February 4 — A Brief Overview of Hempcrete
The last few days, the apprentice team has been dividing into some individual research on various natural building materials and methods. We were each assigned a material from the following list: Strawbale, Cob, Hempcrete, Timber Frame, Bamboo, Adobe. Personally, I have been looking into hempcrete as a material option. I will be presenting to the other apprentices on Tuesday, but I figured I might as well share what I have learned here as well.
**Disclaimer: I am in no way an expert on this subject; just a student who is sharing as I learn. If you find the subject interesting, I highly recommend doing your due diligence in further research and or consulting an expert before you get started on any projects.**
What is Hempcrete?
A hemp-lime composite construction material made from the chopped stalk of industrial hemp (also known as the cannabis plant) and a lime-based binder. More specifically, the hemp shiv (or the inner woody system which has historically not been used for much) is utilized in the hempcrete mixture.
Hemp is a fast growing, annual plant that stands 1.5 to 4 meters tall with bark fibers that range from 1.2–2.1 meters long and are extremely strong.
History and Use
We have recordings of hemp being grown and used by humans as far back as neolithic times, in products ranging from oil and resin, and cosmetics to food, cloth and rope textiles and more. It is believed to have originated from China and eventually spread throughout the world where it became a major part of the culture of each population it touched.
In the 20th century, because of the popular use of cannabis (or marijuana) as a recreational drug, it became outlawed in much western countries. The unfortunate result of this prohibition was that cultivating hemp in all forms, even non-drug related, was also banned. After being banned in the UK in 1928, the early 1930s began a period where significant effort was applied toward developing cultivars of hemp with very miniscule THC content. This time of experimentation was successful and in 1993, the UK legalized the growth of industrial hemp (containing 0.2% or less of THC compared to the 10–15% contained in varieties used for recreational purposes) followed by Canada in 1995. Since the early 1990’s the western world has been rediscovering the natural and sustainable uses of the extremely versatile plant.
In the mid-1980s, Hempcrete was developed in France, making it a relatively new construction method. It was the result of a successful experimentation process aimed at replacing deteriorated wattle and daub in medieval timber frame buildings. The hempcrete they developed became a non-load bearing, sustainable, breathable (vapor permeable), and insulating material that could be applied to form walls, floor slabs, ceilings, and roof insulation in new construction as well as renovation and restoration projects. Since the 1980s it has spread, first across Europe and then the world.
Despite being formally “developed” in the 1980s, there is evidence of previous construction with hemp. One example is a Hemp House in the Miasa Village in Japan that was built in 1698 and still standing in good condition today. This structure serves as an excellent case study pointing to the longevity potential of such structures when properly maintained. It is also highly likely that other previous civilizations had discovered and utilized the use of hemp in construction, but it would be difficult to find evidence of or prove this hypothesis because of the materials ability to be composted and returned to the Earth.
Today, hempcrete is an attractive natural building option to self-builders at is a relatively low tech and achievable option while still having the potential to result in highly efficient buildings. Yet, it is also a viable option for larger / commercial projects. The initial cost is comparable to conventional methods, but when considering financial benefits of energy savings over the life of the building, hempcrete construction can result in significant savings.
Where is it Found? Which Climates Would it Work Well For?
The hemp plant itself grows easily in a wide range of soils and climates as long as the pH is 6.5 or higher (that is, a neutral to alkaline soil). Examples of Hempcrete buildings can now be found all over the world. I don’t have a perfect answer yet to the best climates for hempcrete use, but from what I understand so far it is very versatile and suitable for a wide range of climates. There are some important weather and climate considerations for the time of building. For example, in rainy climates, the risk of rain during installation does play a significant role and requires additional planning and precautions. If not properly managed, rain can cause problems with the hempcrete mixture as well as increase the risk of lime burns to those working with the materials. Another concern is temperature. You want to avoid placing hempcrete when the temperature will drop below freezing before the binder has achieved its initial set. Temperature and humidity levels can also affect the drying time required. Ideally, the mixture is allowed to dry fully before applying any finishes that will slow the drying times. This can be a major consideration for projects that have a strict timeline due to commercial or seasonal pressures.
How to Build with Hempcrete
There are 2 main options for building with Hempcrete. Cast in-situ., and Pre-cast., each demonstrating their own pros and cons.
With cast in-situ, the mixture is prepared on site and cast into mods in the exact position they will remain in the final building. Because hempcrete is not a load bearing material, it is always cast around a structural frame; commonly, a Timber Frame. It is not liquid in form so rather than pouring, like concrete, it is either hand placed or sprayed.
Hand-placing is the standard (though labor intensive) method of carefully applying the mixture within a typically temporary mold around the structural frame. This method allows for a high level of quality control.
Spray-application is a fully mechanized method of application that requires expensive equipment and a higher level of skill, but speeds up the process if used properly. With this delivery system, the hemp is sprayed onto a permanent shuttering board. The method does require slight adjustments to be made to the structural frame as well as a finer grade of shiv in the hempcrete composition.
With precast hempcrete, there is a distinct advantage of predictability in the building process. Because the drying is completed off-site, the uncertainty around drying times and scheduling can be greatly reduced if not eliminated. This factor is highly beneficial to large scale or commercial projects, or projects with strict timelines for any other reason. The 2 main options for precast hempcrete are blocks or panels.
Hempcrete blocks are laid be wetting the surface and bedding in a thin layer of mortar (made of hydraulic lime and sand). They are easily cut with a handsaw, but for efficiency and simplicity, designs should be planned around the standard block size as much as possible.
However, there are some arguments against the use of these pre-cast blocks. First, in order to make them with the capacity to withstand handling and transport, the hempcrete’s density needs to be increased, which has the side effect of reducing the insulation performance. Also, because it has to be installed in mortar, you run the risk of creating cold bridges. And finally, the compressive strength of hempcrete blocks is not sufficient to act as a structural member like concrete blocks, meaning they need to be installed around a structural frame.
Hempcrete panels, also produced offsite, are comprised of a timber frame, hemp fiber quilt insulation layer, and breathable vapor control layer. Unlike the blocks, they do not require mortar for installation so there is no setting time to account for before applying finishes to the building. The insulation layer also provides even better thermal performance than the cast hempcrete for a given wall thickness. Some panels also have the capacity to act as structural members. When joined on site, they can serve as the structural component of the building. These are most well suited for large residential, school and commercial buildings, but can be used for any 1–3 story buildings.
Sustainability Notes
Hempcrete, when properly designed for and utilized has the potential to serve as a natural, healthy, local construction material with low-embodied energy. Due to many factors and complexities, further research is needed to truly answer the question of in-situ vs pre-cast in terms of embodied energy and environmental impacts over the products’ life cycles. The material is also better-than-net-0-Carbon. The CO2 taken up by the plant is stored in the woody fibers. At the end of the building’s lifecycle, it can simply be composted and returned to the Earth. *There will be a few more sustainability benefits in my fun facts section*
A few Pros and Cons
In addition to the many benefits I’ve already discussed, hempcrete also has anti-microbial properties, is moisture absorbent and the insulation properties are excellent in both hot and cold climates. The primary draw back / bad press surrounding hempcrete has been related to slow drying time and/or not properly setting. However, if the material is properly understood and well-sourced, this is an avoidable complication. The main mistakes to look out for seem to be: 1) incorrect amount of water, 2) applying finishes too early, 3) Faulty binder (not properly tested / homemade — stick to a reputable and tested binder), 4) Faulty hemp (to much fines or dust in the mix resulting in a weak binding).
Fun Facts
· The word “canvas” is derived from “cannabis”; literally meaning “a fabric made of hemp.”
· Hemp was so important across British and Irish society that naming across the region reflected it.
— Hemel Hempstead in South of England — “a place of hemp” or “hemp pasture”
— Cwm Cywarch in Snowdonia — translates to “the steep walled mountain basin in which hemp is grown.”
— Hemp Mill Walk in Loggerheads in Staffordshire
— Hemp Street in Belfast
· The plant has been used throughout history in religious ceremonies.
· Hemp is a deep rooting plant which breaks up the soil and contributes to soil health.
· Hemp is an effective weed suppressant as it grows quickly and competitively, “winning out” over other species.
Conclusion
That’s all for today! I hope you found that as interesting as I do. :)
