Hemp for building insulation

It is illegal to grow hemp without a license as it is the plant source of cannabis, a class B drug. However the strains used for construction material do not contain significant amounts of THC. Historically hemp fibre has been used to make rope, sailcloth, parachute webbing and heavy duty clothing but recently it has been used as a natural insulation material and as a reinforcing fibre for concrete and plastic. It is even used in making body parts for cars (famously introduced by Henry Ford [1])

Other fibre crops can also be used such as flax and bamboo. However hemp is stronger than either [2].

Hempcrete can be made and used DIY with a little effort.

A few years ago I watched a demonstration of building walls with hempcrete which is made from hemp fibre (or shiv) and lime. The result has excellent insulation properties but is not as strong as traditional masonry and the lime is nasty to work with – it can burn your skin so you must use thick gloves and goggles to protect your eyes from splashing. Also the mixture takes a long time to dry – one week per inch of thickness is the rule of thumb. The demonstration used volunteers to do the work, proving that hempcrete can be used DIY with a little effort. However since hempcrete is much less dense than conventional concrete (it does not contain stone aggregate) the work is light in comparison. Here is a video of how to make hempcrete

Or use ready-made Hemp-products

Alternatively you can buy ready-made hempcrete blocks or you can use hemp ‘wool’ instead of fibreglass insulation. Or you can cast your hempcrete like concrete, by building shuttering to make a mould and filling this with a mix of hemp fibre and lime (mixed with a shovel or mechanical mixer). You can also make mixtures with hemp for 3D printing [3].

Hempcrete is not load-bearing.

Hempcrete is not strong enough for load bearing (unlike masonry) so you can only use it in buildings with a structural frame, typically wood or steel.

Hempcrete is excellent as:

• Thermal insulation Thermal conductivity is similar to mineral wool
• Non-flammable (unlike plastic foam insulation)
• Thermal mass (due to the lime binder rather than the fibre) 
• helps to regulate room temperature during heatwaves and cold spells
• Sound insulation (because it has pores of a range of sizes forming barriers to sound at different wavelengths).
• Moisture management – protecting against mould and improving indoor air quality
• Hempcrete is vapour permeable and absorbs water helping to regulate humidity and condensation and hence reduces mould growth
• Hempcrete is durable. It is resistant to:
• Spalling from freeze-thaw cycles (because it is sufficiently flexible to accommodate low levels of thermal stress.)
• Mould growth
• Potential lifetime is hundreds of years.

Carbon negative

Hempcrete can be considered as carbon negative because it stores carbon absorbed from the atmosphere while the plant is growing. However the storage is not strictly permanent as ultimately it will degrade.

Can be used on uneven  surfaces

Hempcrete can be sprayed onto uneven surfaces whereas rigid insulation  boards such as Celotex cannot. This means you can insulate uneven walls.

Can be used DIY

Hempcrete can be applied DIY. Hemp fibre can also be used in concrete either to replace a proportion of aggregate or to replace steel reinforcement [4].

Internal or external walls

It can be used for internal or external walls. For external walls it needs some kind of render for weather protection – typically lime because this is vapour permeable. Internal walls will also need some kind of vapour permeable plaster finish such as lime plaster. Either way you need a frame to carry the building loads.

Advantages of hempcrete over conventional insulation

Hempcrete can be used as a carbon-negative alternative to conventional insulation such as fibreglass matting or PIR insulation (e.g. celotex) - assuming the GHG emissions from cultivation do not exceed the CO2 absorbed by plant growth. Also those materials do not regulate humidity while hempcrete does – improving indoor air quality.

Hemp is organic whereas most conventional insulation is derived from oil.  It has an advantage over other organic alternatives such as sheep’s wool because it does not need an insecticide: the lime binder works well as a repellent.

References

[1] Hemp In the Automotive Industry (Cannabis museum, Amsterdam)

[2] (Paulo Peças, Hugo Carvalho, Hafiz Salman and Marco Leite, 2018) Natural Fibre Composites and Their Applications: A Review (Journal of Composites Science)

[3] (Thanh Ha Ngo, Shuai Li, Tien Huynh, Y.X. Zhang, Phuong Tran)3D printable hemp concrete: Rheological, mechanical, and microstructural properties (Journal of Building Engineering)

[4] (Chun Hua Julia Liu, Bernardino D'Amico, Francesco Pomponi 2026) Chapter: Advances in the use of hemp fibre in the construction industry In book: ‘Achieving sustainable cultivation of hemp’  Publisher: Burleigh Dodds Science Publishing, London

Post settings Labels No matching suggestions Published on Permalink Location Options Post: Edit