Building with Hemp

After decades of regulated growth, one of the oldest crops in the world is making a comeback in the green building space. Europe is leading the industrial hemp comeback, making major strides in using the crop as a carbon-sequestering, energy-efficient building material in the form of hempcrete and as hemp insulation. The United States also is making strides as barriers to growing and processing hemp begin to fall.

Hemp hurd and lime create one of the greatest building materials known to man – Hempcrete!

Hempcrete is touted as the most energy-efficient construction material on the market, bringing annual energy savings to homeowners and improving the building sector’s carbon-dioxide emissions.

Hempcrete is a mixture of the inner core of the industrial hemp stalk (known as the hurd or core), lime and water. It has a high-insulation value but is non-loadbearing, so it is compressed between structural framing with removable formwork. Buildings can also use the exterior hemp fibers (technical fiber or bast fiber) as an insulation material, either as an insulation batt or as a spray-on application. 

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Hemp Builds, Old and New

Even though Europe has recently led in the designs and construction of hemp-based building materials, building with hemp is not new. The hempcrete design was likely modeled after a sixth-century bridge in southern France, which was built with hemp hurd and petrified lime. Maisa Village in Nagano, Japan, recognizes a hemp house built in 1698 as a national heritage site.

Building using industrial hemp isn’t new all around the world. These are just a few structures designed with sustainability and hemp.

Today, hemp-built projects span the globe. Companies new and old are adopting the material as building inspectors begin to study and accept hemp for construction projects. American Lime Technology has constructed houses and commercial buildings in North Carolina, Florida and the United Kingdom. Organizations like Tiny Hemp Houses and the International Hemp Building Association are establishing open hemp building courses around the world to teach people how to build with hemp.

Tony Budden aided in the design of South Africa’s first hemp home in hopes of spreading the idea of “how a house could be grown.” Now South African architect Oliver Wolf is building with hemp in Cape Town and an individual in Malawi has government permission to begin research trials there.

Costs of Building with Hempcrete

Tim Callahan of Alembic Studio in Asheville, N.C., says materials for building a hempcrete home cost approximately $9 per cubic foot but that overall costs can vary depending on labor and design. Material costs are expected to fall as the United States begins to permit locally grown hemp.

But the hemp hurd is not the most costly material. The largest drops in material costs will come as hempcrete gains popularity, because the high cost of lime should come down at the same time.

On a cost-comparison basis, “Hempcrete is between 7 percent to 10 percent more expensive than stick-built homes” (traditional timber-framed houses with siding veneer) and costs about the same as a brick-veneer home, Callahan says.

Builders on Board with Hemp materials

Hemp building materials are gaining traction, and architects, builders and individuals are hopping on board for a number of reasons.

One is a growing interest in less toxic building materials. A study at a Seattle public-housing complex found improvements in its residents’ health after upgrading the buildings with green building materials.

The U.S. Green Building Council (USGBC) operates the Leadership in Energy and Environmental Design (LEED) certification program, which rates buildings on a point basis depending on its green building performance. Hemp is a relatively new material with non-toxic and high insulation qualities, so a hemp home has the potential to rack up LEED points as an innovative material and for indoor air quality and energy performance.

In addition, studies have shown hempcrete and hemp fibers in composites to have excellent fire-resistant and soundabsorbing properties.

A French manufacturer of Isochanvre (hemp and lime mix) carried out fire tests that resulted in a material classification between M0-M2, considered to be ”non flammable” or ”low flammability” materials. A 2014 Brunel University study in London suggested that the hemicellulose fibers in hemp “produce less ignitable gases than cellulose and polyester.”

The hurd also has excellent sound-absorption properties, though they vary depending on the ratio of hemp hurd to binder. A 2009 study in Polimery found that composites made of polypropylene with 40 percent hemp fibers have about 20 percent better sound absorption potential than pure polypropylene composites at high frequencies. According to the study, “the quality of good sound absorption by polypropylene/hemp composite(s) is likely due to the anatomic structure of the hemp.”

Carbon Impacts of building with hempcrete

According to the United Nations Environment Programme, the building sector contributes up to 30 percent of global annual green house gas emissions and consumes up to 40 percent of all energy.

“Given the massive growth… if nothing is done, greenhouse gas emissions from buildings will more than double in the next 20 years ,” the organization says.

1 Tonne of CO2 represented in Paris during the historic climate negotiations.

Industrial hemp provides both reductions in greenhouse gas contributions from the building sector during construction and lowers energy consumption in the use phase. Unlike most carbon-intensive building materials used today, industrial hemp absorbs carbon dioxide as it grows. In a life cycle assessment (LCA) analyzing the environmental impact of hempcrete, the practitioner found that hemp sequesters approximately 1.7 kilograms CO₂ per kilogram hemp. The practitioner studied the growth of Canadian hemp on a worst-case scenario basis, including land tillage, irrigation and the use of chemical fertilizers and herbicides, including glyphosate.

The processed hemp hurd was utilized within a 120-square-foot hempcrete wall. The inputs in the wall included a timber frame, lime mortar, gypsum stucco, nails and screws, and hempcrete mix at a density of 330 kg/m³. The hempcrete mix is based off of hemp builder Steve Allin’s blend consisting of hydraulic lime, hydrated lime, Portland cement, hemp hurd and water.

Allin says the hydraulic lime adds a “breathing agent to the mix, but also something that holds the water to act as a wetting agent.” No other on-site assembly, wall footings, caulking or infrastructure were included.

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The LCA study found the global warming potential (GWP) in the production, processing and transportation for the materials in the hempcrete wall. GWP is an increase in earth’s average temperatures caused by increases in anthropogenic greenhouse gases emissions. In the LCA study, the materials used in the hempcrete wall emitted 1760-kilograms CO₂, although this does not include the carbon sequestration potential of the materials.

Not only does the hemp store CO₂ as it grows, but over time the materials in the hempcrete wall undergo calcination, absorbing CO_2. Considering the carbon sequestration potential of hemp and other material inputs, the rate of absorption within the 120-square-foot wall is 1090 kg CO₂ sequestered per wall.

Sean Murphy and Steve Allin’s Hempcrete Home, partially under construction.

This reduces the climate change potential to 670 kg CO₂-eq. More important, this does not account for the use phase, which in the long run contributes to the largest emissions related to climate change potential.

In addition to carbon savings of the materials used in a hemp building, energy costs are reduced during the use phase, which in turn reduces carbon emissions. Currently, hempcrete buildings are the most energy efficient due to low thermal conductivity, high thermal capacity and high insulation values (12” wall is ~R26). The monolithic structure of hempcrete creates an airtight mass with minimal heat loss.

In a study presented at the 2015 International Hemp Building Symposium, HEMPSEC found that “HempCell (hemp and lime built structure) had the lowest heat loss coefficient at 9.62 W/˚C when compared to wood fibre, polyisocyanurate, mineral wool and brick and block.”

In addition, it is unlikely that a hempcrete home would drastically lose its insulation value over time because unlike fiberglass insulation, as moisture appears, the hempcrete wall acts a permeable barrier. This makes hempcrete insulation stay energy efficient over the course of its life.

The Home Energy Rating System (HERS) rates homes based on energy efficiency. According to HERS, a standard new home normally scores 100, whereas a typical resale home scores 130. This means that a standard new home is 30 percent more energy efficient than a resale home.

North Carolina hempcrete home created by Alembic Studios.

According to Callahan at Alembic Studios, LLC, a hempcrete home built in North Carolina received a HERS score of 17, meaning the newly constructed hempcrete home was 83 percent to 113 percent more energy efficient than traditional homes.

If this energy efficiency transferred directly to energy costs, then each month a hempcrete home would cost 83 percent to 113 percent less in heating and cooling costs. So while a hempcrete home may initially cost 7 percent to 10 percent more to construct than a traditional home with timber and siding, the homeowner sees large returns in energy savings over the years (see Table 1).

Given the energy savings in Table 1, if a homeowner chose a hempcrete home over traditional materials he or she would see annual energy savings of $1,243.

These energy savings not only save dollars but also translate into carbon savings, as can be realized in Table 2.

The hempcrete home saves an annual of 7,792 kg CO₂-eq emissions. With carbon markets unfolding, these carbon savings could eventually turn into cash. So builders, contractors and homeowners could also have the potential to sell their carbon credits on the open market. Over the past 10 years, U.S. carbon markets have fluctuated between $3 and $10 per short ton. With savings of 7,792 kg CO₂ per year in carbon savings, the homeowner could realize an additional $25.50 to $85.80 per year in carbon credit income just by living in a hempcrete home.

With nations examining methods to cut carbon emissions in order to help keep temperatures below 2˚C, industrial hemp building materials need consideration.

In addition, the ease in building with hemp makes it viable in developing nations and fosters the United Nations’ Sustainable Development Goals. In most regions, nations can grow hemp locally, using the grain as a highly nutritious health food and the fiber for sustainably built homes.

These nations will have the largest impact as they develop over the next 50 years, but building with hemp can curb those emissions.

Author : Annie Rouse

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