Most of us probably don’t think of using plants to build our next house, but plant-based building materials are pretty much essential to an ecohouse.
Plant-based building materials are generally highly sustainable. Typically they will have low embodied carbon, they are a natural part of circular construction because they can simply be composted at the end of their useful life, are unlikely to have any toxic substances that could affect the health of residents or construction workers, and will perform better at moderating the indoor environment than most man-made materials.
If the plants are grown and harvested using sustainable agricultural practices they will also contribute to broader sustainability outcomes, such as wildlife habitat and biodiversity and flood management.
But the “bigger picture” reason for using plants in an ecohouse is because they “sequester” carbon. All plants take in carbon from the atmosphere as they grow and by using them we “lock away” that carbon in the building fabric. They also put carbon into the soil as they grow, and for some plants the amount of carbon sequestered is more than the carbon required to process the construction materials and transport them to our building sites, making the material net-negative for embodied carbon.
Changing large parts of the global economy to bio-based industry is essential to the net zero transition, and it’s not just environmentalists saying this. The chairman of Lombard Odier, one of Europe’s oldest and most respected investment banks, recently stated that “shifting industrial activity into bio-based economy” is a key part of the “very profound economic transition” that we are going through.
The good news is that many plant-based materials are already an integral part of the supply chain for construction, we just tend not to think of them as coming from “plants”, or they may be (incorrectly) considered too “alternative” for mainstream construction.
Trees are by far the most important plant used in construction. Trees are used for structural timber and recent research has developed new construction methods where timber can be used to replace less sustainable materials like steel and concrete. We also use timber for creating the “skin” of our houses – think weatherboard walls, shingle roofs, hardwood floors, tongue & grooved ceilings. We use timber for doors and windows, for skirting boards and architraves, for cupboards and shelves. Trees can also be used for insulation when the wood is shredded into fibres or by using cork. Apart from the plumbing and electrical services, it would be possible to build a house entirely from trees.
Hemp (not to be confused with its close relative cannabis) was once a common agricultural crop in many countries, its fibres providing the raw material for ropes and cloths, and it is now seeing a resurgence. The biggest potential for hemp is as insulation where it can be used to make batts, rolls and slabs to standard industry formats.
There are many different grasses that can be used in many different ways as part of an ecohouse. Sisal is used for floor coverings. Jute is similar, but is most commonly known for its use as hessian sacks, a packaging material often used in construction. Flax is used to make linen fabric for furnishings and curtains. Reeds are used to thatch roofs, giving them very high levels of thermal insulation. And of course straw can be used for insulation. Most people will have heard of straw bale houses and the common perception is they are an “alternative” building material, but manufacturing systems have been developed where straw is used in prefabricated wall panels which are readily integrated into mainstream construction.
Plant resins are a less obvious way where plants are used in construction, but they provide essential chemical elements to many standard products. Paints use resins (from both plants and minerals) as the film-forming material. Hardboard (known by the trade name Masonite in Australia) is created when tree resins are cured through heat and pressure. Linoleum is made from rubber, (technically a sap rather than a resin), a high quality and durable product with net-negative embodied carbon that has been widely used for interior design for decades.
Using plants for building materials is different from using plants or plant-like designs to enhance the character of a building, something called Biophilic Design, which is another aspect of ecohouse design.