Sustainable Construction

Factors for sustainable construction

• Using building materials that have a low impact on the environment.
• Incorporating materials in construction that create less CO2 emissions in their production and application.
• To lessen the energy needed in heating and cooling the building i.e. energy efficient buildings with a high thermal performance.
• The use of non-toxic materials in a building, so there is no adverse effect on the building’s occupants.

What is Hempcrete

Hempcrete is a hemp shiv and lime mixture used for construction and insulation.  Hemp shiv is the inner woody core of the stem.

The hemp core or shiv has a high silica content which allows it to bind well with lime.  This is a unique property that hemp has compared to other natural fibres.  The shiv is mixed with water and a lime binder to form a concrete like material.

The result is a lightweight cementitious insulating material about a seventh or an eight of the weight of concrete. It is not load bearing, so loads would usually be carried by timber stud framing.

Benefits of Hempcrete

Hemp is carbon sequestration crop.

Hempcrete is a natural strong and lightweight material with five incredible properties:

• Excellent insulation and thermal mass
• fireproof
• water resistant
• rot proof
• Hygroscopic, hempcrete naturally regulates humidity and temperature in a building. This ability of hempcrete to passively regulate indoor air quality reduces the need for mechanical ventilation systems, further reducing the energy required to maintain a hempcrete building. The breathability of hempcrete walls makes it great for people with Bronchial problems.

Not only is the high carbon content found in hemp locked into the walls of buildings. The carbon has been absorbed by the plant whilst growing Hempcrete also continues to absorb carbon for decades after the building is finished as the lime carbonates and the walls gradually turn into limestone.

The concrete industry produces 5% of all greenhouse gases in the world. Whereas hemp absorbs carbon from the atmosphere. According to Dr Mike lawrence who is the Director of Bath’s Building Research Park: “Making one tonne of steel emits 1.46 tonnes of CO2 and 198 kg of CO2 is emitted to make one tonne of reinforced concrete. One square metre of timber framed, hemp-lime wall (weighing 120kg), after allowing for the energy cost of transporting and assembling the materials actually stores 35.5kg of CO2.”- See Guardian Article: Hemp wood fibre construction climate change. This figure goes even higher in homes built using a cast-in-situ design.

Traditionally houses were painted with a lime wash which allowed the walls to breathe. Poor air quality is now a major issue in homes due to a variety of reasons including, all of which are covered in Tom Woolley’s book “Building Materials, Health and Indoor Air Quality”. It could be claimed that hemp buildings are the next evolutionary step as they combine the need for moisture vapour to travel through walls with greater insulation properties.

Hemp insulation can be applied both internally and externally. A hemp sprayer speeds up the process of applying the hemp. Here is a video showing the process. Hemp-Lime Spray are a UK company who are experts in this field and could be a good resource for information about how to use this equipment.

Our housing stock needs to be upgraded if we are to reach our emissions targets. Hemp could be grown and processed locally and used to insulate homes both reducing energy use and trapping CO2 in the walls.

The Rediscovery project in Ballymun used hempcrete in their restoration of the walls in the old Boiler House there and it has been a great success. The public are free to walk around the building and observe the hempcrete insulation in action. Watch video of the Rediscover Center Project.

Making prefabricated hempcrete blocks can be very useful, in both new build and retrofit as this can speed up construction as the blocks are already dry.

The French have been building with hemp for decades and are leading the way in hemp construction. This video shows the process of building a large prefabricated building in France from the harvesting of the crop right through to the finished building. See Video

The ideal way to build with hemp is as a monolithic cast-in-situ walling system, around a timber frame, which minimizes the chance of thermal bridging by forming a continuous sheet of insulation material around the building. To overcome this, buildings could be designed to be erected with spacing left for the hempcrete to be shuttered in after they are erected. The hempcrete could then be mixed on site and poured into the shuttering thus overcoming any potential thermal bridging issues.

Hempcrete walls are easier to construct because, unlike contemporary buildings, they don’t require the several layers. You just need a 300mm wall with an inner and outer breathable plaster.

It’s essential that the plaster and paint finishes are breathable or else moisture vapour is not able to travel through the walls. Hempcrete buildings are great if constructed properly but problems can arise if materials are not mixed correctly and applied in the right way. For this reason, it’s essential that anyone who is involved in hemp construction is properly trained, to prevent bad building practices.

Another major advantage of using hempcrete compared to other building materials is that it can be reused or composted when a building is knocked down. It also has good acoustic properties which vary depending on the density of the mix.