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Your Complete Guide To Natural Building Materials

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Photo by Rammed Earth Australia
natural-bldg-mat
Photo by Jess Ahlemeier
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Photo by Viva Living Homes
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Photo by Robyn Rosenfeldt

Clockwise from above left: Rammed earth walls; Mudbrick interior; Strawbale interior; Light Earth wall; Cob house.

natural-bldg-mat
Photo by Viva Living Homes

Whether you’re thinking of becoming an owner-builder or retrofitting your home, you might be wondering which building materials will ensure an effective, beautiful and natural home. Some important factors to consider are: which resources are available to you locally (both on your property and in your area); cost of materials; thermal properties sought – passive solar design, thermal mass and insulation – and how these interact with each other; embodied energy involved; and the ease of material construction. With an introduction to these factors, you will be better equipped to begin choosing the materials that best suit your climate and house design.

LOCALLY SOURCED MATERIALS

Research your local resources – keep an eye out for timber mills (for free/cheap offcuts), dense forest, high-clay soil, quarries for stone, salvage yards, building-grade strawbale manufacturers, and any excess subsoil from building sites. Practise sustainable building methods.

If you’re planning to use earthen materials, test the clay content in your own subsoil first. If you have a clay content of around twenty-five per cent, techniques such as cob, light earth (straw clay), rammed earth and mudbrick would be perfect options in your design.

Another great local resource is trees. Many native Australian hardwoods are suitable for building; ironbark, tallowwood, Victorian ash, blue gum and spotted gum are some. Once you’ve identified the species and its properties (such as strength and durability), the timber can be used for post and beam construction, or milled for weatherboards, cladding, floor joists and furniture.

COST

For many people, cost is the deciding factor in the choice of building materials. Some people assume that natural building is cheap, but this isn’t necessarily true. The major costs in building any home are usually labour and fixtures, such as kitchen, bathroom and internal fit-outs. For example, a straw bale house built by a natural builder can cost anywhere between $1800–2000 m2, with the bales only accounting for fifteen to twenty per cent of the cost. However, you can reduce costs by downsizing, sourcing your materials locally or from your property, using recycled materials, and inviting the community or volunteers to take part in building workshops at your place.

THERMAL PROPERTIES

When assessing thermal properties, passive solar design, thermal mass and insulation are major considerations. In an Australian climate, holding and storing heat and cold effectively, by using insulating materials on the external walls and thermal mass internally, can create an ideal environment for self-regulated temperature control.

Passive solar design is one way to harness the sun’s energy for the heating and cooling of living spaces. This design practice aims to create a self-regulated temperature within the home, instead of installing non-renewable systems such as gas heating and air conditioning. This involves consideration of the orientation and internal layout of the house and, most importantly, the choice of building materials and their strategic placement. By constructing a passive solar design you are investing in lower energy bills and using the naturally occurring thermal properties of your materials. With an understanding of passive solar design you can begin to choose building materials.

Thermal mass stores, and releases or absorbs, heat. When a mass stores heat from a source (such as a fire or the sun), it slowly releases that energy back into the room once the ambient temperature drops below the mass temperature – sometimes delaying the heat flow through the home by up to twelve hours (lag). Materials that have a high thermal mass include brick, stone, cob, mud bricks, concrete and water. You can use these materials for internal structure and to build partition walls, which work well in winter, especially when positioned near a fire.

Insulation stops or slows down the transfer of heat and cold. As a general rule, materials that are fibrous and contain air pockets are good insulators. In Australia, building materials are given an R-value rating, based on the thermal conductivity or resistance of the material. However, efficient design is about more than R-value, for example material used in the walls may have a high R-value but the walls are just one element of the design – only fifteen to twenty-five per cent of heat is lost through walls. A strawbale wall has an incredible R-value of R10.0. Other good insulators (with an R-value around R3.0) include sheep’s wool (expensive at around $25 m2) and hempcrete.

EMBODIED ENERGY

Embodied energy is the total energy consumed by the processes of extracting, processing, manufacturing and delivering building materials to your site. In natural building the embodied energy and carbon footprint (i.e. related carbon dioxide emissions) of materials will depend on your location and what is locally available to you.

The ideal building material for low embodied energy would involve little or no processing of the raw material, and all the energy inputs would be ‘borrowed’ from the earth. Cement, the key ingredient in concrete, is extremely high in embodied energy; however, it’s now one of the most common materials used in conventional building. The main use of cement in buildings is in the foundations and floor. While it can be difficult to avoid using cement when trying to adhere to permits and engineering specifications, there are ways to reduce the use of cement, for example by using: urbanite (reused) concrete; added natural materials such as crushed limestone (e.g. LimeCrete); rubble trenches; concrete strip footings with an internal earthen floor; or by constructing timber posts and frame on top of brick piers.

EASE OF CONSTRUCTION

The diverse range of natural building materials makes for an equally diverse range of construction methods. The key considerations to balance are: your ability to use unskilled labour; speed of construction; and weight of the materials. For example, you could employ a carpenter to erect the structural frame, which means your chosen infill for the walls doesn’t need to be structural, allowing for additional unskilled labour (such as friends or family) to assist in the building process. Strawbale wall installation would be a quick process with unskilled labour, while mudbricks – which are heavy for their size – will take longer to install.

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Photo by Keren Dobia
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Photo by Hempcrete Australia
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Photo by Jess Ahlemeier

Clockwise from below: Mudbrick home; Hempcrete; Timber framing by Evergeen Homes; Cob walls.

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Photo by Robyn Rosenfeldt

MATERIALS [see further details in the table]

Cob

Cob is made from subsoil (sand and clay), water and a fibrous organic material, typically straw. For a strong cob mixture you need around fifteen to twenty-five per cent clay, seventy-five to eighty-five per cent sand and one part straw. Always test samples and different ratios for strength before building, as natural materials are different from place to place. If you can’t use subsoil from your property, try calling local landscapers and building sites or look on community websites for free subsoil. Cob is also extremely fire-resistant.

Mudbrick

Mudbricks are made by mixing subsoil and water, and sculpting the mix into brick-shaped forms which are left to dry naturally. Sometimes straw and other fibres are added to reduce the shrinkage in the drying process. Mud bricks are extremely fire-resistant.

Strawbale

Oat, rye, wheat and rice straw are commonly used materials for making strawbales, but not hay (which can germinate and break down in the walls). Use previously compressed bales (building grade) rather than regular strawbales.

Light earth

Light earth (also known as light straw clay or insulating straw clay) is not structural but used as infill, and is best for retrofitting walls or cavities due to its great balance of insulating and thermal mass properties. It is made from loose straw, lightly coated in a clay slip/slurry, which is tamped into formwork between a post and beam structure. Once dry (two to four months), the walls are rendered with an earth or lime render.

Hempcrete

Hempcrete is made by combining water, hemp fibre and a lime-based binder. It is both fire- and pest-resistant. Similar to light earth, it’s an infill material: you use formwork to pack the material into sections, usually a load bearing timber frame.

Rammed earth

Rammed earth is a mixture of gravel, clay, sand, cement and sometimes lime or waterproofing additives. It is most easily machine-compacted into removable formwork, resulting in a water-resistant, load-bearing, long-lasting wall.

Timber

Sustainable timber is a renewable material that absorbs carbon from the atmosphere while growing, and stores it for the life of the building. Australia has many native hardwoods that are great for building. Timber can be used sawn or left in the round.

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HOW TO GET STARTED

It can be easy to set your heart on a specific style, so it’s important to remember that natural building techniques can be adapted to different shapes and design parameters and, once rendered with lime and/or earth, can maintain a very similar aesthetic. The important first step is to begin to look at your own needs in a home. How do you spend time in a space? How can your home benefit your needs and lifestyle?

Research which materials are readily available in your area, from the natural world and recycled, as well as via websites such as Gumtree and Freecycle. Stay in tune with what your budget allows, and think outside the box when considering fixtures, fittings and labour alternatives.

Most importantly, enjoy this empowering building process as much as the goal. You can build a solid, beautiful and breathable home with earthen, carbon-neutral materials. It’s about realising your needs, observing your resources, and acknowledging local skills and community.

For further information and details of relevantcourses go to:

  • Your Home: Australia’s guide to environmentally sustainable homes at www.yourhome.gov.au
  • The Australian Timber Database at www.timber.net.au
  • Viva Living Homes at www.vivahomes.com.au
  • Huff ‘n’ Puff Strawbale Constructions at www.glassford.com.au
  • Agari Permaculture Farm at www.agarifarm.org
  • Earth Building Solutions at www.earthbuildingsolutions.com.au
  • Milkwood at www.milkwood.net
  • Rammed Earth Australia at www.rammedearthaustralia.com.au
  • Evergreen Homes at www.evergreenhomes.com.au
  • Hempcrete Australia at www.hempcrete.com.au

Adam Hickman owns and runs Evergreen Homes and has been teaching natural building courses all over Australia. He has two upcoming courses: a three week strawbale round house with reciprocal roof in Adelaide and a one week home in Mornington Peninsula.

For more info visit agarifarm.org or contact adam@agarifarm.org

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