HEMPCRETE

Highland Hemp House
Bellingham, Washington
Photo by hempitecture
Highland Hemp House
Bellingham, Washington
Photo by hempitecture
Highland Hemp House
Bellingham, Washington
Photo by hempitecture
The Triangle low energy townhomes | Glenn Howells Architects
Swindon, UK | 2011
Photo © Paul Raftery
The Triangle low energy townhomes | Glenn Howells Architects
Swindon, UK | 2011
Photo © Paul Raftery
The Triangle low energy townhomes | Glenn Howells Architects
Swindon, UK | 2011
Photo © Paul Miller http://www.pmillerphotography.com/
WNDER Workshop project by Bob Escher AIA (Architect), Eric McKee (Owner WNDER Workshop), Left Hand Hemp (Hempcrete contractor), and Mark Cover (General contractor)
WNDER Workshop project by Bob Escher AIA (Architect), Eric McKee (Owner WNDER Workshop), Left Hand Hemp (Hempcrete contractor), and Mark Cover (General contractor)
WNDER Workshop project by Bob Escher AIA (Architect), Eric McKee (Owner WNDER Workshop), Left Hand Hemp (Hempcrete contractor), and Mark Cover (General contractor)

Carbon Impact of Hempcrete

Hempcrete is a monolithic wall system that consists of hemp hurd, lime, and a hydraulic additive over a structural support. Hemp hurds of processed quality are currently available to US markets from the Netherlands, China, and possibly a few small scale processors in the US and Canada.

Lime, in this application uses about 80% less energy to calcine than when used in concrete. As an “air set” material, the lime in hempcrete reabsorbs the CO2 that is driven off in calcining [CaCO₃ -> CaO + H₂O -> Ca(OH)₂ -> CaCO₃ with evaporation of water and sorption of CO₂].

Statistics

hemp based composite sequesters and stores an estimated 325 kg carbon per metric ton of hemp based composite¹

Carbon Smart Attributes

Hemp sequesters carbon

Hemp naturally sequesters carbon both in stalk itself and by storing carbon into the soil. The amount of carbon sequestered depends on the type of hemp, where and how it was grown, and on harvesting methods.

Material Attributes

Hemp is fast-growing

Hemp is a very efficient and fast-growing plant, growing as tall as 15 feet in a season.

Hemp utilizes an agriculture by-product

Hemp is typically grown for fiber, seed, medicinal CBD’s, animal feed and other uses. Hempcrete uses the woody core (hurd) of the hemp plant that is often considered a waste or byproduct.

Thermal Performance

Hempcrete wall systems are highly insulative and act as thermal mass. Walls that are typically 12” thick provide requisite R-Value (~2.5/in.)

Durability: moisture, fire, and insect Resistance

The hempcrete wall system transmits humidity, is fireproof, deters insects and rodents, resists mold, (can recover from water penetration without molding), has excellent acoustic performance, is recyclable, has no VOC or other toxins, and can endure for centuries.

Hempcrete can create an air-tight wall assembly

Finished with lime render, there is no opportunity for air flow when the finish is properly wrapped to windows and door openings.

Hempcrete can be used in multiple applications

Hempcrete could be used in new construction or retrofitted over existing framing. With proper equipment, hempcrete can be sprayed, including onto masonry walls, or as pre-cast panels. There is some speculation that a hemp/lime slurry could be 3-d printed.

Design & Construction Guidance

Hemp hurd should be appropriately processed

Processing of hemp hurd should yield pieces that are less than ¾ inch, have most of the dust removed, and have a suitable moisture content (~10-15%). Specify hemp hurds that meet these standards.

Pay attention to what binder is being used

Some type of binder is needed for hempcrete construction. This may be achieved by using Natural Hydraulic Lime (NHL), by adding Portland Cement, or by including an additive that has been formulated to mimic the Pozzolanic constituents of NHL. If using hydrated lime as a binder, ensure that it has at least 90% calcium content. Reference CONCRETE for carbon-smart cement.

Portland cement should be used strategically

Portland cement and other binders can interfere with the thermal and moisture performance of a hempcrete wall. Understand how the binder used will influence the thermal and moisture performance of your hempcrete mixture.

Acknowledged Challenges, Questions & Unknowns

Hemp hurd processing is currently available to US markets from the Netherlands, China, and a few small scale processors in the US and Canada. The embodied carbon of hempcrete could be greatly reduced if hurd processing and binder formulation were made locally or regionally available.

The various growth habits and varieties of hemp present different processing challenges. Tall plants that are harvested for fiber are the easiest to process for hurd. Bushy varieties grown for seed require a different processing method; these stems may be more suitable for paper and other products

Natural Hydraulic Limes (NHLs) are not typically mined in North America, as there have been few uses for them before.Sources of naturally occurring hydraulic limes are not well studied or documented in the US, though they may exist wherever ancient sea beds were exposed to volcanic activity. Alternatively, correct proportions of silicates and other mineral additives may be formulated to mimic the NHLs of European usage. (Additives are currently available in Europe.) The contents include commonly occurring minerals in the form of clay, silicates/alumina, and ashes.

Proprietary blends of hempcrete binder are available in the US. Price and quality need to be evaluated for practical use.

Hempcrete properties vary depending upon method used to build. Though hand packing can create inconsistent densities, it hasn’t been shown to affect performance.

Studies indicate insignificant performance gain between 9”(22cm) and 12”(30cm) walls in the UK.

Studies need to be conducted to validate properties of hempcrete, and building codes adapted, so that municipalities can permit these buildings. Criteria needs to be adjusted towards performance objectives rather than other industry prescribed conveniences.