Building and Remodeling with Hemp

There’s a lot of information out there now about hemp construction. Since the recent passing of the 2018 Farm Bill in congress, which legalizes the growing of industrial hemp on a federal level, the hempcrete building technique has gained media attention.

But what do you need to know if you are thinking about your new dream hemp house or hempcrete remodel?

hemp building in progress

How Hemp is Used in Construction

Many articles in the press have done a good job of explaining the use of hemp in hempcrete walls. This system involves hemp hurd being mixed with lime binder and then loosely packed into forms that are built around the wall framing. This is applicable in both new builds and remodels. There is lots of room to experiment, as well.

For example, the hemp-clay house we worked on uses the hemp portion of the wall solely as an infill which butts up to the inside of the otherwise more conventional wall. It serves as a thermal break and plaster substrate. Hemp can also be mixed into plasters, earthen floors, and used in creating blocks, with either lime or clay binders.

There are companies that make building products made with hemp that could replace plywood, batt insulation, and even wood framing members. For now, though, hempcrete (or hemp-lime, or hemp-clay) is the king of hemp building.

Benefits of Building with Hemp

There are many benefits to building with hemp and hempcrete systems.

Environment

First off, the environmental benefits. One way hemp is better than other building materials that it may replace is that its production (farming) and processing (making the hemp plant into hurd) can require less energy, water, and time inputs. In fact, due to its long root system, hemp plants can actually improve soil health by increasing organic matter in soil over time and naturally loosening dense soils.

A hemp home may use less lumber, since the wooden frame of the house can be designed with greater spacing than if the home used more conventional insulating batts that are designed for 16″ or 24″ spaces. And since hemp grows much faster than trees, it requires less water and energy inputs, and a smaller area of land to produce an equivalent amount of material.

Hemp also replaces insulation. Batt insulation like fiberglass and mineral wool are manufactured products that need a lot of energy to create and ship. If local hemp hurd is used instead, the embodied energy is much lower. Also, hemp captures carbon from the air as it grows, which is then sequestered and stored in the walls of your building for as long as it stands. This means you get to start out with a carbon neutral or even carbon negative home from the beginning!

Building Health

The rise of “sick building syndrome” is well documented. As homes get more energy efficient with tighter envelopes, indoor air quality can decrease. Particularly when the materials used to build and finish them and the products we bring inside are off gassing chemicals and VOCs, this can result in poor health, lowered cognitive abilities, and potential long term disease. In addition, mold can become a problem from condensation forming within wall systems or inside of poorly ventilated homes.

Hemp can help with this is a few ways. If you follow our plaster site, you may have read about how plasters can improve indoor air quality. The lime (or clay) binder in hempcrete is the same earthen material as in a natural plaster. Also, most hemp walls are finished using plaster on the inside.

Also, hemp won’t contribute to VOC and chemical off-gassing as it is a wholly plant based material which has been dried and processed before being mixed with the lime.

Finally, hempcrete walls are highly mold resistant. The lime binder has a very high pH while wet, which prevents mold. And, once it has cured and been plastered, a hempcrete wall regulates humidity and allows water vapor to pass through it in a way that won’t lead to any points of condensation within the wall. There are also no gaps within the wall through which air can move. This eliminates another source of moisture and mold, and contributes to the longevity of your building.

Boosting Local Economies

Growing hemp is seen as a healthier, safer alternative to other economic activities, like mining and large scale industrial agriculture. Because hemp requires less inputs of pesticides and herbicides than other crops like cotton, soy, and corn, it is a good candidate for replacing or joining into a crop rotation with these plants. Also, parts of the country whose economies were dependent on coal mining or manufacturing jobs could use the recent legalization of industrial hemp to carve out a new living. In Colorado, hemp and marijuana growing is already boosting the economy.

Current Challenges of Building with Hemp in Colorado

Since industrial hemp is a new crop in the United States, we are lagging slightly behind in the manufacturing of products using hemp and in the processing of the raw plant material. The equipment, factories, and machinery required for such a task are expensive and it will take time to catch up to other countries where hemp building has been popular for decades.

For example, when we built our hemp-clay project, using a unique and lower carbon version of hempcrete where clay is used to bind the hemp together in the wall instead of lime, we used imported hemp hurd. There are drawbacks to this, like increased embodied energy. However, the current consistency and quality of processed hemp hurd available locally (if it is available at all) meant that there would be more headaches and room for errors than if we bought from a part of the world where hemp building is well established. In those countries, the processing of the plant into the final product is seamless and reliable.

So, while we are still waiting for Colorado and the US to catch up with other areas of the globe, hemp building is coming to Colorado quickly, including in a studio project that we are starting later in 2019. Keep an eye out for updates!

Hemp-clay Mountain Timber Frame

This beautiful timber frame home at the edge of the mountains has a fun twist: a hemp-clay infill between the timbers of the exterior walls. We installed a 4″ layer of hemp to the inside of the walls, using a clay binder instead of lime to keep embodied energy down. We then plastered over those walls with custom clay plasters.

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