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.


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
hemp plastered wall timber frame

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!

Understanding Materials: Clay

clay plaster

We applied a lot of lime plaster in 2016. But we’re just as amped about clay and clay plasters. Curious about why?

Clay is a fascinating material with many unique properties. Clay is made of very tiny particles of rock which have been broken down over millennia. Because of the small particle size of clay, it has a very large surface area. The surface area of the clay is important because it creates a lot of space for the clay to bond with minerals and hold water. The water holding, or hydrophilic property of clay means that water is drawn to clay and absorbed into its structure.

For building purposes, clay is used as a binder. Some clays are great for building, and others not so much. When you moisten clay, it becomes sticky and pliable. That’s why clay is so useful in making sculptures, pottery, tiles, and bricks. For our purposes, we use almost exclusively unfired clay instead of heating it to set its shape.

When you mix clay with sand and straw, everything gets held together by the sticky clay, but can still be molded into different shapes. Depending on the texture of the mix, it can be spread into a thin plaster, sculpted into a cob bench, or lightly coated onto straw to tamp into a wall cavity. As it dries and the water evaporates, the material hardens and holds its shape. The sand and straw ensure that the finished product is strong and that it won’t crack as it dries.

Benefits of Clay as a Building Material:

adobe house
A traditional adobe home with proper roof and foundation detailing is a durable and affordable home in a warm climate, with a long and rich history.

Clay is a time-tested building material, having been used in some of humanity’s earliest structures. When protected from direct rain and wicking moisture out of the ground, clay-sand-straw mixtures dry into a strong and durable material. Additives like wheat paste can be used in plasters to increase durability and hardness.

cob clay balls
Cob is just clay, sand, and straw and could be used in the yard if you have leftover material.

Clay can be found on-site or nearby almost anywhere that you are building, making it a low embodied energy material. Unlike many other modern building materials, if you have too much on hand, you don’t need to send the extra to a landfill because it is safe and simple to dispose of.

clay plaster window
Clay finish plaster drying around the laundry room window.

Again, unlike many building materials, clay is naturally free from volatile organic compounds (VOCs) and other unhealthy chemicals which can cause poor indoor air quality and adverse health effects. Clay’s ability to absorb and release humidity is also beneficial for improving the indoor air quality and comfort of a living space, particularly in high humidity areas like basements, kitchens, bathrooms, or laundry rooms.

cob oven
A cob oven during a repair process, ready for the final plaster over a recently finished intermediate clay layer.

Unfired clay can be easily rehydrated with water and then reworked. Most bricks and tiles are examples of fired clay, which has chemically changed from its original state due to the heating. But unfired clays can be moistened and will then bond again. This makes it easy to fix cracks, gouges, or other damages, or to replaster or add a layer of clay paint to walls to change color or texture.

Thermal Mass
earth floor clay
An earthen floor being installed. The earth will absorb heat on sunny days and regulate the temperature of the home.

Clay holds onto its temperature longer than the environment around it. This means that clay walls can hold the nighttime coolness in your home during the summer, even after the day begins to heat up. Conversely, a sunny earthen floor will continue to radiate heat into the room even after the sun has gone down in the winter.[divider]

clay plaster kitchen
Clay plaster in a kitchen adds some texture and shine to the walls.

Clay is easy to sculpt into curves or fun shapes to create a unique look. You can also tint clay to a variety of colors.

If this information has piqued your interest in clay or how we can incorporate clay building or clay finishes into your home remodel or backyard design, get in touch.

clay samples
Cheryl labeling clay plaster samples.