Hempcrete – a natural building material

The hemp plant

Marijuana may be great for getting high, but the architectural value of cannabis is in the industrial hemp that forms a vital ingredient of products including hempcrete.

Hempcrete is low-tech natural building material made from the chopped stalk of the hemp plant and a lime-based binder. It is carbon-negative in that it absorbs carbon dioxide and can be recycled. The materials are sourced locally – hemp is grown in South-East England.

A few months ago, a new four-bedroom house in Midlothian designed by Halvorsen Architects was granted planning permission. An application for building warrant has been submitted for the hempcrete home. The client is an organic farmer who powers his milking machines with a large array of photovoltaic panels. He now wants a new house that is as sustainable as his farm management.

Having considered using materials sourced from his farm – including sheep’s wool, straw bales – and having visited several buildings around Scotland using natural building materials, we decided to build the new house in hempcrete. When weighing up the options we visited two hempcrete houses by Sue Manning on Loch Tay. One of these was finished a few years ago and the other is currently on-site, so we were able to see how the first had aged and witness the building techniques with the second. Sue is already designing her third hempcrete house and says she favours the material over all the other natural building materials she has used.

Hand-placed hempcrete wall with shuttering

One of the biggest advantages of hempcrete construction is its monolithic form – the material’s continuity around corners makes it airtight and reduces cold bridging at junctions such as eaves. Another is its simplicity and multi-performance. Hempcrete provides insulation and thermal mass as well as insect, fire and moisture control. Not only that, it is a natural material and therefore, combined with its Gore-Tex-like breathability, gives a healthier (chemical and damp free), more pleasant indoor environment.

Hempcrete can be cast in-situ, sprayed or built-up in brick form. We have opted for the first or standard method – the lowest-tech and most labour-intensive solution that gives the highest level of quality control. It is not load-bearing and so it requires an internal frame, in our case untreated timber.

Internal walls and floors will be insulated with hemp fibre and all the walls finished in lime plaster to maintain the integrity of the breathable and non-toxic fabric.

Finished hempcrete wall, shuttering removed

The principal rooms of the new house face South. We are currently deciding which renewable technology to heat the house with – the most likely options are a district wood-chip boiler using wood from the client’s farm or a ground source heat pump.

We look forward to starting work on what we will believe will be the first house in the Lothians to be built of this remarkable material.

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