Hempcrete: What It Is, Its Real Benefits and Its Limits

Hempcrete is a lightweight building material made from the chopped woody core of the hemp plant mixed with a lime-based binder and water. It is used to fill and insulate walls, floors and roofs, not to hold a building up. Its strengths are real: good insulation, the ability to buffer humidity, resistance to fire and mould, and a low or even negative carbon footprint. Its limits are real too: it is weak in compression, slow to dry, and still unfamiliar to many builders and building inspectors. Here is what hempcrete can and cannot do.

Interest in the material jumped in the United States after news reports in the early 2010s about one of the country's first modern hemp houses, built in Asheville, North Carolina. The CNN report below shows the house and the claims made for it at the time.

What hempcrete is made of

The key ingredient is not hemp fibre but hemp "shiv" or "hurd": small chips from the woody inner stalk, left over after the long fibres are stripped for textiles and rope. These chips are mixed with a binder based on lime, usually with water and sometimes small amounts of cement or pozzolans to help it set faster.

The wet mix is packed or sprayed around a timber or steel frame, or cast into blocks and panels. Lime hardens slowly by absorbing carbon dioxide from the air, so walls typically need weeks to gain usable strength and longer to dry out fully. The finished material is very light, roughly 15 percent of the density of ordinary concrete, which is why the name "hemp concrete" can mislead: it behaves much more like a rigid insulation than like structural concrete.

The hemp house in Asheville, North Carolina. The hempcrete fills the walls around a frame that carries the structural load, and is protected by a lime render outside.

The benefits, checked

Insulation

Early reports said hempcrete might have a higher R-value than concrete. That is true but sets a very low bar, because dense concrete is a poor insulator. Published figures for hempcrete range from about R-1.7 to R-3.0 per inch, depending on the mix and density. That is less per inch than fibreglass batts or foam boards, so hempcrete walls are usually built much thicker than conventionally insulated walls. Their mass also slows the movement of heat through the wall, which helps keep indoor temperatures steady.

Breathability and humidity

Hempcrete is vapour-open: it absorbs moisture when indoor air is humid and releases it when the air dries. This buffering can make a home feel more comfortable and reduce condensation, provided the rest of the wall is designed to let moisture move rather than trapping it behind impermeable layers.

Fire, mould and pests

The lime binder is alkaline, which discourages mould and insects, and the mix chars rather than burning freely. One British supplier reports a one-hour fire-resistance rating for its wall system under UK and EU testing. Those results apply to specific tested assemblies, so builders should check data for the exact product they plan to use.

Carbon

Hemp absorbs carbon dioxide as it grows, and the lime keeps absorbing more as it cures over the years. Life-cycle studies estimate that a hempcrete wall can store more carbon than was emitted making it, with net figures ranging widely from slightly to strongly carbon-negative per square metre of wall. The largest source of emissions is producing the lime binder, followed by transport, so local supply chains make a big difference.

The limits builders should know

• It is not structural. With a compressive strength of only around 0.3 megapascals, hempcrete cannot carry the weight of floors and roofs. It needs a frame. Its low weight and resistance to cracking, however, make it attractive in earthquake-prone areas.
• Drying takes time. Cast-in-place walls can take weeks or months to dry, which affects building schedules and the timing of finishes. Too much water in the mix slows curing further.
• Moisture details matter. Walls need a barrier against rising damp from the ground and a protective lime render or cladding outside to keep rain from rotting the hemp chips.
• Cost and skills. Because few contractors have experience with it and materials are often imported, costs can be higher than for conventional walls, at least until local supply grows.
• Codes and permits. Building with hemp in the US generally requires a permit and an inspector willing to approve it. In 2022 the Hemp Building Foundation submitted a proposal to have hemp-lime recognised in the International Residential Code, a step toward treating it as a standard material.

The most useful way to judge any claim about hempcrete is to ask which complete wall, roof or floor assembly is being described and what it is being compared with. The ingredient alone does not guarantee warmth, savings or durability.

Hemp and the law in the United States

When stories about the Asheville house first spread, the biggest obstacle was legal. Hemp and marijuana are the same species, Cannabis sativa, but hemp contains very little THC, the compound that produces a high. Even so, US federal law treated industrial hemp as a controlled substance, so American builders had to import their hemp from abroad.

That changed in stages. The 2014 Farm Bill allowed universities and state agriculture departments to grow hemp in pilot programmes. The 2018 Farm Bill then removed hemp from Schedule I of the Controlled Substances Act and classified it as an agricultural commodity. Under rules from the US Department of Agriculture, hemp means cannabis containing no more than 0.3 percent delta-9 THC by dry weight. Domestic hemp is now legal to grow under licence, although processing capacity for building-grade hurd is still developing.

Who has been building with hemp

Europe led the way. Hemp-lime construction has been used in France since at least the early 1990s, including for renovating old stone and lime buildings, and it spread to the UK, Belgium, Switzerland and beyond. In the UK, the Tradical Hemcrete system was marketed as a carbon-negative walling material. Canada, especially Ontario and Quebec, followed.

In the US, the North Carolina firm Hemp Technologies was among the early promoters. According to a 2012 HuffPost report, its co-founders David Madera and Greg Flavall toured European projects before holding their first American seminar in 2009, and the company had worked on hemp homes in several states, including an Asheville project known as NauHaus. Other businesses sold related products such as hempboard, a fibreboard made from hemp hurds for furniture, shelving and wall panels.

Smaller pioneers continue the work. One example is the Washington state project described in our article on Pamela Bosch's hemp-building project.

Is hempcrete right for your project?

Hempcrete makes most sense for people who value low embodied carbon, healthy indoor air and a comfortable, stable indoor climate, and who are willing to accept thicker walls, a longer build and a search for experienced trades. It suits timber-frame new builds and the sensitive repair of old masonry buildings particularly well.

It is less suitable where speed, minimum wall thickness or the lowest upfront cost are the priorities, or where no local builder or inspector has dealt with it before. Before committing, look for tested data on the specific product, talk to someone who has built with it in a similar climate, and speak to your local building department early. It will not change the way we build everything, but it is a genuinely promising option for building more sustainably.

Sources: Wikipedia: Hempcrete (composition, thermal data, carbon estimates, US code status); Wikipedia: Hemp (US legal history, 2014 and 2018 Farm Bills, USDA definition); original report by Huffington Post (2012); video report by CNN via YouTube.

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