Hempcrete is a rising star in the building world for more than one reason

We’re hearing more and more about this material, and had to investigate.   What is hempcrete and why is it gaining more attention as a hempcrete as a bio-based alternative to conventional building materials.  

Hempcrete is made of the inner woody core of the hemp plant (hemp shives), a lime-based binder, and water. (Hempcrete)  It can take a long time to “set” or harden completely, at a rate of 1 inch of wall thickness per week, but if both sides are exposed and allowed to dry, then it happens twice as fast (a 12” wall cast in place can be dry in 6 weeks).  (The Drying of Hempcrete Walls) In order to speed up the drying process, sometimes a small amount of cement will be added to speed up the chemical process of hardening.  (Cement and other hydraulic binders harden when water is added.)

Hempcrete is not exactly like concrete because it’s not a load-bearing material. With hempcrete, you still need to have wall framing.  (For this reason some companies call it hemplime).  Hempcrete is used between the framing or next to the framing, and acts as insulation and substrate.  The hempcrete material is then covered with plaster on the interior and exterior wall surfaces. (Hempcrete: A Homeowner’s Guide to Sustainable Building)

Check out some photos of hempcrete installation:

Above is a method of building a solid wall with hempcrete.  The outside is held in place by exterior siding, and wall is built up on the inside with a moveable sheet of plywood.  Source: Hempcrete Wall System @ The HempShed

Above is this method in action, making an even thicker wall by spacing the exterior siding on spacers as well.  Source: Hempcrete - modern solutions for green buildings

Above is a man installing a “Lego-Style Hempcrete Block” by Just Biofiber and Renewabuild.ca.  According to the manufacturer, the block is only about 25 pounds, is pre-cured, and enables a hempcrete wall to become structural on par with high performance concrete but with much better insulation properties.  The hemp-lime mixture is covering a structural frame, which gives the block strength and modular properties. The company makes 2 types of blocks: 1 with ability to construct walls 30 feet high (black frame), and the residential block with ability to construct walls 20 feet high (green frame).  The blocks have holes for electrical conduit, and can be cut with a reciprocating saw on-site. (Amazing Lego-Style HEMP BLOCKS Make Building a House Quick, Easy & Sustainable)  According to the latest company news, Renewabuild is capable of producing 2 of these blocks per minute, and building with them is projected to cost about $50 USD per sq. foot, so that the updated system is designed to compete directly with traditional building methods like concrete masonry units (CMU) rather than just standard wood-frame construction.  (Renewabuild Revives Interlocking Hemp Block with Canada, US Factories)

Here are some advantages to using hempcrete vs. concrete:

• Hempcrete has a high thermal mass, similar to concrete, which means it was shown to increase heating energy consumption and reduce cooling energy consumption due to a reduction in solar heat gain.  

• However, it has a very low conductivity to heat compared to concrete, so that it is 16-18 times better insulation than concrete. 

• Hempcrete is carbon-negative, sequestering roughly 300 kg of per cubic meter, while concrete has a high carbon footprint. (Hempcrete vs. Concrete) Part of the component of concrete’s carbon emissions is its inclusion of Portland Cement, which is a pozzolan.  Pozzolans are compounds that react with the lime in concrete to form the hard paste that holds together the aggregate. Both natural and synthetic pozzolans are available. The natural types include processed clays or shales, volcanic ash, and other powdery compounds. A synthetic pozzolan, fly ash is a byproduct of coal combustion, which is used to generate power.  Fly ash has been substituted for cement intermittently since the building of the Hoover Dam.

• Hempcrete (without cement added) doesn’t release any toxins into the air as it cures or during its lifetime.  Regular concrete, which may be mixed with fly ash as a substitute for Portland cement, has questionable toxic components!  See our explanation in this article. 

• Hempcrete is very resistant to fire.  However, only the companies that send their samples for ASTM E84 testing are certified “fireproof”.  Hemptecture is one such insulation manufacturer, and they were ecstatic to receive the test results: their product received a “0” on a 0 to 450 scale for flame spread and smoke generation, the best possible score. (In US, Hempcrete Isn’t Fireproof… Until Now)

• When encapsulating a wood frame with hempcrete, the substance protects the wooden structure from weather, mold, fire and pests.  (Hemphouse video in Colorado)

Now for some disadvantages:

• Since hempcrete is lighter (less dense) than concrete, it is not used for structural components on its own (except where an internal frame is included, see Justbiofiber.com and Renewabuild.ca).   Due to the high amount of void space in hempcrete, correct tamping is essential to achieve maximum integrity and bonding.

• Hempcrete is resistant to mold while it’s drying due to the high pH of the lime, but this mold protection goes away once the wall has fully cured.  Therefore, if it’s installed in a continually humid environment, hempcrete will certainly sustain mold.  For this reason, Hempcrete is not intended for ground contact or below-ground use, and is even exclusion for bathroom use by some builders (smart decision).  

• Hempcrete costs more than traditional concrete.  However, you’ll need to understand that due to its insulative and heat-transfer properties, less insulation may be needed with hempcrete.  For this reason, it’s better to look at the whole building envelope cost when deciding whether to specify hempcrete.   

Like all building materials, the more available hempcrete becomes, the more familiar builders will become with using it, and the cheaper it will become.  It’s not the “silver bullet” we were hoping for mold-resistance, and most of the installations we’re seeing use a lime plaster coat on the outside (and inside).  Lime plaster is also susceptible to mold after it’s dry (check out our article on lime here).   However, if you’re looking for a natural building material that’s fantastic for energy conservation (think Passive House standards), hempcrete might be for you.

Photo source: Natural Building Alliance