This article provides a definition of renewable materials and considers some common characteristics – carbon sinks, flammability and biodegradability. It derives from PRO 10 Renewable resources: an introduction to materials (Environment Design Guide externallink, 2001), written by the author and now withdrawn.

Introduction

Many materials used in construction are obtained from finite resources which, once exploited as far as is economically feasible, can never be replaced. They are not renewable – at best they can only be recycled in perpetuity. While we have an abundance of many such resources at the moment, extraction costs are rising, and availability is decreasing – at least for some commodities in some countries (G.M. Mudd, The 'Limits to Growth' and 'finite' mineral resources externallink, 2010). In the (very) long term we will be increasingly reliant on a closed loop of industrial recycling, especially for efficient sourcing of these non-renewables.

Demand for construction materials will increase as world population expands and developing nations become wealthier, with raised expectations for life style and affluence. The increasing demand for non-renewable resources may be partially offset by more efficient use of such materials, by discovery of new sources and by development of more efficient extraction processes.

Nevertheless, it stands to reason that finite non-renewable resources should be carefully husbanded so as not to disadvantage future generations. This is the essence of sustainability. One way of taking the pressure off non-renewables is to use renewable construction materials as far as practicable. Use of renewable materials also brings other benefits, and some disbenefits, discussed below.

Architects tend to think of timber as the renewable building material. But there are many others. A host of renewables have been and still are in common use, e.g. bamboo, cotton, flax, hemp, other vegetable fibres, straw, thatch, agar, sea grass, cork, wool, silk, beeswax, lacquer, linseed oil, shellac, tar, tung oil, turpentine, vernonia oil, rubber, casein, rosin, sewage sludge, starch, and rice and peanut hulls. What makes these materials renewable is that they are sourced from living (solar powered) plants and animals, not from mineral deposits or fossil deposits of organic material such as oil, gas, coal, peat and asphalt.

However, although these materials are renewable, they are not necessarily renewable (or being renewed) in a sustainable manner. For example, production of Brazilian biofuels (ethanol and biodiesel) has had adverse environmental and social impacts (E.F. de Almeida et al, The performance of Brazilian biofuels externallink, 2007).

As another example, the Dutch Directorate of Public Works and Water Management decided (in 1996) to source its hardwood imports from Africa instead of buying Western Australian (old growth) karri, on environmental grounds (I. Anderson, Dutch and Australians at loggerheads over forestry externallink, 1996). The WA Forest Products Commission externallinkis only now (2013) seeking certification to the Forest Stewardship Council's Controlled Wood Standard for old growth karri harvesting.

A final example is cotton, which requires disproportionately massive use of insecticides (a quarter of global sales) and water (perhaps more than 20 kL per kg). WWF has some useful resources on these issues.

On a wider front, Friends of the Earth has pointed out that the use of timber is not as 'green' as it should be – demand is likely to exceed supply, waste is under used, other renewables could be substituted and specification practices could be tighter (B. Evans, 'Timber – not green enough?', The Architects' Journal, 4 May, 1995). Clearly, specifying renewable materials is not necessarily a good thing in itself, even if viewed only from an environmental perspective. From a broader perspective, of course, many other issues (e.g. cost, availability, aesthetics, fitness for purpose, societal and economic impacts) need to be considered in materials selection.

Renewable materials can be sourced from the wild, or cultivated. The former is not usually seen as a good thing, but the latter has some drawbacks too, such as loss of biodiversity as possibly vulnerable monocultures are created at the expense of natural ecosystems, and as wild species and traditionally cultivated species are ignored and perhaps lost. Sourcing from the wild is not necessarily all bad in ecological terms. Harvesting can be perpetual, as in shearing (e.g. of sheep) and coppicing (e.g. coppicing of Queensland sandalwood externallink) instead of one-off (that is, by killing the plant or animal), and can often be managed to minimise collateral damage to non-harvested species.

Common characteristics of renewable materials

Renewable materials are, by definition, organic materials – they contain carbon. This feature gives renewable materials some significant common characteristics.

Carbon sinks

Quite apart from their very renewability, organic materials have another potential big advantage – they sequester carbon. Generally, the more bulk a product has, the more carbon, so timber is the most effective material to use in this respect. The best timber to use is that which is replaced by planting (so that more sequestration takes place), and that which is harvested at the most efficient time in terms of carbon take-up. Mature trees, for example, slow down in terms of growth and therefore carbon sequestration (I. Johnson & R. Coburn, Trees for carbon sequestration externallink, 2010). Leaving them standing in plantations longer than necessary is, from this point of view, not desirable. Plants with rapid growth, such as hemp and bamboo, are appealing in this respect.

However, all organic materials contain carbon, and every bit of non-fossil organic material used in long-lived building components will help to remove CO2 from the atmosphere, working towards negating the anthropogenic greenhouse effect. Using fossil-sourced organic materials, as in many plastics and solvents, is a waste of time in this respect – that carbon was sequestered long ago.

Flammability

Virtually all organic materials will burn. Though some, such as wool, are self-extinguishing, others require protection in the form of inorganic fire-retardants (which generally only improve surface burning characteristics) or fire-resistant casing (for example, to protect metal connectors at joints). The shear bulk of large timber sections provides protection as charring externallink is restricted to outer (sacrificial) timber. If properly considered, the essential structural section will remain intact for at least the required safe period for exit. Some timber species are more fire-resistant than others – generally, the denser the wood, the shallower the charring.

Biodegradability

This feature is both a blessing and a curse. On the blessing side, demolished and waste renewable materials, if untreated, will decay – we will not be faced with long-term disposal and storage problems. However, for components of buildings made from renewable materials, treatment to prevent decay is often regarded as essential. Structural materials must be durable – this is an overriding question of life safety. Apart from this sort of threat, rot, insect infestation, termite attack, mildew and so forth are all threats both to the materials themselves and, often, to the health of human occupants.

Some materials are more naturally durable than others. Examples of natural durability of timber species (heartwood) are given in BS EN 350-2:1994 Durability of wood and wood-based products. Natural durability of solid wood. Guide to natural durability and treatability of selected wood species of importance in Europe, in terms of resistance to wood-destroying fungi, common insects, and termites and marine borers. Durability is generally tied to density. So oak is a durable timber (durability class 2/D) at 720 kg/m³, and European birch is a not durable timber (durability class 5/P) at 670 kg/m³ (see TRADA Wood species database externallink – registration required).

Preservatives used to protect biodegradable materials are often noxious (those containing arsenic and chromium were banned from sale in 2006 in the UK, and creosote externallink is restricted to professional and industrial uses) and are rarely themselves renewable.

Treated timber is not, of course, innocuously biodegradable in the way that untreated timber is. One problem is the noxious nature of the preservatives themselves. For example, creosote- and CCA-treated timber has to be treated as hazardous waste and cannot be burnt for fear of poisoning (TRADA Technology & Enviros Consulting, Options and risk assessment for treated wood waste externallink, 2005). The other problem is simply lack of biodegradability – one of the big advantages of using renewables is gone.

One option in the use of renewable materials, therefore, is to live with short life spans for built structures.
Another is to only use durable renewables for structural components, restricting the use of less durable renewables for components which are more easily replaced, less critical, or both. A third is to design such that non-durable materials are protected from degradation, for example through ventilation, sacrificial protection, and traditional timber details such as drip mouldings. Extensive use of renewable materials will clearly affect the design of buildings, perhaps radically.