37% of UK greenhouse gas emissions are from the use of buildings, so new buildings must be efficient. Construction causes GHG emissions too, so building materials need to have low ‘embodied’ emissions and reduce emissions in use.
Hemp locks up carbon as it grows, offsetting the CO2 emissions from lime production, so Hembuild and Hemclad are carbon-neutral building products. If the buildings had been built using normal methods, 2,300 tonnes of CO2 would have been emitted during construction.
With fantastic thermal properties and ability to stabilise humidity, hemp mixed with lime to create hempcrete has been used in sustainable building construction for many years. But until now it has always been cast on-site, giving it limited commercial appeal.
‘Hembuild’ panels are made-to-measure off-site, then arrive at building sites ready to be slotted together like a jigsaw puzzle to form the building’s walls. Aside from making the use of hempcrete economically viable for builders, building users can revel in their superior eco-credentials, a comfortable environment – and lower bills.
The M&S building is revolutionary, and the performance gains from using hemp are phenomenal.
Ed Dixon, Sustainability Manager, Mace Group.
Hembuild and Hemclad have been used in a range of projects to date, including a flagship Marks and Spencer store in Cheshire, which was built to the highest environmental standards, and an archive for the Science Museum, which required carefully controlled temperature and humidity to protect its contents from decay. Over 50 projects have been completed so far, including commercial and domestic properties.
Now that building regulations have substantially cut the ‘in use’ GHG emissions from new buildings, the emissions embodied during construction become more important. Assuming a typical embodied CO2 content of 155 kg per square meter of wall area built using traditional methods, the projects complete or underway using Hembuild and Hemclad have avoided about 2,300 tonnes of CO2 during the construction phase.
As part of a design to minimise a building’s energy use, Hembuild and Hemclad can contribute to significant CO2 savings – the M&S store built using Hemclad showed energy use reductions of 40% compared to a standard store. The extra savings due to using Hembuild or Hemclad instead of a traditional insulation product with a similar U-value are up to 17%. Government calculations indicate that including Hemcrete in the external envelope of a house can cut space heating requirements by 50%, with more dramatic energy savings for schools and commercial buildings that may also need cooling, as Hemcrete tackles the overheating problem that can result from lightweight construction methods.
How is it manufactured and installed?
Hemcrete is a mixture of lime, water and the woody core of the hemp plant known as ‘shiv’, which contains many fine capillaries that can store water. Once these ingredients are mixed, along with small amounts of other additives, they form a wet composite which can be moulded and shaped in a similar way to concrete. However, Hemcrete does not have the compressive strength of concrete, partly because it is 85% less dense, so must be used within a structural frame. More importantly though, the carbon locked up in the hemp offsets the CO2 released during lime manufacture, resulting in a carbon neutral product, while concrete or bricks are very carbon-intensive.
How does it work?
Hembuild and Hemclad provide four key services to a building: insulation, thermal inertia, breathability and humidity control.
- Hemcrete itself has insulating properties, but it is the layer of hemp-based Breathe insulation which provides most of the benefit here – a completed wall with 120mm Hemcrete and 180mm Breathe insulation has a U-value of 0.14 W/m2K, similar to 300mm of mineral wool insulation.
- The hemcrete layer provides ‘thermal inertia’ – it takes time to cool down and warm up, absorbing and releasing substantial amounts of heat as it does so. This is in part
- due to the mass of the Hemcrete, but is also due to the capillaries within the hemp shiv, which can absorb and release moisture in a process that also stores and releases heat.
- Breathability performance is maintained because none of the materials used in the construction act as a complete barrier to moisture or air – they won’t let a draught through, but water vapour can migrate through them.
- The breathability combined with the moisture-holding capacity of the hemp shiv allows Hemcrete to regulate humidity as well as temperature, by releasing or absorbing water vapour from the air in the building.
The net result is that Hembuild and Hemcrete deliver a thermal performance in excess of what would be expected from their U-values. As well as cutting heat transfer they also reduce daily and seasonal fluctuations in temperature and humidity.
How much does it cost?
Hembuild and Hemclad prices vary according to the scale and specification of a project, but tend to be slightly higher than competing products with the same U-value. However, these other products usually have low thermal inertia, and are unable to stabilise temperature and humidity in the way that Hembuild and Hemclad can. For a commercial building, the extra cost to the developer of using Hembuild or Hemclad is offset by the savings from being able to use lower capacity HVAC (Heating Ventilation and Air Conditioning) systems, leaving them with a building that can be marketed as cheap to run and ‘green’ at no extra cost.
The development of Hembuild and Hemclad has brought Hemcrete into the mainstream construction sector for the first time, and there is significant scope for Hemcrete Projects to expand sales. It is currently working on the construction of schools in Ealing (London), Milton Keynes and Grimsby, a second artefacts storage facility in Scotland, 15 domestic dwellings and has a pipeline of varied commercial and domestic projects to work on in the coming years.