Volatile wood prices, declining quality of framing lumber, and environmental concerns have led many builders to consider alternatives to wood.
Wood frame construction has been the unchallenged norm for residential building in North America for decades. Over the last few years, however, volatile wood prices, declining quality of framing lumber, and environmental concerns have led many builders to consider alternatives to wood. A 1993 survey done by the National Association of Home Builders (NAHB) found that about 1% of new homes were framed entirely with steel, and that 5% used steel in non-load-bearing applications.
These figures represent a dramatic increase from January of 1992, when a survey by the American Iron and Steel Institute found steel use to be 1⁄4 % and 1⁄2 % of new home construction, respectively. AISI has established a goal of capturing 25% of the market by 1998.
Framing with light-gauge steel is widely promoted for a number of reasons, not least of which is a perceived environmental advantage over wood. But is steel framing really environmentally preferable to wood? What are its real advantages and disadvantages? As industry associations, manufacturers, and trade groups take sides in this highly polarized issue, EBN finds that switching to steel may make sense in some cases, but not as a simple substitute for wood.
Why Light-Gauge Steel?
There are many alternative building systems available to builders today, ranging from structural stress-skin panels to straw bales, from masonry to adobe. So why has steel framing suddenly garnered so much attention? Part of the answer may lie with a well-funded campaign by the American Iron and Steel Institute (AISI), which provided a complete model house at last year’s NAHB convention in Las Vegas. The NAHB Research Center’s resource conservation house also featured steel framing. But steel framing has gained prominence so quickly primarily because it offers price stability and a simple piece-by-piece substitution for wood. Steel studs replace wood studs, steel joists replace wood joists, and steel channel sections replace wall plates and band joists. Builders can adjust to using the new material without worrying about learning a whole different approach to framing. In fact, contractors with experience in commercial buildings are usually already comfortable with steel-framed partition walls, so the switch is quite easy.
There are other reasons that steel framing is attractive. Liza Bowles, director of the NAHB Research Center, cites quality control as a key factor. Steel is manufactured to exacting specifications, without any of the twisting, warping, and other defects that can plague framing lumber. Its lighter weight is also an advantage, making assembled walls easier to handle. Steel’s inherent insect resistance is a big plus in termite-prone areas.
Finally, advocates of steel framing cite performance advantages in fires and earthquakes, though these are challenged by the wood industry (see Table 1).
Environmentally, steel has a number of selling points. It is praised for its recycled content, although the levels of recycled material in steel framing are often exaggerated. Steel isn’t associated with the highly publicized and emotionally charged concerns about forest management and timber supply. Some people with extreme chemical sensitivities prefer steel because the terpenes emitted by softwoods can be irritating, and because chemical treatment with preservatives isn’t needed. Steel has one glaring handicap, however, that has the potential to override all its advantages: thermal performance.
Steel itself is over 400 times more conductive of heat than wood. A 20 gauge (0.04-inch-thick) standard steel stud conducts roughly 10 times more heat than a 11⁄2-inch-thick wood stud. This high conductivity causes severe thermal bridging wherever steel spans from the inside to the outside of the building envelope. If appropriate measures aren’t taken to control this thermal bridging, it can cause a number of problems: excessive energy use for heating and cooling; the need for larger space conditioning equipment to handle the larger loads; and condensation of moisture on the warm side of a wall, leading to dust or mildew stains. In areas with significant heating or cooling loads, the energy penalty from using steel studs in exterior walls may be the single most important environmental factor to consider.
Measuring the Problem
Heat flow through a building envelope is usually calculated using an approximation called the parallel path method. The parallel path method assumes that the only way heat flows is straight through the wall. The problem with this assumption is that as a rule heat doesn’t flow straight through the wall; it moves in the path of least resistance. For wood framing the parallel path assumption is close enough, but for steel framing it’s way off.
In effect, any heat in the vicinity of the steel moves sideways through the wall to the stud and then travels through the steel (see heat flow diagrams).
Table 2: Impact of Framing on Wall R-values
Applies to C-Channel metal studs of 16 gauge or thinner
1. 16" on center spacing assumes 11.9% of wall area is framing.
2. 24" on center spacing assumes 8.9% of wall area is framing.
Source: Values for steel from ASHRAE Standard 90.1, values for wood calculated using parallel path method.
Other heat-flow calculations commonly used by engineers have also failed to estimate the performance of steel stud walls accurately, according to Ed Barbour of the NAHB Research Center. Only sophisticated computer modeling based on finite difference calculations may be up to the task, and even these must be verified by actual tests. That’s one goal of a study now underway by Enermodal Engineering of Waterloo, Ontario, for the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE). Alex McGowan of Enermodal is analyzing thermal bridging in building envelopes, and trying to come up with a simplified procedure for estimating the heat flow through such bridges.
ASHRAE’s 90.1 Committee published a series of corrected values for steel framed walls for use until a simplified calculation procedure is developed (see Table 2).
The committee recognized that these corrections may be revised based on further testing, but felt it was important to alert people to the problem. “The table that we put out there was intended to serve as a wake-up call,” said Committee Chair Merle McBride. These values show that wider spacing between studs reduced the thermal penalty, as one might expect. It’s also clear from these factors that cavity insulation in steel-framed walls suffers from diminishing returns: as more insulation is added to the cavity, a bigger fraction of that insulation is circumvented.
The issue is further complicated by the fact that real walls have corners, band joists, and rough openings. All the actual tests performed to date have used only sections of simple wall area, ignoring the effects of junctions and framing for rough openings. André Desjarlais and Jan Kosny of Oak Ridge National Laboratory (ORNL) ran a series of computer simulations to model the effect of these framing details on the overall wall R-value for a simple ranch-style house. In a paper published in the July 1994 issue of the
Journal of Thermal Insulation, they report that for a simple steel-framed wall with studs on 16" centers, the overall wall R-value is 22% lower than the R-value of a section of wall with studs only.
Many people are looking into potential solutions to the thermal bridging problem. Most solutions offered to date fall into one of three categories:
1.Modifying the steel studs.
2.Adding insulation or strapping to walls outside the steel studs.
3.Using steel in a different framing configuration, unlike platform framing.
In the first category, at least two manufacturers are reportedly preparing modified steel studs for residential use. Products being discussed rely primarily on perforations or other gaps in the web of the stud to reduce heat transfer across the cavity. Some of these products also have small nubs on the flanges that reduce the contact area between wall sheathings and the studs. Both these modifications have been reported to improve the performance somewhat, though not as dramatically as initial claims suggested. Angeles Metal Systems of Los Angeles, California is preparing one such stud for market. “Preliminary test results are very encouraging,” says Jeff Olmstead of Angeles, but he adds that they’re still “doing further modifications” to the product.
The second approach, adding insulating sheathing to a steel-framed wall, does not “solve” the problem of thermal bridging, but it does increase the wall’s overall R-value. In fact, recent tests indicate that it may actually increase the wall’s R-value by as much as 20%
more than the R-value of the added sheathing. Though some researchers dispute this result, others find it convincing. According to Desjarlais of ORNL, it’s not impossible that the insulating sheathing actually does compensate to some extent for the thermal bridging.
The NAHB Research Center has just completed a series of full-scale tests and computer models of various steel-framed wall sections. Funded in part by AISI, these tests looked specifically at possible solutions to the thermal bridging problem. Preliminary results provided to EBN show that the walls with insulating sheathing consistently tested at a higher R-value than the sum of the R-values of the framed wall and the sheathing. While this increased R-value does not come close to fully making up for the thermal bridging through the steel, it does suggest that current estimates should be revised upward by about R-1 when insulating sheathing is used over steel studs.
In the third category, some researchers argue that steel is a fine framing material, but it shouldn’t be used as a piece-for-piece replacement of wood framing. Steel’s uniformity and high strength, together with its poor thermal characteristics, suggest that it should be used in a system that requires far fewer framing members, spaced farther apart. “The way that we’re building with steel today is not correct,” says André Desjarlais of Oak Ridge National Lab (ORNL). “We’re building with steel as a direct substitute for wood. There are good ways to build with steel, though.” Desjarlais and others at Oak Ridge have reportedly developed a system of trusses and foil-faced insulation that wraps around a steel frame to prevent thermal bridging. Details of the system have not yet been released.
Manufacturing the Materials
One of the reasons people often give for choosing steel over wood is concern for the environment. With all the recent publicity about old-growth forests, endangered species, and timber supplies, it’s not surprising that any material offered as an alternative to wood will get attention. (Concrete and PVC industry associations have made similar claims.) Add to that the consistently high recycling rate of steel and you have an environmental winner, right? Not necessarily. The extraction of raw materials used to make steel can have serious environmental impacts, and the manufacturing process, even from recycled steel, is extremely energy intensive. Wood, on the other hand, is naturally renewable, requires less processing energy, and is ultimately biodegradable. There is no simple answer to the question of which is better.
The production of both steel and lumber happens in two distinct phases: resource extraction and manufacturing. The environmental impacts of the manufacturing stages can be quantified relatively accurately in terms of energy use and pollution emissions to air, water, and land. The resource extraction stage is much harder to analyze because it involves human activity in interaction with complex natural ecosystems.
The resource base for framing lumber is coniferous trees: primarily several species of spruce, fir, and pine. The environmental impacts of extracting this resource vary tremendously from region to region, company to company, and even between operations within the same company. Wayne Trusty is the project manager for a comprehensive Canadian research project coordinated by Forintek Canada Corp. entitled: “Building Materials in the Context of Sustainable Development” (see EBN
Vol. 3, No. 2 for a review of a report from this project). Trusty has recently surveyed a number of industry experts on the environmental impacts of resource extraction for wood, steel, and concrete products. He reports that, of the three industries, “there is the least amount of agreement with [the impact of] timber harvesting.” Trusty adds: “There is an enormous wealth of information on timber harvesting. Just the sheer bulk of material tends to make timber come to the top as the big problem area. It’s not necessarily very fair.”
Framing lumber used in North America comes in large part from several productive regions: the Pacific Northwest and British Columbia, the western intermountain region, eastern Canada, and the southeastern U.S. The wood species, forest types, and forestry operations differ widely among these regions. In general, timber harvesting affects a very large area. The intensity of the impact varies from mild to severe, depending on (among other things) the sensitivity of the ecosystems, the topography, and the practices used. “You can renew trees,” Trusty explains, “but not forests.”
Especially inappropriate practices can lead to habitat destruction, loss of biodiversity, and siltation of streams. Good forest management practices on less sensitive sites, on the other hand, may have very little negative impact. For details on some practices and concerns in the Pacific Northwest, see “The Northwestern Timber Debate” in EBN
Vol. 2, No. 3. Plantations of southern yellow pine in the southeastern U.S. a more akin to an agricultural crop than to natural forest management. Chemical herbicides and fertilizers are used in most operations, and species diversity is minimal. The negative environmental impact of these plantations may not be as great as the inappropriate harvesting of some natural forests only because the most significant ecological damage has already been done.
Resource Extraction for Steel
The raw materials used to make steel framing include iron ore, limestone (or another source of calcium carbonate), coal, and zinc.
Large quantities of oxygen (separated from ambient air at the steelmaking facility) are also used, and supplemental heat comes from fuel oil or natural gas.
The vast majority of domestic iron ore comes from the Mesabi Range of northern Minnesota. Although seams of highly concentrated ore have been exhausted since mining began there in the 1890s, enough ore remains for several centuries, according to John Ewing, managing director of business development at US Steel (a division of USX Corporation). Nevertheless, as the quality of North American ore has declined and mining costs have risen, an increasing fraction of the ore (about 7% in 1993) is being imported from Venezuela and Brazil.
Limestone is generally quarried near the steelmaking plants, while the coal used in steelmaking comes primarily from Appalachia (western coal doesn’t have the required metallurgical properties). Zinc, used in the galvanized coating of steel framing materials, is mined in various regions of North America. Several other metals, including nickel, manganese, and chromium, are commonly used to make some types of steel, but are not used in significant quantities in steel framing materials.
All the above-mentioned materials are non-renewable substances mined from the earth. Large pit mines and strip mines are the primary sources of the metal ores and limestone, while most of the coal used is mined underground. The American Institute of Architects’
Environmental Resource Guide (ERG) reports that according to EPA estimates, mining sites cause surface erosion at a rate of 48,000 tons annually per square mile. As the earth’s surface is completely disrupted by open mining, pre-existing ecosystems are effectively destroyed. Large tailings piles have in the past leached sufficient quantities of metals and minerals to harm regional waters. In the U.S., environmental controls have greatly reduced the likelihood of further damage according to the steel industry. Conditions in South American countries may be less carefully controlled, however.
Table 3: Comparing Materials & Pollution from Manufacturing
1. Value from discussions with Jamie Meil of Forintek and others, not included in Forintek report.
2. Includes energy used to make electricity at Canadian average conversion efficiency of 37.6%.
Source: Data adapted from
Building Materials in the Context of Sustainable Development: Phase II Summary Report by Forintek Canada Corp. and Wayne B. Trusty
Making dimension lumber from logs entails debarking, sawing, planing the surfaces, and drying the lumber in a kiln. Kiln drying is the most energy-intensive stage of this process, though fuel for the kiln is often bark and scraps from the milling operation. Common estimates for the embodied energy of kiln-dried softwoods range from 52,000 to 92,000 Btu/cu.ft.
See Table 3 for an estimate of energy use and pollution emissions.
In termite-prone areas where framing with preservative-treated lumber is common, additional manufacturing steps must be considered. Arsenic-based preservative treatments such as CCA and ACZA represent potential environmental damage from the industry, and there are also concerns about possible hazards to builders and occupants of treated homes (see EBN
Vol. 2, No. 1 for details). Borate-treated framing lumber is much safer, both for the environment and for users, and may be appropriate as long as the wood is protected from weather. Embodied-energy figures for pressure-treated framing lumber are not available.
To make steel, iron ore is first processed at the mine into pellets or sinter, increasing the iron concentration. Limestone or other minerals are baked to make lime, and coal is converted to coke. The coking process uses high temperatures in the absence of oxygen to remove volatile organics and other contaminants from the coal. Volatile gases removed from the coal are used to heat the coking ovens and for other processes in the plant. As it emerges from the furnaces, coke is quenched with water, which is then treated and reused or discharged.
Table 4: Recycled Content in Domestic Steel Overall & in Steel Studs
1. These data represent average recycled-content of steel produced in the U.S. Significantly more steel scrap (about 11% of the total manufactured) is collected for recycling than is reflected in these figures because much of that scrap is exported.
2. As per lifecycle conventions and RCRA recovered material definitions, includes scrap returned to the mills from processors and product manufacturers but does not include scrap generated within steel mill.
3. BOFs currently produce 62% of all U.S. steel and about 95% of the coated sheet steel used to make light-gauge framing members.
4. EAFs currently produce 37% of all U.S. steel and about 5% of the coated sheet steel used to make light-gauge framing members.
Source: 1993 data from the U.S. Bureau of Mines and Fordham University, provided by Bill Heenan, president, Steel Recycling Institute.
Iron ore, lime and coke are loaded into a blast furnace, where burning coke provides the heat to melt the iron. Other minerals in the ore combine with the lime flux to form slag. The molten iron and blast furnace slag are removed periodically from the furnace via tap holes at the bottom. Though much blast furnace slag is landfilled, it is also used to make mineral wool insulation and ceiling tiles, and may be processed into aggregate for roads or railroad beds. Blast furnace gas is also created, then cleaned and used as a heating fuel within the plant.
Molten iron from the blast furnace is saturated with carbon and contains various contaminants. To make steel, most of the carbon and other materials must be removed. This conversion is usually done in a basic oxygen furnace. The older, open-hearth furnace technology is no longer used in North America to any significant extent. Twenty-five to thirty percent steel scrap (including in-house and recycled scrap) and some lime or other flux are added to the basic oxygen furnace along with the iron. Pure oxygen injected into the furnace reacts with carbon, while other impurities in the iron and scrap bond to the lime to form steel slag. All the heat needed for the process comes from the molten iron (which enters the furnace at 2500°F) and the chemical reactions of the oxygen. In fact, “you add the scrap to keep it cool,” said Ewing of U.S. Steel.
To make cold-rolled steel—thin sheets that are later shaped into steel studs and joists—hot steel is drawn out of the furnace in a long slab. These slabs are rolled out into thinner sheets, after which the surface of the steel is treated (pickled) in a bath of sulfuric acid or hydrochloric acid. After the steel has cooled, it is rolled to the actual finish gauge of the metal.
Over the last two decades an increasing number of steel mini-mills have bypassed much of the process described above by remelting and forming old steel into new. These mini-mills use electricity to melt the steel scrap in electric arc furnaces. Unlike the large integrated plants, however, mini-mills do not have the machinery required for hot and cold rolling of light-gauge steel. As a result, nearly all their output goes into more massive steel products including rebar and steel plate used to make structural I-beams and H-beams.
This situation is changing, however. A revolutionary new Nucor, Inc. steel mill is forming the molten steel from its electric arc furnace directly into sheets, bypassing the complicated rolling process. Although Nucor’s sheet steel has surface imperfections that make it unusable for automotive or appliance skins, it is perfectly appropriate for light-gauge framing. Managers at Nucor couldn’t identify the end uses of their material, but Ewing estimates that Nucor currently supplies as much as 5% of the steel used in residential construction. The fraction of steel framing materials coming from these mini-mills is expected to increase as more plants using this sheet molding technology go on-line.
Although a 66% recycling rate for steel is often quoted by the industry, this figure is somewhat misleading for several reasons. First, it includes steel scraps that never leave the mill. These “home” scraps are commonly reused as a matter of course in almost any industry and are not considered recycled in most lifecycle analyses, nor are they included in the definition of “recovered” materials in the Resource Conservation and Recovery Act. Second, it includes steel scrap that is collected in the U.S. and exported. While this material is recycled, it does not return as recycled content in products manufactured in the U.S.
The average recycled content of American steel is approximately 46% (see Table 4).
The recycled content of particular steel products depends on the manufacturing process used. While heavy steel items made in mini-mills often have as much as 91% recycled content, the sheet steel used for steel framing contains an average of 24%. Most steel framing actually contains about 20% recycled material, but a small fraction (from the Nucor sheet-steel plant) contains about 91%. As more mini-mills begin producing sheet steel, the average recycled content of steel framing materials will increase significantly.
Another development that is likely to change the environmental picture for steel is an experimental new direct steel making technology. This process bypasses the traditional blast furnace production of pig iron, instead producing steel from iron ore in one step. Direct steel making requires less energy and utilizes raw materials more efficiently. Although several companies are working on this technology, none is yet producing steel commercially in this way, according to Ewing.
Sheet steel for framing is hot-dip galvanized. The rolled steel is coated as it goes through a bath of molten zinc. Steel framing members are typically 3% to 5% zinc by weight.
Metallic zinc for galvanization is made from zinc ores by smelting, which can be done with an electrolytic or a pyrometallurgical process. The AIA’s
Environmental Resource Guide (ERG) reports that wastewaters from zinc smelting facilities can contain a number of heavy metals including cadmium, toxic organics, and chlorinated compounds (from reactions with the chlorine present in the water supply). According to the ERG, “a number of zinc smelter facilities have been listed as Superfund sites.”
Although supplies of zinc ore in the U.S. are plentiful, metallic zinc is frequently imported, due a to shortage of domestic smelter capacity. George Vary of the American Zinc Association (AZI) cites environmental problems as one of the reasons that smelters in the U.S. have closed down, but he blames the problem on the nature of multimetal smelting operations that were once common. Currently zinc smelters in the U.S. produce only some cadmium as a by-product, according to Vary. About a third of U.S. consumption is produced domestically, with the rest coming from Canada (another third), Mexico, and other countries.
Of the zinc produced domestically, about a third comes from reclaimed or recycled materials. Residues from galvanizing operations are a major source of reclaimed zinc, as is brass scrap. Also, when galvanized steel is remelted as scrap, the zinc evaporates (along with most other contaminants) and is largely captured by collectors with the furnace dust. Figures for the U.S. are unavailable, according to AZI, but throughout the industrialized world about 8% of reclaimed zinc is extracted from such furnace dust.
Although wood is renewable, traditional harvesting practices have raised many environmental questions.
Forests that are managed primarily to maximize timber supply tend to perform poorly in terms of wildlife habitat, recreational uses, and even global climate control. Because timber comes from forests that cover a very large area, such sacrifices of the other functions of forests can have broad and severe impacts. Additionally, significant overcutting in the 1970s and 1980s led to shortages in timber supply in some areas, leading some to question whether U.S. timber demands can be met in a sustainable manner.
In those rare (but increasing) instances where lumber is available from sources with a certified commitment to sustaining healthy forests for multiple uses, such lumber may well be the most environmentally attractive building material available. The low energy requirements for processing it into lumber and the minimal pollution from such processing are big advantages.
Engineered wood products are providing viable alternatives to solid lumber for some applications. By using fast-growing and underutilized species, they avoid many of the forestry concerns of solid wood products, and they tend to be more stable and uniform in quality than new lumber. However, the manufacturing processes for these products have environmental drawbacks such as increased processing energy and use of fossil-fuel-derived binders. Also, engineered products are still too expensive to replace most lumber in a standard house.
The steel industry in the U.S. has made tremendous strides in improving its environmental performance over the past fifteen years. According to data from Scientific Certification Systems (SCS), since the early 1980s CO2 emissions have dropped by more than 28%, and SOx emissions, responsible for acid rain, have been cut by nearly 95%. Dr. Stanley Rhodes, president of SCS, is leading the study, which includes a detailed lifecycle analysis of the entire steel industry. This study is scheduled for release by the end of the year. A companion SCS study on the renewability of the wood resource, commissioned by the Western Wood Products Association (WWPA), is due out at the same time.
From the perspective of a full lifecycle analysis, the ease with which steel can be recycled is an important advantage. Rhodes argues that the significance of initial impacts from iron ore mining and coke production is greatly diminished when you recognize that the resulting steel can be reused indefinitely. Even with a high level of recycling, there are significant energy factors to consider in the collection and remelting of the scrap, but recycling dramatically improves the overall picture for steel.
Although a full lifecycle analysis for lumber production is beyond the scope of the SCS study, Rhodes feels that there are significant forest management problems that the timber industry must address. “It’s on the resource end that wood has enormous problems,” he says. “Once you get past the forestry issue there is nothing to [worry about in relation to] wood.”
In the overall picture the thermal performance question looms large, because resource and embodied energy costs are paid just once for a material, while operating energy for a building is expended year after year. The combined heating and cooling loads of typical American houses—48 to 90 million Btus per year (according to the EPA)—is comparable to the embodied energy of the framing materials (53 million Btus for steel and 42 million for wood, from Table 3). Thus, over a number of years the environmental impacts of the heating and cooling energy will dwarf the energy impacts of material manufacturing.
Looking at the full lifecycle, “the thermal issue is extremely important,” Rhodes points out. If it isn’t resolved for steel framing, it will “take steel right off the table.” Rhodes sees the recent tests on the advantages of insulating sheathing as encouraging. Including foam insulation as part of a wall system, however, means that all of the lifecycle impacts of that foam must be factored in when comparing the two framing materials. Those impacts could be significant, since most foam insulations are made with HCFCs, which contribute to ozone-depletion and global-warming, and all are fossil-fuel derived.
Both on the construction site and during demolition, the ease with which steel can be recycled is another important advantage. Markets and stable prices for steel scrap are virtually guaranteed, and in many cases even the zinc in the coating is reclaimed from the steel furnace dust. By comparison, efforts to promote recycling of wood waste are not yet very effective in most places. Wood treated with arsenical preservatives (CCA, ACA, and ACZA) presents a serious disposal problem.
There are no perfect materials. Wood and steel each have their drawbacks, both in terms of environmental impacts and performance as framing members. For wood, the main area of concern is in the harvesting and forest management practices. Unless these can be dramatically improved, wood will become less and less viable and a material for many uses. For steel, the heavy industrial manufacturing processes have impacts that must be considered, though with ongoing improvements and the high rate at which scrap steel is recycled, those impacts will diminish. Most important, however, the thermal performance issues must be addressed. While exterior foam insulation is one solution, there may be others that increase the envelope’s performance without using more energy-intensive and environmentally harmful materials.
Increasing and preserving the ecological value of our forests is an important goal. If this goal can be met while still producing lumber from those forests, it is a good material to use. If we choose to replace that lumber with a material like steel, we should take advantage of steel’s strengths and work around its weaknesses. The wood-framing model, in which studs and joists provide both the structural strength and the insulation cavities, may not be the best way to use steel. If you’re considering switching materials, stay open to the possibility of changing framing systems as well.