Timber From The Tropics

The first shipments oftropical timber cameto Europe via Venice during the fifteenth and sixteenth centuries. This timber was mainly extracted from the rainforests, which covered about 14% of the Earth's surface at the beginning of the twentieth century. This has now been reduced to about 6%. It has been estimated that about 25% ofall global emissions of carbon dioxide are caused by deforestation in the tropics (Gielen, 2000). This accelerates the process of global warming, also creating an increased strain on theforests themselves including extinction of vulnerable animal and plant species.

Tropical timber is used for many purposes, such as windowand door frames, interior panelling, floors and furniture, both as solid timber and veneers. Some timbers such as azobe, irokoand bankiria, have qualities useful to ecological building.They have a strong resistance to rot and can therefore be used in very exposed situations without chemical treatment. Despite this, using rain forest timber should beavoided altogether, unlessthe timber is managed sustainably.

For almost all purposes it is quite feasible to use temperate hardwoods instead of tropical timber. Oakand chestnut are examples. Many other timbers have excellentdu-rability and aesthetic qualities. New solutions with heat treated coniferous timber can attain very much the same appearance as hardwoods, and can fulfil at least half of thesamefunctions (Gielen, 1997).

The production of plant-based building materials is mainly local or regional. Energy consumption for processing and transportare relatively low, as is pollution occurring at the cultivating, harvesting and refining stages (Tables 10.3 and 10.4). This favourable environmental profile will be reflected in the building's overall ecological footprint, as well as by a good indoor climate. Most of these materials also have quite a long life, in particular wherever the conditions of use are fairly dry, and where they can dry out quickly after exposure to rain or humidity. Global warming is expected to cause significantly more rot damage in northern latitudes, whereas Western and Southern Europe will experience a decrease in risk (Noah's Ark, 2006).

When a building decays, organic materials will quickly return back to the natural environment. Alternatively, some of the materials can be recycled for re-use or as a source of energy.

Seen in a global climate perspective, building materials based on plants act as a store of carbon - a 'carbon sink'. One kilogram of dry timber contains about 0.5 kg carbon which is 1.8 kg carbon dioxide bound through photosynthesis. This quantity is thus removed from the global atmosphere for as long as the plant or timber itself lasts -until it rots or burns. Large quantities of timber and other plant products are used in construction, and the amount can be greatly increased. This can thus provide a significant contribution to reducing global warming. A conventional timber frame house contains about

Table 10.3 Embodied energy1 in basic plant products

Product

Manufacture (MJ/kg)

Combustion value (MJ/kg)

Embodied energy (MJ/kg)

Timber, squared and air dried

0.5

16

16.5

Timber, planed and air dried

0.7

16

16.7

Timber, planed and kiln-dried

3.0

16

19.0

Sawdust and wood shavings

3.0

16

19.0

Straw, loose fill and bales

0.5

14

14.5

1 Note: The embodied energy is defined as the sum of the energy used to manufacture a product and the combustion value of the product. It is assumed that raw material inputs which are not left in the material are used as energy source or as raw material for other products.

1 Note: The embodied energy is defined as the sum of the energy used to manufacture a product and the combustion value of the product. It is assumed that raw material inputs which are not left in the material are used as energy source or as raw material for other products.

150 kg/m2 of timber. Thus a 120 square metres house 'stores' about 32 tons of carbon dioxide. If a building is constructed in logs, or the increasingly popular system of massive timber then this can be increased to about 550 kg/m2. This means carbon storage of nearly 120 tons of carbon dioxide. This amount almost equals the normal total greenhouse gas emissions of a house during its lifetime, including both its construction and heating. However, the carbon sink must be maintained for a minimum of 100 years in order to be accounted fully in a climate calculation. Long life and maximum re-use of timber and plant-based components are, therefore, important. Where plant products have a lifetime shorter than 100 years, they can only be accounted as a reduced amount of climate reduction.

Whilst most organic materials have this healthy environmental profile, there are a few exceptions. Cultivating plants can involve the use of insecticides, fungicides, hormone additives and synthetic fertilizers that can be responsible for increased erosion, polluted ground water and the damage or destruction of local ecological systems. This type of cultivation can also produce defects such as enlarged and mouldy cell growth in timber. Gene manipulation has been suggested as a solution to reduce the need for fungicides. By adding genes of a more resistant plant, it is possible to reduce the amount of insecticide sprayed on a crop during cultivation. This gives the modified species an 'unfair' advantage over other species in the ecosystem, however, and may lead to the collapse of the whole system. This kind of solution is at present too dangerous to accept as a long-term environmental strategy.

Plant products may also be impregnated against mould, glued with synthetic glues, treated with flame retardants and with chemical paints and varnishes. Many of these additives are problematic, both for nature, for indoor climate and as final hazardous waste.

Generally it can be said that it is desirable to increase the use of organic materials in the building industry. This can considerably reduce the overall climate footprint of production and operation of buildings (Burnett, 2006; Goverse et al., 2001). This is both due to the carbon storage, and the generally lower energy and pollution effects of production. Only a small percentage of the potential organic building materials available are used today. The use of more varied species will stimulate different methods of application and a richness and diversity of species within forestry and agriculture. This is beneficial both to the farmer and to nature.

Table 10.4 Material pollution from plant materials

Materials

Polluting substances

Trees and grasses (processing)

Wood dust1; formaldehyde (coniferous trees)

Cellulose

Lye; chlorine

Fatty acids

Colophony

Terpenes

Turpentine; limonene

Wood tar

Aromatic and aliphatic hydrocarbons

1 Notes: Dust from some trees can be slightly carcinogenous, for example red cedar, oak and several tropical species.

1 Notes: Dust from some trees can be slightly carcinogenous, for example red cedar, oak and several tropical species.

BIOPLASTICS

Trees and plants contain many substances that can replace fossil resources In the manufacture of plastics.The building blocks for plastic include methanol, starch, lignin, oils, proteins and cellulose. Most types of plastics could be produced from plants with the required consistency, chemical and mechanical qualities (Gielen,1997). At present bioplastics are little used except in fairly short-lived packaging products. For buildings, technical requirements are often difficult to assess and production is still more expen-sivethan plastics from fossil oils. Dwindling fossil resources are likely to changethis picture fairly quickly (Stevens, 2002).

Production of the most robust bioplastics requires about as much energy as conventional production (Patel, 2002). Monomers, solvents and additives are likely to have similar toxicological characteristics as those used today, and they are not necessarily easily degradable.Their essential difference from today's plastics is in being produced from renewable resources.They can thus also be regarded as climate neutral, since burning them after use will release the same amount of carbon that was originally bound by the plants from the atmosphere.

BIOCOMPOSITES

By composite materials is meant all materials comprising at least two ingredients, this applies in other words to a majority of building products. However, the term is normally applied to cement or plastics-based materials that are reinforced with fibres. The term 'biocomposites' is used where this reinforcement consists of natural fibres such as wood, hemp, flax or cellulose. If the binder itself is a bioplastic then one has a purely vegetal product. Use of natural fibres has increased in recent years since they often display the same technical performance as synthetic fibres of poly-esteror fibreglass.Their tensile strength is often slightly lower but this is largely compensated by lower weight (Mohanty et al., 2005). An important use is in cement-based sheets. In many developing countries with scarce timber resources, plastics-based sheeting materials reinforced with natural fibres such as sisal and jute have been marketed for facades and roofing. Other building elements have been developed based on recycled plastic reinforced with wood fibre that can be recycled from demolition products.

Seen from the point of view of resources, substitution with natural fibres is a positive step. The production energy will be considerably lower, and can be halved for some sheet products (Patel, 2002). Generally, composites are not very desirable since they involve irreversible mixing of materials, making material recycling practically impossible. In the case of bioplastics, this picture provides openings for improvement. In some cases the products may even be entirely compostable.

Table 10.5 Bound carbon dioxide in plant products versus moisture content

Moisture content (%)

Bound carbon dioxide (kg CO2/kg)

0

1.87

8

1.73

12

1.67

18

1.28

20

1.56

30

1.44

100

0.94

(Source: Beyer etal., 2001; Esser, 1999)

(Source: Beyer etal., 2001; Esser, 1999)

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