Many of the building projects covered in BPN are structures of concrete, brick, steel and glass. Every now and then we’ve used the Environ to remind readers that other material options exist. In the past we’ve covered strawbale construction, rammed earth, multi-storey wooden apartments, and their ilk. Here again, we highlight materials that haven’t been particularly rowdy in announcing their presence on the building market stage.
Rice
Australia has the potential to produce over a million tonnes of rice annually, and our rice farmers have shown themselves to be very efficient in its cultivation, using 50 per cent less water for the crop than the global average. Rice needs to be hulled to remove the outer husk from the eatable inner grain. The waste generated in the process is significant - it has been calculated that in excess of 100,000,000 metric tons of rice hulls are created globally.
These little shells, which make up approximately 20 per cent of the weight of harvested rice, have shown themselves to have rather remarkable properties. They are considered to be naturally flame retardant, requiring no chemical additives to provide their self extinguishing characteristics, partly due to their high silica content. They resist the growth of fungi and mould. In the USA, rice husks have been classified as Class A insulation material for their high thermal resistance value. In building construction the husks can be hand-filled, or machine blown into the cavities of prefabricated noggin-free wall trusses.
Cork
Another exceptional material, Cork is a naturally renewable and biodegradable product, harvested every nine years as bark of the Quercus suber, or cork oak tree. Almost half of cork’s composition is a honeycomb air-like gas cell of a fatty acid, known as Suberin, which is largely fire, water and insect resistant.
We all know that cork has long been used in resilient flooring, either alone or as an additive to linoleum. But less well known is the application of cork in external walls. It can be also used as a rigid insulative sheet board in cavity walls, or more creatively as external wall cladding, up to 300mm thick, either exposed or subsequently protected by a layer of render. Indeed, there is even a granular cork and water-based resin product than can itself be sprayed directly onto metal or masonry surfaces. Look for brands like SecilVit, SwisTherm, Corkwall, and MDFACHADA, amongst others.
Pictured: Expanded Insulation Cork Board from Amorim Isolamentos in Portugal.
Mushroom
Or more correctly, mycelium fibres. Ecovative’s mushroom materials contain about 13 km of the tiny fibres for every cubic 2.5 cm block of the stuff. In brief, agricultural by-products are inoculated with the mycelium and left for about week at room temperature, without the need of light, watering or petrochemical inputs. The resulting natural foam is dehydrated and heat treated to reign in the rampant growth, and reduce spore and allergen concerns.
Although still in the commercial trial phase, the Ecovative process is being developed for a range of building applications. These include: structural composites where the mycelium fibres bind to substrates like hemp, jute, wood veneers, fibreglass, and even carbon fibre. Being able to bond directly to wood, the mycelium is being considered for Structural Insulating Panels (SIPs), where it can replace both the usual Expanded Polystyrene and adhesives. A mushroom insulation is said to have been crafted which achieves a class 1 fire rating (USA), without requiring toxic fire retardants.
Pictured: Ecovative’s Mushroom Insulation
Sawdust
A while ago the Cement Concrete & Aggregates Australia released a report entitled “Use of Recycled Aggregates in Construction,” where they noted that although no applications of organic materials have been reported in Australia, “in the US, sawdust has been used as an aggregate for more than 50 years for floor, wall and roof units.” Going on to say that, “Sawdust concrete has a good insulation value, resiliency, low thermal conductivity, and can be sawn and nailed.”
A few years back the ARPN Journal of Engineering and Applied Sciences published a paper on sawdust (and palm kernel shells) as aggregate replacement. Their testing suggested that substituting 25 per cent sawdust could produce “lightweight reinforced concrete slabs which could be used where low stress is required at reduced cost” with a weight reduction of 14.5 percent. A builder in the US made a Sawdust concrete house, over 30 years ago, from sawdust concrete (and some diatomaceous earth and clay) which is reputed to have up to 14 times the insulation value of standard concrete.
Cob
Mix some clayey soil with some wheat or barley straw and form into it a big mud pie. Presto, you have Cob. Whilst it is pliable, and a little wet, add it to your wall and shape it as desired. Repeat. Until the wall is complete. Depending on the building site’s inherent soil structure and any local agricultural waste supply, it may be possible for home builders to get their materials almost for free.
However, such hand-built structures are obviously quite labour intensive. Additionally they tend to take months to cure and dry out. Plus the walls need to be 600 to 1000mm thick to support the weight of materials, and this may reduce the volume of interior space available. On the upside, the thick walls mean that “typically a cob house will use approximately 20 per cent less energy to heat compared with a typical modern house,” according to building pioneers who have rediscovered the medium that has served the walls of some British homes for more than 500 years.
Earthbag
The military and emergency services are very familiar with this form of building construction. They know them more simply as ‘sandbags’ and employ them to ward off explosive ordinance or floodwaters. Adventurous owner builders have been keen to experience the many opportunities that Earthbags present.
Although these days lightweight polypropylene sacks, such as normally used for seed or agriculture feed, are more commonly used than the old hessian bags, the principles remain much the same. With a few twists. One being the use of several lines of barbed wire laid down between courses of Earthbag ‘bricks’ to reduce bags slipping to the side. Being rather malleable for a building material, Earthbags have often been deployed to make vaulted domed structures, reminiscent of igloos. If filled with soil, or mineral based materials a wall with high thermal mass is created. However when stuffed with the likes of rice hulls or volcanic rock, the result is one with higher insulation qualities. Finished structures are often rendered to give a smoother exterior and interior.
Quincha
Quincha is a South American version of one the oldest forms of wall construction known, in this case a mud plaster is applied to a lattice woven of stout cane reeds. In Europe, where the practice is at least 3,000 years old, the English phrase is Wattle and Daub. A term that sounds familiar to Antipodean ears because Australia’s native acacias were found to be very suitable by early settlers for wattling, hence the common name for these flexible trees.
The technique, and its many variants, go by many other monikers, including the delightful Palisade and Pug. In some instances straw or fibrous animal dung is also added to mud plaster. Although not in common usage in developed countries today, the method was once widespread. (This writer once arrived at work in an inner Sydney shop to find the ceiling had collapsed. It was clearly a timber lattice structure filled with a plaster of paris pug.) Building conservators and natural building enthusiasts keep such time honoured traditions alive.
Image: Serra Talhada (Pernambuco), Brazil. House of taipa. Image: Wiki-commons: Patrick-br
