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Insulation Alternative: Hempcrete

Creating locally sourced lime and clay binders for use in hempcrete.

Hempcrete (a mix of chopped hemp hurd and a lime-based binder) is a relative newcomer to the North American insulation market, and most of the R&D and testing work to date has been done in Europe using proprietary formulations from the companies that pioneered the material for commercial use. This has been great for the development of hempcrete use on an ever-growing array of European projects, but the dependency on imported materials has made the use of hempcrete in North America prohibitively expensive for most.

At the Endeavour Centre, we have developed the use of locally-sourced binders for hempcrete, and paired that with hemp hurd sourced in Canada. At this point, our homemade hydraulic lime binder is well-tested and we feel it works as well as the imported binders, at a fraction of the cost.


Hempcrete mix

Our hempcrete binder is composed of 50% hydrated lime (we use Graymont’s Ivory Finish Lime) and 50% Metapor metakaolin from Poraver (created as a by-product of the company’s expanded glass bead production). The metakaolin causes the lime to have a partially hydraulic set, speeding up hardening time and promoting faster drying of the material.

We mix our wall-insulation hempcrete at a ratio of 1 part chopped hemp hurd by weight, with 1.5 parts of the binder by weight. After translating these weights to volume measurements, it approximates 4 buckets of hemp hurd going into the mixer with 1 bucket of binder (1/2 lime, 1/2 metakaolin).

Using a mortar mixer, we dry mix until the hurd is well coated with binder powder. Water is then misted (not sprayed) into the mixer until the mix is just moist enough that if we pack it like a snowball in our gloved hands it keeps its shape, but is still fairly fragile (ie, can be broken with a bit of a squeeze). It is important to not over-wet the hempcrete, as this will greatly extend the drying time once the hempcrete has been packed into a wall.

Hempcrete is placed into formwork on a frame wall, using light hand-pressure to compact the mix just enough to ensure that the binder will stick all the individual pieces of hemp together.

Our last workshop participants were able to mix and place hempcrete at a rate of over 6 cubic feet of material per hour!

Hemp insulation

Hempcrete recycling

We have long touted the no-waste benefits of hempcrete. We’ve speculated that insulation removed from a building during renovations or demolition could be broken up and recycled into a new mix with new binder added. We put that theory to the test at a recent workshop, demolishing one of our sample walls and adding the broken up hempcrete into our new mixes at a ratio of 3 parts new hemp to 1 part recycled hempcrete. The resulting mixes were impossible to distinguish from the all-new mixes! Hemp-clay shows lots of promise!

Hempcrete insulation is almost always made using a lime-based binder. But at the Natural Building Colloquium in Kingston, New Mexico last October, we were doing a hempcrete demonstration right next to a straw/clay demonstration, and we took the opportunity to mix up a block of hemp hurds with a clay binder.

The success of that demo block led us to try this combination on a slightly larger scale, and we machine mixed the clay and the hemp to fill one tall wall cavity with this hybrid material. Using the same mixing methodology as typical hempcrete, we dry mixed hemp hurd and bagged clay before misting with water. Interestingly, we were able to use less clay binder compared to lime binder (3/4 bucket of clay to 4 buckets of hemp hurd) and the resulting mix was stickier and easy to form than with the lime, and with the addition of noticeably less water.

The key difference between the two binders is in their manner of setting. Hydraulic lime binders cure chemically, and consume water to change the chemical structure of the mix as it solidifies. Clay binders simply dry out and get hard. In practice though, the two mixes had nearly identical drying times.

Clay binder with hempcrete offers some advantages over lime-based options, including a significantly lower carbon footprint and none of the caustic nature of lime that can cause skin burns when handling. The clay-based binder creates a mix that is much stickier during installation, which means less packing/tamping to get the material to cohere in the forms. Less water means that it was almost impossible to over-compact the mixture. We will definitely be exploring this option in a serious way!

About the Author: Chris Magwood is the author of Hempcrete Construction, a new book from New Society Publishers.