Metals have become such a common element in so many building applications, from nails to plumbing fixtures, that it would be hard to imagine building without them. Metals are strong, durable, and generally do not cause indoor air quality problems. (Airborne dust from lead paint is a notable exception.)
Sometimes metals are just one of several viable material choices, in which case it is instructive to compare options. Structural framing is one such example. The debate over which is the “greener” framing material — steel or wood — has no unanimous resolution. Although steel is highly recyclable and its raw materials are plentiful, wood is a renewable resource, is recyclable and biodegradable, and has much lower embodied energy than steel (even recycled steel has five times the embodied energy of kiln-dried wood). Wood is also a natural insulator, whereas steel is a conductor. (It is 400 times more conductive than wood.) The “thermal bridging” that occurs at exterior walls where steel studs span from the inside out can halve the overall R-value of a wall with cavity insulation (as compared to the R-value of the same wall framed with wood). This presents a major energy-efficiency problem for steel-framed exterior walls. Providing a layer of continuous exterior insulation, while it does not completely solve the thermal bridging problem, can significantly increase the overall R-value of the steel-stud wall.
On the other hand, steel framing is lighter than wood, more regular and dimensionally stable, and offers the advantage of resistance to insects. It does not require (as wood does) treating the soil with termiticides, and therefore is better for air quality. Steel is easily separated at the demolition site using a magnet, and steel scrap has a ready market. The overall recycled content of U.S. steel (on average for all steel products) is 46%, but this doesn’t account for the steel scrap that is exported (11% of the total manufactured steel) rather than re-manufactured in the U.S.
Both the wood and steel industries have caused serious environmental problems. Clear-cutting forests has caused habitat destruction and siltation of streams (and pesticide-laden, monoculture plantation forests are not much of an improvement). Strip-mining for the iron and limestone used in steel has caused severe erosion, ecosystem destruction, and leaching from tailings piles into water systems. Fortunately, both industries are making environmental and efficiency improvements.
In an application that allows the use of either wood or steel (especially if untreated wood can be used), wood from a certified, well-managed forest would be the most environmentally sound choice. Overall, its manufacturing process uses much less energy and creates less pollution and environmental degradation than mining and processing steel.
The mining and manufacture of other metals presents environmental concerns similar to those associated with steel, and often much more severe. For instance, the embodied energy of copper is about twice that of steel, while virgin aluminum has as much as seven times the embodied energy of steel.
Like steel, other metals used in building are highly recyclable. Although remanufacturing metals uses significant energy, it is much less than the energy and environmental impacts of starting with the virgin resource. Because metals are highly durable and could be recycled indefinitely, their environmental impact (extraction from the earth and the fact that they are nonrenewable resources) is significantly reduced.
Finally, metals offer clear advantages for certain applications. For example, if water collected from a roof surface is to be used for drinking, a steel roof will not leach petro-chemicals into the water, as an asphalt-based roof might. Although stone or clay tile roofing could also be used, their greater weight would require more structural support than the lighter steel.
Alexis Karolides, AIA, the author of this article, is a registered architect and principal with the Rocky Mountain Institute (RMI) in Snowmass, Colorado. This article was adapted from Green Building: Project Planning & Cost Estimating, 3rd Edition, available through RSMeans.