Monday, June 17, 2013


Wood: A Renewable Resource

Wood is the only major structural material that is renewable.

In the United States and Canada, tree growth each year  greatly exceeds the volume of harvested trees, though many timberlands are not managed in a sustainable manner.

On other continents, many countries long ago felled the  last of their forests, and many forests in other countries are  being depleted by poor management practices and slash- and-burn agriculture. Particularly in the case of tropical  hardwoods, it is wise to investigate sources and to ensure  that the trees were grown in a sustainable manner.

Some panel products can be manufactured from rapidly  renewable vegetable fibers, recoverable and recycled wood  bers, or recycled cellulose fibers.

Bamboo, a rapidly renewable grass, can replace wood in  the manufacture of ß ooring, interior paneling, and other nish carpentry applications. In other parts of the world,  bamboo is used for the construction of scaffolding, concrete formwork, and even as the source off  brous material for structural panels analogous to wood-based oriented  strand board (OSB), particleboard, and fiberboard.

Forestry Practices

Two basic forms of forest management are practiced  in North America: sustainable forestry, and clearcutting  and replanting. The clearcutting forest manager attains  sustainable production by cutting all the trees in an area,  leaving the stumps, tops, and limbs to decay and become  compost, setting out new trees, and tending them until  they are ready for harvest. In sustainable forestry, trees are  harvested more selectively from a forest in such a way as to  minimize damage to the forest environment and maintain  the biodiversity of its natural ecosystem.

Environmental problems often associated with logging of forests include loss of wildlife habitat, soil erosion, pollution of waterways, and air pollution from machinery ex- hausts and burning of tree wastes. A recently clearcut forest  is a shockingly ugly tangle of stumps, branches, tops, and  substandard logs left to decay. It is crisscrossed by deeply rutted, muddy haul roads. Within a few years, decay of the  waste wood and new tree growth largely heal the scars. Loss  of forest area may raise levels of carbon dioxide, a greenhouse gas, in the atmosphere, because trees take up carbon  dioxide from the air, utilize the carbon for growth, and give
back pure oxygen to the atmosphere.

The buyer of wood products can support sustainable forestry practices by specifying products certified as originating from sustainable forests, those that are managed in  a socially responsible and environmentally sound manner. 

FSC-certified wood products, for example, satisfy the  requirements of LEED and all other major green building  assessment programs.

Mill Practices

Skilled sawyers working with modern computerized sys- tems can convert a high percentage of each log into marketable wood products. A measure of sawmill performance is  the lumber recovery factor (LRF), which is the net volume  of wood products produced from a cubic meter of log.

Manufactured wood products such as oriented strand  board, particleboard, I-joists, and laminated strand lumber effciently utilize most of the wood fiber in a tree and  can be produced from recycled or younger-growth, rapidly renewable materials; finger-jointed lumber is made  by gluing end to end short pieces of lumber that might  otherwise be treated as waste. The manufacturer of large,  solid timbers generates more unused waste and yields fewer  products from each log.

Kiln drying uses large amounts of fuel but produces  more stable, uniform lumber than air drying, which uses  no fuel other than sunlight and wind.

Mill wastes are voluminous: Bark may be shredded to  sell as a landscape mulch, composted, burned, or buried in  a landfill. Sawdust, chips, and wood scraps may be burned  to generate steam to power the mill, used as livestock bedding, composted, burned, or buried in a land.

Many wood products can be manufactured with significant percentages of recoverable or recycled wood, plant ber, or paper materials.


Because the major commercial forests are located in  concentrated regions of the United States and Canada,  most lumber must be shipped considerable distances.

Fuel consumption is minimized by planing and drying the lumber before it is shipped, which reduces both weight  and volume.

Some wood products can be harvested or manufactured  locally or regionally.

Energy Content

Solid lumber has an embodied energy of roughly 1000 to  3000 BTU per pound (2.3 to 7.0 MJ/kg). An average 8-foot-long 2 4 (2.4-m-long 38 89 mm) has an embodied energy  of about 17,000 BTU (40 MJ).
This includes the energy ex- pended to fell the tree, transport the log, saw and surface the  lumber, dry it in a kiln, and transport it to a building site.

Manufactured wood products have higher embodied  energy content than solid lumber, due to the glue and resin  ingredients and the added energy required in their manu- facture. The embodied energy of such products ranges from  about 3000 to 7500 BTU per pound (7.0 to 17 MJ/kg).

Wood construction involves large numbers of steel fas- teners of various kinds. Because steel is produced by relatively energy-intensive processes, fasteners add considerably  to the total energy embodied in a wood frame building.

Wood does not have the lowest embodied energy of the  major structural materials when measured on a pound-for-pound basis. However, when buildings of comparable size, but  structured with either wood, light gauge steel studs, or concrete, are compared, most studies indicate that those of wood  have the lowest total embodied energy of the three. This is  due to woodÕs lighter weight (or, more precisely, its lesser density) in comparison to these other materials, as well as the rela- tive efficiency of the wood light frame construction system.

Construction Process

A significant fraction of the lumber delivered to a construction site is wasted: It is cut off when each piece is sawed to size and shape and ends up on the scrap heap, which is  usually burned or taken to a landfill. On-site cutting of lumber also generates considerable quantities of sawdust. Construction site waste can be reduced by designing buildings that utilize full standard lengths of lumber and full sheets  of wood panel materials.

Wood construction lends itself to various types of prefabrication that can reduce waste and improve the efficiency of material usage in comparison to on-site building methods.

Indoor Air Quality (IAQ)

Wood itself seldom causes IAQ problems. Very few people are sensitive to the odor of wood.

Some of the adhesives and binders used in glue-laminated lumber, structural composite lumber, and  wood panel products can cause serious IAQ problems by  giving off volatile organic compounds such as formalde-hyde. Alternative products with low-emitting binders and  adhesives are also available.

Some paints, varnishes, stains, and lacquers for wood  also emit fumes that are unpleasant and/or unhealthful.

In damp locations, molds and fungi may grow on wood  members, creating unpleasant odors and releasing spores  to which many people are allergic.

Building Life Cycle

If the wood frame of a building is kept dry and away  from fire, it will last indefinitely. However, if the building is  poorly maintained and wood elements are frequently wet,  wood components may decay and require replacement.

Wood is combustible and gives off toxic gases when it  burns. It is important to keep sources of ignition away from  wood and to provide smoke alarms and easy escape routes  to assist building occupants in escaping from burning buildings. Where justified by building size or type of occupancy,  building codes require sprinkler systems to protect against  the rapid spread of fire.

When a building is demolished, wood framing members  can be recycled directly into the frame of another building, sawn into new boards or timbers, or shredded as raw  material for oriented-strand materials. There is a growing  industry whose business is purchasing and demolishing  old barns, mills, and factories and selling their timbers as  reclaimed lumber.

A study commissioned by the Canadian Wood Council compares the full life cycle of three similar office buildings, one each framed with wood, steel, or concrete and all  three operated in a typical Canadian climate. In this study,  total embodied energy for the wood building is about half  of that for the steel building and two-thirds of that for the  concrete building. The wood building also outperforms  the others in measures of greenhouse gas emissions, air pollution, solid waste generation, and ecological impact.

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