Because it breathes
A few decades ago Charles Rassetti, a French artisan went looking for a way to repair historic buildings that had been made of wattle and daub. The modern cement based repairs to these buildings were creating condensation problems, deepening the damage. Rassetti was looking for materials that would “breathe” instead of trapping moisture and humidity. He discovered that adding the inner stalk, called “hurd” or “shiv” of the cannabis plant to a traditional building material, lime, gave him those desired properties.
Building with hemp and lime quickly spread. First across France, to the UK, throughout Europe, and around the world. The properties that made hemp a solution for Rossetti made it desirable in new construction as well. And, as it is with the multifarious usefulness of hemp, the spectrum of benefits offered to building practices is astoundingly positive
First of all, hemp is an efficiently renewable annual crop, growing in about 90 to 120 days to yield 4-6 tons of carbon sequestering bio-mass per acre. The carbon that is locked up in the plant offsets that which is released by heating the lime, the other main ingredient used to make hempcrete. Lime, like cement, is calcined to drive off Co2. Then, as a powder, it can be reconstituted, cast, and molded. It sets up by taking back the molecules that were driven off in the heating. Cement is heated high enough to make a solid set that will not deteriorate under water. Lime, on the other hand is fired to a temperature that encourages a slower set, curing over a long period of time—50-100 years! As it cures, the lime continues to pull carbon from the atmosphere in a process of calcification.
A special mix
The part of the hemp plant that is generally used to make “hempcrete” is the woody core, the part that remains after the fibers are removed (and after the seeds are harvested as the primary crop.) Some experiments have been done that include the fibers in the mix, but since the fibers can bind up the mixing equipment, most hemp builders use just the hurd.
It is chopped into pieces of varying size, from about half an inch and smaller. (Unless sprayed, another option. Then the pieces need to be small enough to flow through the spray equipment.)
With care taken to get the moisture/binder/ hurd in the right proportion and consistency, a light, fluffy mix is obtained that can be packed into forms around a stick framed structure. Spacers are used so that the hempcrete surrounds and protects the framing, resulting in a wall of congruous material from inside to outside, compatible with and ready to receive a “lime render”, water repellant finish on the outside, and a lime plaster finish on the inside. (Or it may be left to reveal the hempcrete texture.)
The structural strength of the building does not come from the hempcrete, but from the framing. However, the encompassing support and thickness of the walls reduces the amount of lumber needed by 25-30%. What hempcrete does provide is an amazingly unique blend of insulation and thermal conductivity.
eR-value can vary depending of how densely the material is packed. Stats are posted at this site. In general it is not difficult to achieve building code standards, but comfort and energy efficiency in an interior space are more complicated than the heat exchange rates that are measured in a laboratory. (R-value) Hempcrete walls don’t pull heat from your body; they mediate heat and moisture. Also not measured by testing materials alone, hemp lime walls are monolithic, too dense for drafts, but nevertheless breathable, because the air pockets in the plant structure that attract and store moisture are dispersed throughout the wall. The effective thermal performance is much higher than what our building standards currently measure.
Another benefit of walls that absorb and release water from the air is that molds and spores that thrive in moisture don’t have the opportunity to grow. Unlike with cement and other nonporous materials, condensation doesn’t build up, but is taken into the wall. Good ventilation is an important consideration in a hemp-built structure because air flow is microscopic, but not macroscopic—or windy; the thick walls are airtight. If hempcrete gets wet, it must be dried out, but it does so very effectively with proper ventilation.
The indoor air quality, therefore, is much healthier than it is where moisture barriers are used, where indoor humidity is hard to control, where the bad air doesn’t have anywhere to go, and where failure of the membrane is common.
Non-toxic indoor air is also a consequence of a lime based material. Since lime itself is chemically caustic, a natural biocide, coating the already microbial deterring hemp with lime, inhibits the growth of micro-organisms. Neither is lime inviting to ants, termites, or rodents. Care should be taken with skin and lungs in the construction phase of working with raw lime powder and wet lime, but once set, there are no dangerous fumes, formaldehydes, PCBs or NO2. Instead the walls absorb CO2. Imagine how many health conditions might be controlled if more indoor spaces were made of healthy, breathing walls.
Lime has wonderful qualities to pair with the inner beauty of hemp. It’s been used to build with for thousands of years. Nature used its composition (CaCO3) to build sea shells and other fossils that decomposed on ocean floors over millions and billions of years. Lime is everywhere on earth where there were oceans. When it is heated, driving off some if the CO2, the lime cycle is kicked off. In simplified description, the powder of crushed rock becomes CaO (quicklime) or CaOH, (hydrated lime).
In these forms, adding water and then allowing it to dry out, the lime keeps grabbing the CO2 from the air in order to become once again like the shell or skeleton of sea creature from whence it came. Thus it calcifies the structure of the hemp’s cellular structure, keeping those little water-storing rooms and capillary hallways open for receiving air and water.
When humans dwell in rooms whose walls are made of natural lime plaster, it is possible to feel the gratitude of ancient sea beds standing erect in renewed functions of sheltering ever more complex organisms. (The author is growing weary of mostly technical information and thought that you, perhaps, may also be drifting into considering the more poetic and esoteric aspects of this bio-chemical union. Please do.)
Oh, and hempcrete is fireproof!