We get asked a lot about the hemp component in hemplime or hempcrete, the material we used to insulate Project PA Hemp Home. This component is called hurd, and it is the inner woody core of industrial hemp stalk after processing by decortication.
Hemp hurd is mixed with lime and water to create a bio-composite, non-structural insulating material which can be used in walls and ceilings. The hurd for hempcrete contributes incredible thermal properties, and to maintain that must be clean and free of the fiber. The fiber content of the hemp hurd will have a large impact on the qualities and performance of the final material. Having a higher percentage of fiber increases density and strength, but at the expense of thermal performance. This is why most hemplime mixtures use little or no fiber. Hemplime has an R value of 2 per inch and has a much slower release performance than typical petrochemical-based materials.
For this project, we had to source the hurd from France, where hemp production and processing has been uninterrupted, unlike in the USA where we have to learn to grow and process all over again. DON Processing is working to bring the technology for large-scale decortication to western PA so the hurd can be produced domestically.
New York-based Parsons New School Healthy Materials Lab, one of the project partners, is a great source for information about building with hemp and lime: https://healthymaterialslab.org/tool-guides/hemp-lime-1