Hempcrete, or concrete made of hemp in architectural engineering

Hempcrete (hemp + concrete) is an ecological building material – not so much innovative and new, but today needed more than ever before. In the 21st century, every industry is looking (or at least should) for solutions that are as environmentally friendly as possible. Due to the low ecological footprint compared to other materials, hempcrete is becoming more and more popular among proponents of natural, more sustainable engineering. What is hempcrete exactly and why is it so popular?

What is hempcrete?

The colloquial name hempcrete, or hemp concrete, mainly refers to the properties of the concrete itself – the two substances, however, differ in composition and production process. Hempcrete is basically a very simple material that is created by mixing hemp shives (i.e. cut, woody parts of the plant) with a lime-based binder. The high silica content in hemp allows it to bind effectively to lime. Still, in some cases, such a mixture also contains sand, pozzolans or clay [1].

hemp shives

After about 2-3 months, when the material dries properly, it can be used in construction [1]. Hempcrete was originally designed as a material for repairing damaged buildings made of old straw. However, it quickly gained popularity (mainly, but not only) as an efficient and durable insulation for walls, floors and roofs. What speaks in favor of hempcrete?

Hempcrete has many advantages…

There are many advantages of using hemp in construction. They can be seen at every stage of the building’s life cycle – from growing industrial hemp to the possible demolition of a hemp structure.

Hemp is a (quickly) renewable raw material

Industrial hemp (Cannabis sativa L.) is very productive. With a relatively low environmental costs (they do not require intensive use of pesticides and fertilizers), we can get high yields every year. For example, in Great Britain, around 5,500 kg of hemp material is harvested from 1 ha of field per year [1]. About 70% of this value, i.e. 3,850 kg, is shives. To insulate a small building with a floor area of ​​about 40 m2 and walls of about 90m2, one needs about 8000 kg, i.e. annual harvest from 2 ha of hemp crops [1].

hemp cultivation

Moreover, hemp shives are not a cultivation goal in itself. They are usually waste generated during the acquisition of hemp fiber. Here, according to the idea of ​​upcycling, they cease to be waste, and become a valuable component of high-quality building material.

Hempcrete is strong and light

The big advantage of hemp concrete is its strength. It is a material of relatively low density, therefore it is resistant to cracking. In contrast to concrete, during movement, instead of large cracks, microcracks create, which, in contact with moisture, stick together again [2]. Such property of hemp-limestone building material is a huge advantage in places where earthquakes occur.

At the same time, hempcrete weighs about ⅛ of what “traditional” concrete weighs [4]. Thanks to this, its application is easier, the foundations can be shallower, and the transport of the material itself is less emissive.

Thermal and acoustic insulation

As it turns out, hempcrete is a great thermal insulator. Less heat flows through it than through other insulators, e.g. wool or cotton. In winter, the temperature of the outer walls of hemp houses can be around 5C lower than in the houses (heated the same way) with standard insulation [1]. This means that less heat leaves the interior of the building. Indeed, with comparable heating intensity, houses insulated with hemp-lime material maintain an average temperature higher by 2C. Furthermore, hempcrete can behave like a heat sink that absorbs or emits it, depending on the ambient temperature [1]. The use of hempcrete can therefore significantly increase the energy efficiency of a building.

hempcrete

In addition to thermal insulation, this material also provides good sound insulation. Although there are materials that work better in this respect, hempcrete attenuates sound waves at a satisfactory level [1].

Walls that breathe

The big advantage of such a material is its water vapor permeability. Hempcrete is porous, so it can drain excess moisture, accumulating it in hemp cellulose [5]. A similar mechanism works here, as in the case of thermal energy – if the humidity in the room decreases, the fibers “give it back”. This property improves air quality, and thus also indirectly the well-being of the household members.

Although hemp fiber is able to accumulate moisture, due to the presence of lime with a naturally high pH, ​​hempcrete hinders the growth of bacteria and fungi [1], [5]. Mold in this case is rather a rarity, which appears only if the drying process was carried out incorrectly. In addition, lime is non-flammable, which increases the building’s security.

Eco-demolition

Therefore, both hemp extraction and the use of a house built with them is ecological (and economical). And what if you demolish such a house? For the environment – absolutely nothing (well, apart from the exhaust from the machinery used for demolition). Hempcrete is completely recyclable. What is more, it can be composted or even left as debris. None of its components are toxic, and each of them is biodegradable [4]. Such harmless “waste” can even be used as a fertilizer [6].

… and some disadvantages

Unfortunately, no material is perfect. Hempcrete has several disadvantages (although much less than advantages) as well. However, they come down not to the hemp itself, but to the second key component of the material, i.e. lime.

Atmospheric conditions

The first thing that works against the whole material is the length of curing. The whole process takes up to three months. During this time it is exposed to adverse weather conditions, which may delay the process or even damage the structure [1]. As we all know, this is quite a random factor, so you need to be careful in planning the construction date. This is especially advised against during the winter months and during frosts – lime is particularly sensitive to low temperatures.

Limestone mining

Limestone, i.e. sedimentary rock from which lime is obtained for the construction industry, obviously requires extraction. As you can easily guess, it is invasive to the environment. Limestone is mined on a very large scale, which is associated with intensive land transformation, and thus also a loss of biodiversity in a large area [1].

limestone mine

Heavy machinery (driven by fossil fuels) in constant motion is also a considerable noise pollution. In addition, limestone is processed in a place distant from the mine, which is associated with transport over long distances and further CO2 emissions.

Carbon footprint of hempcrete

Of course, no material is completely emission-free if we consider all its production processes. Hempcrete is no exception. It is worth noting, however, that hemp absorbs significant amounts of CO2 during growth, which accumulates in their tissues in the form of carbon and so “trapped” in concrete does not return to circulation. Each ton of hemp absorbs about 1.8 tonnes of carbon dioxide (500kg of coal) [1]. Other sources say that the hectare of growing plants equals, on average, 2.5 tons of CO2 absorbed [6]. The balance of greenhouse gas emissions from hemp crops depends on many factors and should also take other substances released into the environment into account. As a rule, however, hempcrete carbon footprint is largely neutralized by the sequestration of carbon dioxide by the plants themselves, as well as by the carbonization of lime over time [1].

The entire archtectural engineering industry is responsible for around 10% of anthropogenic carbon dioxide emissions today. Such statistics do not please anyone. No wonder then that we are looking for more “green” materials. Even if there is no perfect solution, it is worth looking for the best – hemprecte seems to be one of them.

Sources

  • [1] Rhydwen R., 2020: Building with Hemp and Lime [received 08.06.2020].
  • [2] Hempcrete Direct: The Incredible Benefits of Hempcrete [received 08.06.2020].
    [4] Bedlivá H., Isaacs N., 2014: Hempcrete – An Environmentally Friendly Material?. Advanced Materials Research, 1041, 83-86.
  • [5] Hempcrete Direct: Hempcrete Insulation [received 08.06.2020].
  • [6] Gołębiewski M., 2017: Hemp-Lime Composites in Architectural Design. KNUV, 4, 54, 162-171.
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