Hemp Concrete – Why Do I Like It?

When writing this blog, I drew on publicly available information. I am neither a physicist nor a chemist, but an ordinary person with common sense J

The goal of environmentally sound construction is to build structures that naturally create a healthy indoor climate while causing no harm to the surrounding environment. The best way to achieve this goal is to combine modern scientific knowledge and use natural materials in construction that leave behind minimal or no environmental impact.

Natural building teaches us to use natural (and above all, raw) and locally sourced materials that possess low „embodied“ energy (by embodied energy, I mean the energy required for their production or extraction). Thanks to this, we can attribute to them an extremely low carbon footprint as well as a minimal level of environmental pollution.

Lately, it has become very trendy to build „eco-houses“, „eco-buildings“, and „enviro-whatever“, but when we take a closer look at what such an „eco-house“ actually is, we discover that it is just a basic timber-frame structure sheeted with OSB boards, lined with a vapor barrier, backed by 30 cm of hyper-modern turbo insulation, and boarded up on the outside with carcinogenic polystyrene. And I haven't even asked how much CO2 is released during the manufacturing of such a house, or whether the OSB boards happen to contain formaldehyde…. I honestly don't know, or rather, I wouldn't dare to answer. Quite simply, the perfect „eco-building“.

This entire kind of „ecological“ building seems dubious to me, and I honestly don't know whether to laugh or cry over such blatant parasitism on the word ecology.

When building a home, we must not forget that we will be living in it for our entire lives! There is no reason to rush the choice of materials and we must truly understand that the „U“ and „lambda“ values are not everything. Other factors also need to be taken into account—factors that people somehow forget to mention when talking about today's modern „eco-houses“. What heat capacity does polystyrene have, for instance? And to what extent can a structure covered in OSB boards and wrapped in a vapor barrier be vapor-permeable? These are just a few of the crucial physical properties that ultimately determine how living in the house will feel and what kind of living comfort it will provide.

When it comes to natural building, one might feel that clay mixtures do not offer such high thermal insulation properties, while straw subconsciously evokes a sense of something impermanent that mice will chew through and water will rot. The reality is that you can build a genuinely cozy and comfortable home out of clay and straw, but…

Hempcrete

Hemp concrete, as its name suggests, is a mixture of industrial hemp and a lime-based binder. When we talk about hemp, please keep in mind that industrial hemp and marijuana are two fundamentally different things, and comparing them is akin to comparing the traits of a Miniature Pinscher to a Pit Bull…

Hemp concrete is a vapor-permeable material that is also hygroscopic. When there is excess ambient humidity, it absorbs it, and conversely, when the air is dry, it releases moisture back. Much like clay, it can naturally balance the humidity levels inside your home, fostering a pleasant indoor climate. This eliminates the risk of mold growth, bacterial infections, allergic reactions, and asthma.

Regarding thermal insulation properties, a 45-centimeter exterior wall (the standard thickness across all „eco-houses“) achieves a heat transfer coefficient „U“ value of 0.16 W/m2.K, categorizing hemp concrete homes as low-energy builds. And lo and behold, all of this without any supplementary insulation…

Hemp concrete can also accumulate thermal energy and store up to twice as much heat as brick, while simultaneously being able to release it as rapidly as water. The pairing of these properties with low thermal conductivity means that this material insulates exceptionally well, does not allow heat to escape, and on top of that, radiates warmth back into the room during a chilly night after the fire in the fireplace has died down. Fantastic!

Beyond its remarkable physical characteristics, hemp concrete has one other property that no other material possesses: it has a negative carbon footprint. A building constructed with hemp concrete produces no excess carbon dioxide; on the contrary, it sequesters it from the atmosphere. Throughout its growth cycle, the plant captures and stores more CO2 than is generated by the manufacture and installation of hemp concrete. Multiple academic studies have been published on this subject, but it is best to consider the median value – 165 kg of pure CO2 captured in 1 m3 of hand-cast hemp concrete. The reason hemp absorbs such a massive quantity of carbon dioxide is that it grows exceptionally fast and dense, requiring a sturdy stalk where the majority of carbon is sequestered.

Having read the previous paragraph, you might wonder: how can hemp concrete be a „carbon-negative“ material when lime is added to it? During lime production, carbon dioxide is naturally released as it is calcined at 900°C. BUT! As my high school chemistry teacher used to say: „Chemistry is life“. We are talking here about a closed carbon cycle, the outcome of which is that your house walls continually consume carbon dioxide while turning into stone (limestone – CaCO3) (Figure 1).

In the past, I considered building a home using straw bales, but then I discovered hemp concrete. At present, hempcrete is my top choice, mainly because straw gives me (as it probably does to many people) a feeling of impermanence. I have encountered the view that a straw-bale house is more ecological because whenever it gets demolished, it will pose no environmental burden. But why would I demolish my home??? And is limestone, into which hemp concrete transforms, an environmental burden? Hardly.

Hemp house + eco wastewater treatment plant = thumbs up from planet Earth J