Hempcrete is a bio-composite made of the inner woody core of the hemp plant mixed with a lime-based binder. The hemp core or “Shiv” has a high silica content which allows it to bind well with lime. This property is unique to hemp among all natural fibers. The result is a lightweight cementitious insulating material weighing about a seventh or an eighth of the weight of concrete. Fully cured hempcrete blocks float in a bucket of water. It is not used as a structural element, only as insulating infill between the frame members though it does tend to reduce racking. All loads are carried by internal framing. Wood stud framing is most common making it suitable for low-rise construction. Hempcrete buildings ten stories high have been built in Europe.
The material is mixed in mortar mixers for 1-2 minutes and stuffed by hand into the wall cavities. The wall is slip-formed with temporary wooden or plastic “shuttering” forming the inner/outer surface forms. The material is lightweight and can be moved easily about the site in tubs and passed up bucket-brigade fashion to workers filling the cavities. Site clean-up is easy. Simply till it into the soil.
The material is finished on the outside with a hard render coating about 20mm thick to protect it with a final colored topcoat finish added. The end result, appears like any stucco finished building. The inside can be left natural or finished with lime plaster for a traditional look.
Hempcrete was discovered in a bridge abutment in France built in the 6th century. Since its re-discovery it has seen growing use in Europe. Industrial hemp is grown by certified commercial growers so the crop can be certified to be very low in THC. Hemp is not psychoactive. Given it has survived 14 centuries, people expect hempcrete buildings will have a long life.
Hemp itself is a beneficial crop requiring no fertilizer, weed killer, pesticide or fungicide. It grows so thickly that weeds cannot grow. Farmers grow it in rotation with other crops such as barley or rye. The crop following the hemp requires no weed killer because the hemp has driven weeds out. The hemp seed is harvested as a nutritious food supplement rich in Omega-3 oil, amino acids, protein and fiber. It is considered a “super food”. The outer fibers are used for cloth usually as a blend with 45% cotton. The woody inner core is chopped to uniform size for our use. It has been traditionally used as animal bedding. Thoroughbred horse owners demand it.
For over 60 years, growing hemp in America has been illegal, because it resembles its psychoactive cousin marijuana. It is grown legally in Europe, the UK and Canada through a system of licensed growers producing seed certified low in THC (<.05%). Farmers must buy their seed from these seed growers and their fields are inspected periodically. Our hemp must be imported from the UK. A new plant to process the hemp core is being built in Canada so there should be a North American supplier soon.
This added shipping reduces the carbon-negative feature of hempcrete in North America and adds to production costs. It causes additional logistic costs since we had to buy a 40-foot container of material and store it on site. You must have people waiting for the truck with rented equipment to empty it, and you need dry weather. The truck charges nearly a hundred dollars an hour wait time while you unload for half a day. We filled a two-car garage with our material leaving a 4 foot margin for work around the edges. You have to keep this material dry until used so plan for adequate tarps.
For technical details and sources, check the Wikipedia entry for Hempcrete.