The Hemp Clay Experience

We’re working in a home where we used a hemp and clay mixture between timbers to add insulation and thermal mass. Why did we choose this, and how is it different from the hemp-lime or hempcrete building processes?

Typical Hempcrete

Hempcrete, or hemp-lime, as you may know, is a popular form of wall infill that’s a mixture of hemp hurd, hydraulic lime, and water. As it cures, the lime in hempcrete chemically changes back into the same composition as limestone, making it rock solid. Frank has taught at a few hempcrete workshops around Colorado with John Patterson of Tiny Hemp Houses.

hemp building workshop
At a hempcrete workshop Frank co-taught at in Colorado.

The Good:

Hemp! It’s great. It grows fast, and needs less chemicals while growing: all reasons it may be more ecological than other building materials. Hemp is also better for soils than most other plants, with its deep roots that aerate soil. Additionally, the hemp stalk is composed of about 50% carbon by dry weight. This means that the carbon sequestered from the atmosphere during photosynthesis can be locked into our building like a carbon sink, not being released until the building is demolished much further down the line. This enables us the possibility to build a “carbon negative” wall system. If you’d like to learn more about carbon sequestering and building for climate change, we will be posting a blog soon.

The Not-So-Good:

Lime, while a natural and healthy building material, requires a lot of energy input to be created. Also, we import a lot of the natural hydraulic limes from overseas, increasing the embodied energy of the material. Alternately, a mix with cement is used, which also has very high embodied energy and accounts for an absurd amount of greenhouse gas emissions. There are other additives that can be mixed with lime to make it hydraulic, such as different types of pozzolans and geopolymers. These have their own benefits and drawbacks, but it comes down to manufacturing processes, local availability, and toxicity.

What about Hemp-Clay?

Colorado is blessed with beautiful and strong clay, an alternative binder to lime or cement. The best part of this is that the energy required to dig up and screen local clay is minuscule compared to burning lime.

We made a test brick with hemp hurd and clay slip, and the result was strong and lightweight – a perfect combination of insulation and thermal mass (especially once clay plaster is added).

The Installation

The hemp clay installation process went very similarly to hemp-lime. Forms were packed with wet material and then moved up. It goes pretty quickly if you can make your mix dry enough, but still sticky and workable. That way, forms are moved up and the packed in hemp-clay sticks in place without slumping.

hemp building process
Forms and the tamped hemp-clay mixture.

Drying

Since clay does not set chemically, like lime or cement, it has to dry naturally, with time. With several fans and dehumidifiers placed around the home, it still took a while to fully dry. We used a moisture meter to check deep within the walls, and later patched those spots where we had to put the probe in.

hemp wall drying
Hemp wall drying in the basement. Some forms had to stay up a little longer for extra reinforcement of thinner areas.

This step is very important because if you seal the hemp up with plasters before it’s dry, although it can still breathe through the plasters, there is a greater chance that some moisture will get stuck deep in the wall. Over time, this could lead to mold.

Another thing to note is the clay tends to shrink as it dries. This led to some cracking and pulling away from timbers. We took an extra half day to come back and fill those cracks in to prevent thermal bridges and loss of insulation in those places.

hemp clay dried
The dried hemp-clay infill shrunk and pulled away slightly from the timbers. Nothing plaster can’t cover.

Plaster Prep and Plastering

This step is again just about the same as with hemp-lime or hempcrete. We had a few places where the mix was too dry or didn’t have enough clay, as well as fragile corners around windows where we used an expanded metal lath to shore up the hemp clay. Landscape staples were used to attach the lath to the hemp, where needed.

We used clay plaster and our sprayer to get a base coat up first. The texture is perfect for plaster to stick to. Although clay plaster is the safest bet for a strong bond, a lime plaster or lime stabilized clay mix would also key in well to the rough surface.

hemp clay wall
Before
hemp plastered wall timber frame
After

We’re pretty happy with the results and process. It’s not too dissimilar to a woodchip-clay infill wall. The fact that it’s a low embodied energy and carbon sequestering solution is exciting, but the amount of time it takes to dry is a challenge. However, working in the summer could speed that up easily. We would also consider adding a small amount of cement or lime to the mix in order to create that chemical set and allow us to fill higher and faster.

We used a mortar mixer for mixing, which only allows a certain amount of minimum moisture. If you wanted the mix dryer, a horizontal drum mixer would be a better option.

Unsurprisingly, we’re not the first to try this. Check out Chris Magwood and the Endeavour Centre Blog below for their experience.  Scroll to the bottom if you just want to read the hemp-clay part. I think that the hemp-clay block shows the most promise. Because they are small, that minimizes the risks of cracking and pulling away that can happen during the drying of a large wall.

Hempcrete developments

Thanks for reading, and let us know your latest hemp building experience, or if you are interested in trying it out for yourself, or in your home, backyard studio, or shed!

Green and Low Carbon Building Interview

lime plaster skylight

Frank participated in a brief interview on green building in Colorado with Boulder, CO radio station KGNU. Both Living Craft and Rodwin Architecture were represented after our joint presentation at The Eco-Social Solutions 2018 conference at CU Boulder. Click below to give it a listen, and be sure to read the whole article at KGNU’s site, linked below.

Green Building and Carbon Reduction

lime plaster skylight

Case Study: A Straw-Cell Addition

straw wallsLiving Craft Design has been working on a straw bale addition to our friends’ home in Boulder. This family is already well-versed in the benefits of natural building, so their addition incorporates passive solar heating with large, south-facing windows and a concrete slab floor to hold all that sun-generated heat. The addition has the high insulation value (R-value) that is a great feature of straw bales as a building material. Additionally, the family chose to renovate the original rooms of their home and filled the walls and ceilings with densely-packed blown cellulose as part of that process. This home will be comfortable year-round and have low energy bills as well as a reduced need for running heating and cooling systems.

 

A Straw-Cell Design

We jumped into the project starting with straw bale installation. This house has what’s called a straw-cell wall, with an entire wall system behind the bales, complete with wood studs, recycled denim insulation, and exterior wood siding. The bales then are stacked on the inside of this wall, meaning that labor is reduced because you don’t have to cut or trim as many bales to fit within and around the walls’ wood frames. You also only need to apply plaster to the interior side of the bales.

Low Carbon

A great thing about the insulation materials used in this home is that they are all carbon-based, and will now be locked away in the walls of this home for a very long time. This is actually a form of carbon sequestration, one technique to keep atmospheric carbon down and help mitigate climate change. This is in direct contrast to insulation materials like foam, fiberglass, and mineral wool, which consume a lot of energy to create. This means that the manufacture of those materials produces a lot of greenhouse gas emissions, upping their carbon footprint.

Healthy, Natural Plasters

Applying a base layer of clay plaster at the work party.
Applying a base layer of clay plaster at the work party.

Our next step was to start the plasters. A base coat of local red clay was applied to cover the bales first. At a fun summertime work party hosted by the family, we added a leveling coat which will provide the base and shape for the finish plasters. We introduced some folks to the techniques and tools for mixing and applying natural clay plasters and played in the mud with good friends. We also started to build out the windowsills into their final shape using a lime and clay mixture with lots of straw for strength.

Ben talks building with some volunteers while others work on shaping the windowsills.
Ben talks building with some volunteers while others work on shaping the windowsills.
The finish plaster drying around the laundry room window.
The finish plaster drying around the laundry room window.

The final wall finish was customized to get the exact color and texture that our clients wanted, and this last, thin layer of clay-based plaster went on like a dream. Thanks to additions like wheat paste, it dries into a hard and durable finish that will last for years, and also be easily reparable in case of accidental damages.

During the wall finish process, we were preparing for and creating the tadelakt windowsills that will become perfect benches for sitting on and reading in the natural light. This Moroccan finish plaster is created from lime, which is why the base coat for the sills incorporated lime with the clay. This results in a tight bond between the materials. The smooth layers of lime were applied, soaped, and burnished using stones to create a shiny finish. The sills were then waxed as the final step to create a long-lasting surface that can withstand some use.

The burnished and waxed tadelakt windowsills have an amazing shine and reflection, as well as feeling silky smooth.
The burnished and waxed tadelakt windowsills have an amazing shine and reflection, as well as feeling silky smooth.

The End Result: A Beyond Green Building

We’re departing this project with lots of hope for the future which will unfold in this family’s happy, healthy, non-toxic, and environmentally friendly home. Not only will it be a beautiful space to raise a family, but also a good model for other front range homeowners looking to sustainably add some space and renew their original home.

finished window and wall plaster