• How Does Wood Compare?
• Prescription vs. Performance

An Obvious Choice for Sustainable Design and Construction

Concern about the world’s environment is encouraging the use of materials and designs that improve the environmental performance of building construction and operation. As well as leaving a lighter footprint, green structures are often healthier for occupants and more cost efficient to operate.

Studies show wood outperforms other materials when considered over its lifetime using measurable indicators such as global warming potential, resource use, pollution and solid waste. Current thinking in the green building movement has shifted toward an emphasis on this kind of measurable performance. Life cycle assessment is a means to this end because it allows the impartial comparison of materials and assemblies, over the course of their entire lives, based on quantifiable indicators of environmental impact.

 

LCA is used to assess building materials from extraction and processing through manufacturing, transportation, use, maintenance and disposal or recycling.

Life cycle assessment clarifies the environmental trade-offs associated with choosing one material over another and, as a result, provides an effective basis for comparing alternate designs.

In North America, there are two tools that provide life cycle assessment results for whole buildings and assemblies, both offered by the Athena Institute. The ATHENA® Impact Estimator for Buildings is capable of modeling 95 per cent of the building stock in North America, including industrial, institutional, office and residential designs, and simulating more than 1,000 assembly combinations. The ATHENA® EcoCalculator for Assemblies provides instant life cycle assessment results for common assemblies based on detailed assessments previously conducted using the Estimator.

How does wood compare?

Study after study has shown that wood outperforms other materials when considered over its lifetime using life cycle assessment. One study, conducted by the Consortium for Research on Renewable Industrial Materials (CORRIM), compared the environmental impacts of homes framed with wood and steel in Minneapolis and homes framed with wood and concrete in Atlanta – the framing types most common to each city.

The chart below illustrates how each of the designs performed against five key indicators of environmental impact. With two exceptions, the wood-frame homes performed substantially better than their non-wood counterparts. The steel design produced slightly less solid waste and there was no significant difference in emissions to water in Atlanta.

ATLANTA DESIGN	Wood	Steel	Difference	Other design vs. wood (% change)
Embodied Energy (GJ)	398	461	63	16%
Global Warming Potential (CO2 kg)	21,367	28,004	6,637	31%
Air Emission Index (index scale)	4,893	6,006	1,114	23%
Water Emission Index (index scale)	7	7	0	0%
Solid Waste (total kg)	7,442	11,269	3,827	51%

Another study conducted by the Canadian Wood Council compared the life cycle impacts of three 2,400-square-foot homes designed primarily in wood, steel and concrete over the first 20 years of their life spans. Relative to wood, the steel and concrete homes were predicted to:

• Release 24 per cent and 47 per cent more air pollution
• Produce eight per cent and 23 per cent more solid waste
• Use 11 per cent and 81 per cent more resources
• Require 26 per cent and 57 per cent more energy (from extraction through maintenance)
• Emit 34 per cent and 81 per cent more greenhouse gases
• Discharge four and 3.5 times more water pollution

Although these differences may seem small, it is important to note that only a small portion of the materials in a house (by weight) are involved in framing. The impacts are many times larger when components made from different materials are compared directly.

 

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Prescription versus Performance

Green building programs have traditionally taken a prescriptive approach, whereby certain practices or materials are “prescribed” based on conventional environmental wisdom – which may or may not stand up to objective analysis.

Without the kind of quantifiable data generated through life cycle assessment, choices are often subjective. For example, some people assume it is better for the environment to use materials produced locally. On the surface this makes sense, since less energy will be required to transport the materials. But there are many factors that influence whether one material is better than another, including the source of its components, type of manufacturing process and mode of transportation.

Another example is the common preference for rapidly renewable materials. Intuitively, it seems like a good idea to favour materials that regenerate quickly. But does it still make sense once the use of fertilizers and pesticides (including their production and transportation as well as environmental impacts), water requirements, manufacturing process and other factors are all given due consideration?

With life cycle assessment, information is gathered on every aspect of a material at every phase of its life – and viewed through the lens of defined measures such as global warming potential, energy and resource use, air and water pollution, and solid waste. As a result, design and construction professionals are able to make the kind of informed, scientifically based choices that result in better buildings.

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Sources

Athena Sustainable Materials Institute: www.athenasmi.ca

Canadian Wood Council: www.cwc.ca

• Energy and the Environment in Residential Construction www.cwc.ca/NR/rdonlyres/FBEC3574-62E5-44E0-8448-D143370DCF03/0/EnergyAndEnvironment.pdf

Consortium for Research on Renewable Industrial Materials: www.corrim.org

• The Environmental Performance of Renewable Building Materials in the Context of Residential Construction: www.corrim.org/reports/2005/swst/3.pdf