Hemp Hurd

Hemp hurd is the woody inner core of the hemp stalk — the part of the plant that remains after the long bast fibers on the outside have been stripped or retted away. It looks and feels like coarse wood chips, varies from cream to light tan in color, and weighs almost nothing in the hand. By mass it accounts for somewhere between 60% and 75% of the dry stalk, depending on variety, growing conditions, and how cleanly the bast has been separated.

For most of the 20th century, hurd was the part of the plant nobody wanted. Fiber processors stripped it off and burned it, composted it, or sold it cheaply for animal bedding. The economic asymmetry was severe — the long bast fibers were the valuable product, the hurd was the disposal problem. The contemporary recovery of hemp as a building material has substantially inverted this: hurd is now the principal commercial product of many hemp operations, and bast is the byproduct.

What it does

Several material properties make hurd unusual:

• Porosity. Hurd is roughly 50% air by volume. The pore structure is irregular, with both macropores (visible voids) and micropores (within the cell walls). The porosity is what gives hempcrete its insulation value; it is also what allows the material to manage moisture by absorbing and releasing water vapor through the wall assembly.
• Lignin-rich, silica-rich. Hurd contains substantially more lignin than most agricultural residues, and its silica content (concentrated in the outer cell walls) gives it excellent fire resistance — hempcrete walls are documented to resist flame spread for substantially longer than standard wood-frame assemblies.
• Bond geometry. The irregular, slightly fibrous shape of properly decorticated hurd particles creates mechanical interlock with the [[lime-binder|lime binder]]. Smoother, more spherical aggregates would not bond as well.
• Low density. Hurd weighs roughly 100–150 kg/m³ in bulk. Hempcrete made from it weighs 250–500 kg/m³ depending on the binder ratio — substantially lighter than concrete or even most masonry.

How it’s produced

Hurd is the output of decortication — the mechanical process of breaking the dried hemp stalk and separating the long bast fibers from the woody core. Historically this was done by hand, then by water-retting followed by mechanical scutching, and now (in the contemporary industrial supply chain) by various dedicated decortication machines. The bottleneck for North American hemp building has been substantially this: until recently there were almost no commercial decorticators operating in the U.S., and hempcrete projects had to import hurd from Europe at substantial cost. Domestic decortication capacity has grown since approximately 2018, but the supply chain remains thin.

Quality matters. Well-decorticated hurd is consistent in particle size (typically 5–25mm), low in residual bast fiber (which interferes with bond), low in dust (which affects worker health and binder consumption), and dry (high moisture content during storage produces mold and degrades performance). Inconsistent hurd is one of the principal causes of hempcrete projects underperforming the material’s documented properties.

Other uses

A non-exhaustive list of what hurd is good for besides hempcrete:

• Animal bedding. The principal historical use; hurd is highly absorbent (3–4× its weight in liquid), low in dust, and naturally pest-resistant. Equine, livestock, and small-animal applications.
• [[mushroom-substrate|Mushroom substrate]]. A growing application; hurd’s pore structure and lignin content support a range of cultivated species.
• Pulp. Hurd can be pulped for paper and bio-composite production. The 1916 USDA bulletin Hemp Hurds as Paper-Making Material (Lyster Dewey & Jason Merrill) remains a continuing reference.
• Bio-composites. Hurd-polymer composites are an active research area for automotive, packaging, and consumer-product applications.
• Soil amendment. Composted hurd improves soil structure and water-holding capacity.

See also

Auto-generated from this entry’s typed relations: frontmatter, grouped by relation type so the editorial signal isn’t flattened.

• Subset of: [[hemp]]
• Enables: [[hempcrete]]
• Member of: [[ingredient]]

Sources

1. Lyster Dewey & Jason Merrill. Hemp Hurds as Paper-Making Material. USDA Bulletin No. 404, 1916. Source class: primary / historical USDA technical bulletin.
2. Rachel Bevan & Tom Woolley. [[hempcrete|Hemp Lime]] Construction. IHS BRE Press, 2008 (rev. 2017). Source class: book / definitive technical reference.
3. Steve Allin. Building with Hemp. Seed Press, 2005 (rev. 2012). Source class: book / practical builder’s reference.
4. International Hemp Building Association — technical bulletins. https://www.hempbuilding.org. Source class: institutional / continuing technical body.

Lenses still to grow

• Particle-size distribution and how it affects hempcrete performance.
• Domestic decortication capacity in the U.S. and Canada.
• Hurd in [[mushroom-cultivation|mushroom cultivation]] as a continuing application.
• The historical pulp question — what the 1916 USDA bulletin actually claimed.