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Building Materials: Natural Selection

building-materials
This striking rounded entrance is produced using straw bale wales. By Robyn Rosenfeldt

Whether you’re planning a new build or renovating your existing house, there are lots of natural materials to ensure an efficient, beautiful and healthy home.

When deciding which natural building material is best suited to your needs, there are things you need to consider. Local availability, the cost of materials compared to your budget and what thermal properties you need it to achieve. The ease of construction is something you also need to take into account, and some people may also be swayed by a particular product’s embodied energy, which is the total energy consumed by the processes of extracting, processing and delivering building materials to the site.

Know The Source

Research your local resources – much like the food you eat, your build will be far better for you, the planet and your community if it has been sourced locally. 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 25 percent, 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 Australian hardwoods are suitable for building; ironbark, tallowwood, Victorian ash, blue gum and spotted gum are some. Once you’ve identified the species and that its properties (such as strength and durability) meet your specific needs, the timber can be used for post and beam construction, or milled for weatherboards, cladding, floor joists and furniture.

Size Up The Price

For many, budgets have the final say when it comes to choosing building materials. Some people assume natural materials are inexpensive compared to more conventional products, but this isn’t necessarily true. For a lot of builds, the most expensive items are usually labour and fixtures.

For example, a straw bale house built by a natural builder can cost anywhere between $1800–2000 per square metre, with the bales themselves only accounting for 15–20 percent of the cost. Costs can be reduced by downsizing, sourcing your materials from your own property, using recycled materials or inviting volunteers to take part in building workshops at your place.

hardwood-timbers
Australia boasts many hardwood timbers suited to building. By Keren Dobia

Function Over Form

Passive solar design, thermal mass and insulation are three major considerations when determining a product’s thermal properties. Capturing and storing both heat and cold can create an ideal environment for self-regulated temperature control and save you a lot of money in heating and cooling using non-renewable systems.

Passive solar design is a popular way to harness the sun’s energy. This involves consideration of the orientation and internal layout of the house and, importantly, the choice of materials and their strategic placement. A passive solar design invests in lower energy bills and harnessing the naturally occurring thermal properties of your materials.

Thermal mass stores and releases 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 12 hours. Materials with a high thermal mass include brick, stone, cob, mud bricks, concrete and water.

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 just an R-value. Material used in the walls, for example, may have a high R-value but the walls are just one element of the design – only 15–25 percent of heat is lost through walls.

Carbon Footprint

Embodied energy is the total energy consumed by the process of extracting the material in its rawest form all the way through to it being delivered to your site. In natural building, the embodied energy of materials will depend on what is locally available to you.

The ideal low-embodied energy material would involve little or no processing, and all the energy inputs would be ‘borrowed’ from the earth. Cement, the key ingredient in concrete, is extremely high in embodied energy, but also one of the most commonly used materials. And while it can be difficult to avoid using cement when adhering to permits and engineering specifications, there are ways to reduce its use.

Labour And Love

The key considerations to balance are your ability to use unskilled labour, the speed of construction and the 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
Straw bale is versatile and resistant to fire and pests. By Kel Buckley
hempcrete
Exposed hempcrete with coloured swathes.

Materials

COB

Cob is made from subsoil, water and a fibrous organic material, typically straw. For a strong mixture you need up to 25 percent clay, up to 85 percent sand and up to 15 percent straw. Always test 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 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. Mudbricks are extremely fire-resistant.

STRAW BALE

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

LIGHT EARTH

Light earth 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 slurry, which is tamped into formwork between a post-and-beam structure. Once dry, 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 has both fire- and pest-resistant properties. Similar to light earth, it’s an infill material.

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.

Open Minded

Research which materials are readily available in your area, stay in tune with your budget and think outside the box with fixtures, fittings and labour. 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.

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