre who also provided the hurd. While we wait to continue on the exterior, construction on the interior can already commence, because the blocks dried off-site before they were installed.
The next step on the interior is lime plaster. Lime plaster creates an airtight barrier, which is critical for overall energy-efficiency. The base coat was installed by a team of plasterers led by Hempstone. To make sure it’s connected to other building parts like floor and ceiling, we used tape with a mesh, so the plaster adheres to the mesh. Caulking around the windows completes the air barrier where the plastered trim sits directly against the window frame.
We used only spray-applied material in the second micro home. It was sprayed from the outside against temporary walls of 2'x8' form work. This was incredibly fast and actually more cost-effective than the blocks. But we do have to wait much longer for these walls to dry. In the meantime, other work can happen on site, like installation of windows, interior walls and roof insulation.
There are less than 20 examples of buildings in the American Northeast that are built with hemp & lime. The few people who know how to do it are a small, passionate group who support each other and support anyone who is willing to be part of this experiment. We are grateful to have worked with them and to contribute a tiny part of spreading the knowledge and welcome any questions and comments related to building with hemp.
Below is a list of additional resources on this topic. At the top of this list –and a great introduction to the topic– is the Hemp + Lime Guide and Podcast produced by Parsons Healthy Materials Lab.
Alex Sparrow, William Stanwix (2014): The Hempcrete Book, Green Books, Cambridge, UK
Chris Magwood (2016): Essential Hempcrete Construction, New Society Publishers, Gabriola Island, Canada
The US Hemp Building Association supports and advocates for hemp building professionals, projects and materials in the United States and holds an annual event, the US Hemp Building Summit
Hempstone offers trainings and workshops and shares a lot of their own lessons learned on their blog
Lancaster Farming’s Podcast on Industrial Hemp covers all sorts of topics touched on in this and the next episode.
The Endeavour Centre in Ontario, Canada offers workshops and resources on building with biogenic building materials.
The Hemp Building Association keeps an inventory of structures built with hemp. This summer, our hempcrete builders also completed The Cape Cod Hemp House, which used hempcrete even in its roof.
Notes:
¹ Interview With Darshil Shah, Senior researcher at the Centre for Natural Material Innovation at Cambridge. in Dezeen, June 2021
² Calculation of CO² was done using a beta version of the BEAM Calculator by Builders For Climate Action and the EPA’s Greenhouse Gas Equivalency Calculator
Unless otherwise noted, all images by Kaja Kühl, Graphics by Bianca Bryant
