What’s all the fuss about hempcrete?

After all, sometimes even kings and queens live in castles made from dirt.

Photo by David Hiser, Public Domain, Wikipedia

Don’t get me wrong, I’m a huge fan of using hemp in most any way you can dream up (yes, even that way) but too often new ideas are heralded as solving problems that don’t exist, or could be solved in much better ways. Such is the case with hempcrete — a sort of concrete made with hemp rather than rock, sand, and other aggregates.

For starters, hemp doesn’t address the bigger problems of sand and cement shortages, or the vast pollution and fuel usage required to make either available to use. Let’s start with how much cement is needed.

“To make 1 cubic yard of concrete, you’ll need about 5.5 to 7 bags (94 lb) of Portland cement for a standard mix, or approximately 45 to 90 bags of pre-mixed concrete (depending on bag size), plus sand and gravel, with the exact amount depending on the desired concrete strength (PSI) and mix ratio (e.g., 1:2:4). A standard 1:2:4 mix uses around 560 lbs of cement per cubic yard.”

So, how much cement is needed to make 1 cubic yard of hempcrete?

“To make 1 cubic yard of hempcrete, you’ll need roughly 5 to 10 bags (40–60 lb each) of lime binder, as hempcrete uses lime-based binders, not typical cement, with quantities varying greatly by desired density (insulation vs. structural), but expect around 300–600 lbs of binder for a yard, plus hemp hurd and water, forming a mix with density far lower than concrete.”

At first glance, it appears I’ve lost my argument, but as I happen to know that lime is the primary ingredient in cement, I have to ask, how much lime is in 94 lbs of portland cement? The answer:

“In a 94lb bag of pure Portland cement, the actual lime (calcium oxide, CaO) content is around 60–67% by weight, meaning about 56 to 63 pounds of lime, as lime is the primary component in cement…”

Are you seeing what I’m seeing? And don’t forget, less density means lower strength. Unlike concrete, Hempcrete alone is not load-bearing, meaning it cannot hold up a roof.

So, how much sand is required to make 1 cubic yard of concrete?

“To make 1 cubic yard of concrete using a standard 1:2:4 mix (cement:sand:gravel), you’ll need roughly 0.44 cubic yards of sand, which translates to about 12 cubic feet or around 1,200 pounds, though this varies by material density and mix design…”

And how much sand is needed to make 1 cubic yard of hempcrete?

“Hempcrete generally does not require sand in its formula, as it is a mixture of hemp hurds (the woody core), a lime-based binder, and water. The inclusion of sand is optional, often used only for specific applications like floor slabs or high-load areas to increase compressive strength.”

So generally speaking, hempcrete’s lack of sand leans in its favor, but is that enough?

Promoters of Hempcrete like to cite recyclability as a plus, but while hempcrete is recyclable in theory, in practice, it will rarely happen before the hemp portion begins to rot, vastly reducing the overall volume recycled, causing the cement to be forever lost — a victim of biodegradability. By comparison, concrete made from cement plus sand and stone can be infinitely recycled. Having operated a concrete grinding machine and managed a construction material recycling business, I know that from demolition to grinding, materials can sit out in the weather for months during peak season. And where does it go after grinding? On the ground or a concrete pad until it is sold.

Concrete can also be made from cement plus various aggregates, including waste glass, plastics, styrofoam, slag (waste from recycling metals), ceramics, bricks, blocks, bone, and fly ash.

Hempcrete zealots like to point out that Hempcrete draws in and sequesters carbon as it dries, but concrete does the same. Plus, extra carbon can be injected into concrete, making the concrete stronger as it sequesters even more carbon. Add to that, crushed carbon in the form of charcoal can replace some of the gravel in concrete if a lower-density concrete is desired.

Unlike concrete, Hempcrete cannot be used to make load-bearing structures, walls, or foundations. That’s right, foundations for Hempcrete walls are made from concrete. Hempcrete walls require post-and-beam-construction or Hempcrete blocks reinforced with steel or wood.

There is also the issue of places to recycle Hempcrete. You see, as concrete recycling is the obvious best use of old quarries that have been mined out, concrete recyclers are popping up all over the world. No less than 5 are currently located in the very North Carolina county I live in. The recycling of Hempcrete cannot be commingled with the recycling of concrete, as Hempcrete is far weaker than concrete. Thus requiring separate recycling facilities that do not yet exist. Don’t worry, I fully expect Hempcrete recycling facilities to be built mostly from concrete and steel, in about 50 years or so, when Hempcrete buildings start getting torn down, not because they are worn out, but to make way for new construction.

As previously mentioned, Hempcrete must also be kept dry throughout the entire recycling process, as the hemp will begin to rot when it gets wet, then dries again. Concrete can remain outdoors for centuries while waiting to be recycled.

Proponents of Hempcrete like to point out that Hempcrete can be ground fine and used as a soil amendment for farming. The same is true of traditional concrete; it just becomes a different amendment. You know what else gets ground up and spread across farm lands? Sheetrock, aka, drywall. I used to haul tractor-trailer loads of waste sheetrock to a grinding facility in Randolph County, North Carolina. They ground it up and sent it to farms. You see, gypsum (calcium sulfate dihydrate) is used to make sheetrock and is commonly found in limestone (calcium carbonate) quarries.

None of this is to say that hemp isn’t an excellent building material. Hemp is a great building material, just not a great alternative to concrete. Great uses for hemp include hemp fiberboard panels, hemp wood, hemp insulation batts that replace fiberglass batts made from fossil fuels, hemp plasters, carpets, and hemp composites for window frames, roofing, and more. Hemp spray-on insulation replaces plastics. Hemp-based plastics can replace the PVC, ABS, and resin-based plastics used in construction today. I would love to see more and more hemp used in construction, but Hempcrete? Not so much.

Hempcrete can be used to make alternatives to brick and block, provided the brick and block also contain wood or steel. While a block made from Hempcrete and steel can be ground up and the steel recycled, a block made from Hempcrete and wood can only be ground up and spread across the ground.

Those things said, there is a type of “hempcrete” I’d like to see used more often, that being hemp-based cob construction in places where straw isn’t so readily available. It makes no since to make hemp cobb when straw is easy to get. You see, traditional cob construction uses straw, water, and sand mixed with native clays to build walls. Cob construction dates back to prehistoric times (at least 10,000 years) and has been used all over the world. Yemen has 700-year-old cob towers. What’s that? You think it won’t hold up in rainy locations? In Southwest England (Devon/Cornwall) and Wales, thousands of cob homes are still standing from the 14th to 19th centuries. Proper design, construction, and maintenance will make any home last hundreds of years, and when all you need to make repairs can be found growing nearby, maintenance costs are far less expensive. The original builders of those cob homes in Devon and Cornwall got everything locally and did all maintenance and repairs themselves.

In some places, straw is readily available and cheap, but in other places, conditions don’t support the growth of the grains from which straw is a waste product, making hemp a better choice. Builders call hemp based cob Hempcrete, but no cement or lime is needed to make cob so Hempcobb would be a more correct description.

Hempcrete is a good material for construction, but like straw-based cob, it has its limits. Roads, sidewalks, bridges, and tall buildings require more strength than Hempcrete can provide. Roads and sidewalks made from Hempcrete or cobb would quickly fall apart as organic materials like hemp and straw expand when wet and shrink as they dry. Plus, they rot.

Previously, I mentioned sand and cement shortages. Most people don’t realize it, but the only kind of sand that makes good concrete or Hempcrete has to be sourced from rivers and beaches. Such sand is in such short supply worldwide that… Well, I’ll let Google AI tell you.

“While exact “stolen” amounts are hard to quantify, illegal sand mining is a massive global issue, with the illegal trade valued at hundreds of billions of dollars annually, driven by demand for construction, with some sources estimating illegal removal from coasts and rivers at tens of billions of tonnes each year, a fraction of the total 40–50 billion tonnes extracted legally and illegally worldwide. The illegal trade is a major criminal activity, comparable to water scarcity, fueled by its high value and difficulty in regulation, making sand a strategic resource like gold.”

And,

“The black market for sand is estimated to be worth $200-$350 billion annually.”

Entire beaches have been stolen.

“It is true that entire beaches have been stolen, most famously a case in Jamaica in 2008 where an estimated 500 truckloads of sand vanished overnight. This phenomenon, known as beach theft or large-scale sand extraction, is a serious environmental and criminal issue driven by the global demand for sand in construction and luxury resort development.”

While Hempcrete uses less sand than traditional concrete, it still requires very large volumes of sand. That’s why cob, be it Hempcrete or traditional cob, is a better choice. Did I mention cob is made from locally available clays? Locally available materials greatly reduce transportation cost and the fuel needed for processing and transportation.

Previously, I mentioned cement shortages. This is not an issue everywhere, but there are numerous cement shortages around the world. These shortages are due to logistics, transportation, insufficient production capabilities, and in some places, a shortage of available limestone from which to make the lime needed to make cement and Hempcrete. Limestone also has numerous competing uses, including making paper and,

“Limestone is a versatile rock used heavily in construction for cement, concrete, mortar, and road base; in industry, as a flux in steelmaking and for making lime for glass, paper, and chemicals; and for environmental uses like neutralizing acidic soil and water. It’s also a decorative material for building facades, rock gardens, and even fillers in paints, toothpaste, and plastics.”

Limestone is also used to make the scrubbers that capture carbon emissions from power plants and other industrial emitters. Bale construction, be it hemp, straw, or any other baled material, only needs lime to make the stucko coatings.

In places where hemp waste is plentiful, and straw is not, hemp bale construction might be the best way to build a home. Straw bale homes have been around since the first baling machines were invented in the early 1800s. The oldest known straw bale homes still standing date to the 1890s to 1910. Straw bale construction uses traditional wood post and beam construction with straw bales serving as non-load-bearing walls. Straw bale construction was once quite common in Nebraska and other places where lots of grain were grown. Here is a comparison of Hempcrete vs straw bale:

“Comparing directly the price of straw bales and only the raw hemp shiv the prices are fairly similar but where hempcrete increases in costs is the addition of lime as a binder. Straw bale would also be the cheaper to source with fewer materials involved and would be more likely to be sourced from the nearest supplier. This has additional benefits such as cutting down on transportation, and supporting local businesses. Hempcrete is a little more difficult to source in terms of who is providing the raw hemp shiv and at what cost — where they are based, the delivery involved and that some companies prefer to sell you both their hemp shiv and hemp lime binder together as a package potentially limiting options and monopolising the market.

Both products can be purchased outside of the season they are cut as long as they are stored in a dry environment. Hemp shiv has to be processed slightly more than straw bale to remove dust and smaller particles of hemp which will render the hempcrete mix unsuitable for building. The dust and fines absorb most of the water depriving the binder or the water it needs to take its initial set. Based on my research I would hazard the opinion that hemp shiv would be a little less likely to suffer from any potential damage if exposed to moisture during storage, as water is ultimately part of the hempcrete mix, however straw bale would definitely need to be stored dry.”

Another option that beats Hempcrete hands down is building homes from bales of waste plastic in the same way that straw bale homes are built. Waste plastics are impervious to water, locally available, sequester more carbon than any other type of construction, and vastly reduce plastic pollution. While Hempcrete sequesters more carbon than concrete, it cannot compare to any type of bale construction, and the cost of bale construction is much lower and has higher insulation values.

Hempcrete has its place, but like every other material, it is far from the best choice in every situation. Homebuyers, builders, developers, and local governments need to look at all the options. For example, plastic bales are likely the best choice for single-family one-storyhomes in cities as cities have the most plastic waste. In Nebraska, straw bales might be best. In places where lots of hemp waste exists, I’d lean towards hemp bale construction while using a Hempcrete plaster to cover the bales.

All these things said, Hempcrete might be the best green choice, depending on where you plan to build. Or not, as the best choice is usually what is available locally. If you’re building a cabin in the woods, a log house might be best. While hardwoods are desired for traditional log cabins, pines and cedar can be used for square logs. Traditionally, rock houses were built where the right kinds of rocks were available locally. Rammed-Earth, also called pisé, has been used for thousands of years and works anywhere clay-based soils can be found. Rammed-Earth resists earthquakes and can last for thousands of years. Earthships are a form of rammed-earth construction invented about 50 years ago. The photo at the top of the page is the first Earthship ever constructed by Earthship inventor, Michael Reynolds. in 1974.

While it has yet to be done, I believe Dogfennel (*Eupatorium capillifolium)** ***would make excellent construction bales. Dogfennel is perennial, and can grow 10' tall in a single season before everything above ground dies off in late Autumn. The dead stems could easily be harvested and baled with no processing needed. The stems are woody, 1/4" to 3/4" in diameter, and have a white core much like bamboo. Dogfennel is very slow to rot, usually requiring several seasons even when left lying out on the ground. I have a 10' length of Dogfennal that I’ve kept on my back porch for 3 years, and it remains hard. I’m curious as to how I might use it in woodworking. I currently use short pieces Dogfennal as mini punji sticks to deter neighborhood cats from digging in my garden. It seems to deter insects as well so I keep some growing in containers. It doesn’t hurt the cats but they tend to avoid it.

Unlike perennial hemp and straw, fields of Dogfennal could be harvested every year with no replanting needed. Did I mention the plant is native to the United States, and spreads via self-seeding and roots that pop-up through the ground every few feet? Hemp is native to Asia. While invasive like hemp, Dogfennel is a very showy plant sometimes used for landscape borders. Both hemp and Dogfennel are sometimes called Ditchweeds. It is posionus but nothing seems to want to eat it and it repells mosquitos and other pests. Dogfennal tends to crowd out competing plants, tends to take longer than a year to compost, and is best controlled by regular mowing and tillage. Like hemp, Dogfennel can be strewn about as a means of insect control so having walls made of the same material might deter insects and other pests from entering your home.

“Insect Repellent: Historically placed in dog pens and houses to deter fleas and ticks due to insecticidal compounds (alkaloids).

Strewing Herb: Used to repel various pests, insects, and even reptiles.”

Who knows, maybe Dogfennal will become the next big thing in green construction. You certainly wouldn’t have to worry about animals eating your house. Even termites leave it alone.

As consumers, we tend to fall in love with shiny new things and new ideas — especially if they appear to align with our values, but for those of you who can choose what kind of house you want to live in, I beg you to make the best choice, depending not on trends but on availability, locally sourced materials, and long term committment to green values. After all, sometimes even kings and queens live in castles made from dirt.