Volume 15, Issue Number 1 May 12, 2009


Wrapped In Stainless Steel: Sustainable Curtain Walls And Roofing

by Catherine Houska, CSI

Table of Contents
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An exterior view of Beijing's Poly Plaza. Photo courtesy A. Zahner Co.
When stainless steel is properly selected and maintained, it can remain attractive for centuries. After this long service life, stainless steel's high scrap values and low corrosion rates ensure its end-of-life recapture and recycling rates are very substantial, making reuse possible. This article explains the material's use in sustainable roofing and curtain wall projects.

Interest in sustainable design has grown significantly around the world. Consequently, material comparisons for exterior applications now frequently include recycled content, durability, maintenance requirements, and impact on energy and water consumption.1 When these analyses are done, stainless steel consistently garners high marks, particularly in structures designed for thirty or more years of service.

Stainless steel limits negative impacts on the environment in many ways. Bare stainless steel panels produce no emissions, and roofing, curtain walls, and sunscreens can be used to minimize building energy use. Further, cleaning stainless steel to restore its appearance typically requires neither environmentally hazardous nor dangerous chemicals. Older buildings such as the Chrysler and Empire State Buildings have been restored to their original appearance with a mild detergent and water solution and fine, environmentally neutral abrasive powders.

As there is a direct correlation between a material's corrosion resistance and its long-term sustainability, careful selection and specification are important. Corrosion can lead to aesthetic or structural failure, necessitating premature replacement. When any material has to be replaced, the building's total environmental impact is substantially increased. Material mass lost from corrosion has to be replaced with new production and that increases total energy and emissions impact.

Environments with higher pollution levels, de-icing or coastal salt exposure, and/or acid rain accelerate deterioration of most common construction materials. For example, the corrosion rates of copper and aluminum are typically 10 to 100 times that of stainless steel in environments with salt and pollution exposure.2

When stainless steel is properly selected and maintained, it can remain attractive for decades, even centuries. After this long service life, stainless steel’s high scrap values and low corrosion rates ensure its end-of-life recapture and recycling rates are very substantial, making reuse possible.

Recycling
Stainless steel producers use as much recycled content as possible in making all of the metal’s product forms, but the material’s long service life (typically 20 to 30 years) limits scrap availability. Consequently, the industry uses both scrap stainless and carbon steel in its production. In 2002, the International Stainless Steel Forum (ISSF) estimated typical worldwide recycled content to be about 60 percent.3

The Specialty Steel Industry of North America (SSINA) reports the average post-consumer recycled content of 300-series stainless steels, which are the most commonly used stainless steels for architectural applications, is between 75 and 85 percent in North America.4 Stainless steel is 100-percent recyclable with no down-cycling, so it can be indefinitely reused to produce more stainless steel.

Designers should also consider the probability of a component being recycled or recaptured at the end of its service life so building waste is diverted from landfills. Several factors must be considered when comparing materials. Some construction materials can only be diverted into secondary, less-demanding applications. If this latter application is not recyclable, then the material will ultimately be sent to a landfill or incinerated. In other words, initial recycling only delays an inevitable fate. It can be more environmentally responsible to employ materials that may indefinitely be used to manufacture more of the original product.

The other factor to consider is corrosion loss to the environment. If a significant percentage of the construction material corrodes away, it will be lost into the environment and must be replaced with virgin materials. Stainless steel has virtually no metal loss during its service life, so there is no negative environmental impact associated with metal deterioration.

Due to its high scrap value, more than 80 percent of stainless steel is diverted from landfills and recycled into new stainless steel. Designers can increase end-of-service recapture rates by using designs facilitating component material separation. For example, combining different metals in a composite panel design could limit recycling if the materials cannot be easily separated at the end of service life. In comparison, this is not an issue with single skin panels.

Energy reduction
Stainless steel roofing and wall panels can help reduce building energy requirements and urban heat islands (UHIs). Bare stainless steel finishes can meet the solar reflective index (SRI) heat island reduction requirements for steep-sloped roofs in the U.S. Green Building Council's (USGBC's) Leadership in Energy and Environmental Design (LEED) program. (Sustainable Sites [SS] Credit 7.2, Heat Island Effect—Roof, requires SRI greater than or equal to 29 for a slope > 2:12.)

For example, this author knows of popular, proprietary, low-reflectivity, bare stainless steel finishes that have SRI values between 39 and 45 at moderate wind levels and between 55 and 61 at higher wind levels when applied to austenitic (e.g. Types 304 and 316) and lean duplex stainless steels. Polished finishes—such as a No. 4 sheet finish under ASTM International A 480, Standard Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip—have even higher SRI values. One proprietary variation on a No. 4 finish has an SRI value of 60 at moderate wind levels.

The same high-emissivity and SRI-value proprietary coatings applied to aluminum and galvanized steel can also be used with stainless steel. As long as the metal's surface is smooth, there is no significant difference in coating SRI value from one metal to another. Therefore, stainless steel with a proprietary high SRI value coating can also meet LEED’s low-sloped roof requirements (i.e. SRI > 78). The primary benefit of using coated stainless steel relative to other metals is the significantly higher corrosion resistance of the substrate, which increases service life in demanding environments with higher pollution and salt exposure levels and minimizes reliance on coating maintenance.

This makes stainless steel’s performance similar to that of other uncoated metals and substantially better than that of some common roofing products (Figure 1). Further, stainless steel's low corrosion rate helps retain SRI values over time (with occasional cleaning to remove dirt buildup). Since these values vary with the stainless steel finish and alloy, product-specific data should be requested from manufacturers. As with all bare metal finishes, SRI data is generated for low, medium, and high wind levels.

Figure 1
Solar Reflectance and Thermal Performance of Roofing*
Product Solar Reflectance Index (SRI)
Galvanized steel, new bare 46 percent
Aluminum, new bare 56 percent
Metal, proprietary white coating 107 percent
Clay tile, red36 percent
Concrete tile, red17 percent
Concrete tile, white90 percent
Asphalt, generic white26 percent
Asphalt, generic blackOne percent
Wood shingle, brown stain 22 percent
*Source: Lawrence Berkeley National Laboratory's (LBNL's) Cool Roofing Database

Coil-coated stainless steel is produced by several firms and, to this author’s knowledge, they all use Kynar coatings. (For example, Malaysia’s Kuala Lumpur Airport roof is Kynar coated Type 316 stainless steel.) As mentioned, when the Kynar (or a competitive high-SRI coating is applied) the substrate metal is irrelevant as long as the surfaces are equally smooth. It is simply a matter of specifying a high-SRI coating when ordering coil-coated stainless steel.

The initial cost of using stainless steel is higher, but for projects seeking sustainable design goals, lifecycle cost should take precedence. In severe environments with industrial pollution and/or salt exposure, coating life is shorter and galvanized steel life is dependant on rigorous coating maintenance over time. For example, this author is aware of a painted galvanized steel roof that perforated in three years in a Texas coastal location with significant industrial pollution exposure. (It has since been replaced with stainless steel.) Site conditions, building design life, and probable maintenance should be significant factors in material selection. There are appropriate applications for every type of metal.

Protecting the environment
Numerous studies examine runoff from a wide variety of roof materials (e.g. asphalt, metal, tile). The primary purpose has been to determine whether the runoff is potentially toxic to humans, plants or wildlife, but it is also an important consideration when water will be captured for potable use.

A Swedish study compared stainless steel to other metal roofing materials, using rain acidity levels representative of Stockholm’s relatively low pollution levels and moderate coastal exposure (Figure 2).5 The goal was to determine atmospheric corrosion’s influence on roof runoff levels, bioavailability, and eco-toxicity.

Figure 2
Swedish Metal Roof Runoff Study
MaterialAverage annual runoff
Zinc*1900-2500 mg/m2(1588-2090 mg/sq. yd)
Type 304 stainless**
Nickel0.12-0.52 mg/m2(0.10-0.43 mg/sq. yd)
Chromium 0.18-0.57 mg/m2(0.15-0.48 mg/sq. yd)
Iron 10-140 mg/m2 (8.40-117 mg/sq. yd)
* In the form of galvanized steel and zinc sheet.
** In many samples, nickel and chromium levels were below detectable limits. The average concentration per liter was well below typical drinking water levels.

The runoff rates for nickel and chromium from stainless steel were extremely low and, in many samples, below detectable limits. All samples were well under typical drinking water concentration limits. These tests suggest nickel and chromium are released from stainless steel roofs at such low rates they do not cause eco-toxicity. The zinc runoff levels were much higher. Many specifiers do not realize zinc can be a potential biocide.

Metals are also found in the runoff from other common roof materials. Figure 3 summarizes data from studies done in Australia, Washington, and Oregon for several roof materials.6 Eco-toxicity is possible as water concentrates during dry periods. Therefore, some roofing materials are not suggested for environmentally sensitive locations.

Figure 3
Metal Runoff from Various Roofs in Australia, Oregon, and Washington (concentration in micrograms/liter)
Roof typeCopperLeadZinc
Rusty galvanized2030212,200
Plywood w/tar paper11101980
Tar roof w/aluminum paint2510297
Galvanized ironND*1003600
Concrete tileND901600
Concrete tileND50200
*not determined

The U.S Environmental Protection Agency (EPA) sets maximum contaminant level goals and secondary guidelines for drinking water.7 Stainless steel roofing's extremely low runoff levels make it suitable for both environmentally sensitive areas (e.g. wetlands or a low-water-replacement-rate harbor with a fragile environment) and locations where runoff will be used as a source of potable water. Biocides can kill plant life, fish, and crustaceans.

Long-term performance
Although stainless steel is a relatively new construction material, it has had a tremendous impact on international design. There are many high-profile structures where stainless steel has already provided 40 to 80 years of service without deterioration in appearance or requiring metal replacement.

When properly selected, fabricated, and maintained, stainless steel should last the life of the structure, even if that life extends over centuries. This extremely long life makes stainless steel an attractive, cost-effective and environmentally friendly choice for long-term, potentially iconic designs. Stainless steel is in the European, Australian, and Japanese structural design codes and is included in widely used international standards such as ASTM.

The stainless steels most commonly used in architecture are:

  • Types 304/304L (UNS S30400/S30403, EN1.4301/1.4307, SUS 304);
  • Types 316/316L (UNS S31600/S31603, EN 1.4401/1.4404, SUS 316); and
  • 2205 (UNS S32205/S31803, EN 1.4462, SUS 329J3L).

Numerous articles and industry association brochures can be very helpful in stainless steel selection.8 Types 304 and 316 provide identical strength levels and are easily formed and welded into virtually any architectural product. The former is appropriate for most indoor and mild outdoor applications with low levels of urban pollution, while the latter is the most commonly used stainless steel for low to moderate coastal and de-icing salt exposure applications and/or moderate industrial or higher urban pollution levels. Type 304 can be used for applications with de-icing or coastal salt exposure, as long as a smooth finish is specified and regular cleaning removes aesthetically unacceptable surface corrosion staining.

In some very aggressive coastal locations, more corrosion-resistant stainless steels than Type 316 or frequent cleaning should be considered when surface staining is unacceptable. This includes applications subjected to regular splashing or at least occasional immersion in saltwater. Other particularly corrosive locations include coastal areas with very little rainfall and those with frequent salt fog or light, misty, high salt concentration rain. All these circumstances can lead to very high salt concentrations on surfaces and surface corrosion staining of Type 316. Locations with a combination of high industrial pollution levels, acid rain, and coastal or de-icing salt can also be quite destructive.

Duplex stainless steels are significantly stronger than Types 304 and 316 and common structural carbon steels. The most commonly used duplex stainless steel, 2205, offers a significant increase in corrosion resistance over Type 316 and should be considered for locations subjected to high levels of industrial or urban pollution or significant salt exposure, particularly if there is less frequent maintenance cleaning. While these types of stainless steel are most often employed, many others could be suitable.

In the severest locations, even 2205 may not remain stain-free and a more corrosion-resistant stainless steel may be necessary to meet aesthetic requirements unless regularly cleaned. If there is any doubt, the advice of a stainless steel atmospheric corrosion expert with architectural experience should be sought.

Smoother surface finishes (i.e. Ra 20 micro-inches [Ra 12 microns] or less) accumulate fewer corrosive deposits (i.e. salt or pollution) and their specification improves corrosion performance and minimizes the possibility of unattractive staining. Regular cleaning to remove corrosive deposits helps prevent surface staining. In salt-laden environments, it is important to seal Types 304 and 316 mechanical joints using welding or good quality construction sealants to prevent crevice corrosion.

Case studies
Most designers are familiar with older stainless steel curtain wall and roof applications that have performed well. However, recent examples of current innovative and cutting-edge curtain wall and roof applications illustrate how today’s designers are using stainless steel’s advantages in sustainable design.

Wayne L. Morse United States Courthouse
Federal courthouses are designed to provide maximum security and safety to building occupants and users and provide long service life. DLR Group (Portland, Oregon) was selected as the architect of record for the Wayne L. Morse United States Courthouse in nearby Eugene. They partnered with the design architect, Morphosis (Santa Monica, California), to create a sustainable Level IV security facility that emphasizes government and community interaction.

The 24,805-m2 (267,008-sf), five-story building houses both courts and government offices. Its design made extensive use of high-recycled content materials, including stainless steel detention equipment, furniture, and exterior skin. The Type 304 stainless steel exterior wall panels covering 6500 m2 (70,000 sf) should last the life of the building and require minimal maintenance, making it a sustainable choice.

This view of the Wayne L. Morse US Courthouse entrance and corner details show the soft glimmer of the Angel Hair finish on Type 304 stainless steel. Photo courtesy A. Zahner Co.
Further, the fine grain of the proprietary, non-directional 'Angel Hair' finish minimizes dirt retention and cleaning frequency while softly reflecting its surroundings.

Eugene is an inland city so there is no coastal salt exposure and pollution levels are low. With less than 178 mm (7 in.) of snowfall a year, de-icing salt exposure is minimal, making Type 304 stainless steel an appropriate choice. The Wayne L. Morse Unites States Courthouse was awarded LEED Gold in 2006.

This interior view of Poly Plaza's large cable-net wall and rocker system illustrates the simple elegance of this cutting-edge design. Photo courtesy A. Zahner Co.
Poly Plaza
China Poly’s new headquarters in Beijing, China was designed by Skidmore Owings & Merrill (SOM) to establish a civic presence for this state-owned company that would be reminiscent of Rockefeller Center in New York. In addition to the company’s headquarters, the 100,000-m2 (1 million-sf) building houses office space, retail shops, restaurants, and the Poly Museum. It is a simple, monolithic triangle with two distinct cable-net walls facing south (50 m [164 ft] tall) and northeast (90 m [295 ft] tall). The northeastern cable-net wall is one of the largest in the world.

A conventional design using large trusses would have disrupted the view of the city. Instead, the wall is supported by an innovative V-cable counterweighted by the museum structure using a specially designed pulley mechanism that compensates for movement during a seismic event. The glass and stainless steel wall was also designed to withstand 100-year winds and can deflect up to ± 0.9 m (3 ft) under maximum wind load.

Stainless steel was selected because of its high strength and corrosion resistance. Beijing has a corrosive environment with high levels of industrial pollution and there has been significant increase in winter de-icing salt use. Specification of corrosion-resistant stainless steels made it possible to avoid high-maintenance coatings and the bare stainless steel is a sculptural, structural design element.

The cable-net walls are supported by 26-mm (approximately 1-in.) vertical and 34-mm (1.3-in.) horizontal Type 316 stainless steel cables. The cable net intersection points are connected with high-strength, custom cast duplex 2205 stainless steel clamp fittings. The rods between the main cable and the cable net are Type 316 stainless steel. The support armature is a high-strength duplex 2205 stainless steel casting with a glass bead-blast finish. The exterior plate is slotted onto the interior plate and bolted through the glass-to-glass joint.

Early design calculations indicated large deflections would occur during high-wind loads due to the movement of the cables and glass. SOM addressed this problem with a hinged stainless steel glass support channel that allows free rotation of up to seven degrees without applying stress into the glass. The hinged channel is connected back to the cable net and held off the diagonal bridge cables by high-strength duplex 2205 stainless steel rods, which are allowed to rotate in their assembly.

University buildings
Academic buildings are designed for longevity and low maintenance, and sustainability is an increasingly common design requirement. This has made stainless steel a popular choice for interior and exterior applications. Portland-based Zimmer Gunsul Frasca Architects (ZGF) regularly uses stainless steel in its designs, including three recent university buildings.

The stainless steel cladding and roofing on Pacific Lutheran University's Morken Center for Learning and Technology (Olympia, Wash.) helped the project achieve its LEED Gold Certification. Photo courtesy Zimmer Gunsul Frasca Architects.
ZGF has designed several buildings for Pacific Lutheran University in Olympia, Washington, including the 4900-m2 (53,000-sf) LEED Gold-certified Morken Center for Learning and Technology, housing the School of Business and the Mathematics and Computer Science and Computer Engineering departments. The University and ZGF kept LEED standards in mind throughout the design and building process and set a goal of a 100-year life span.

Morken Center's low-maintenance brick and stainless steel exterior has a very modern look, but fits in well with the university's older early-1900s buildings. The design makes extensive use of renewable and materials with high levels of recycled content, such as stainless steel roofing and cladding.

Pacific Lutheran University’s Morken Center’s low-maintenance brick and stainless steel exterior has a modern look. Photo courtesy Zimmer Gunsul Frasca Architects.
Considerable attention was paid to reducing the building's operating costs. The Type 304 stainless steel tile roof and cladding are 'heat-neutral,' so they do not contribute to the urban heat island effect and minimize energy requirements. They are also low maintenance and will last the life of the building. Type 304 stainless steel was an appropriate choice for this inland temperate location.

Although it is not LEED-certified, the new Natural Science and Engineering Research Building (2006) at the University of Texas in Dallas was designed to be a sustainable, energy-efficient building using materials that provide long service life and low maintenance.

The Type 304 electrochemically colored shingles on the University of Texas' Natural Science and Engineering Research Building help increase its sustainability. Photo courtesy Zimmer Gunsul Frasca Architects.
The 22,000 electrochemically-colored Type 304 stainless steel shingles that cover a significant percentage of the building's exterior helped designers meet these goals while creating a visually striking exterior. This coloring method is not affected by ultraviolet (UV) radiation and the hues will remain unchanged over the building's life with minimal maintenance. Type 304 is appropriate for this inland location with moderate pollution and minimal or no de-icing salt exposure.

Kaneko Residential Commons was the first project at Willamette University in Salem, Oregon after the Board of Trustees endorsed green building practices for all new construction and renovation projects. Built in 2006, it consists of a substantial residential suite and apartment addition to the old Kaneko Hall and enlarged the dining facility, along with meeting and activity spaces. ZGF's LEED Gold-certified design minimizes energy and water requirements and used materials with high recycled content that would provide long service life and minimize maintenance.

The embossed stainless steel tiles on the exterior of Kaneko Residential Commons helped the building achieve its LEED Gold Rating. Photo courtesy Zimmer Gunsul Frasca Architects.
Embossed Type 304 stainless steel exterior tile cladding was selected because of its high-recycled content, longevity, and low maintenance. The stainless steel panels also help minimize building energy requirements. Type 304 was an appropriate choice for this inland, moderately corrosive location with minimal winter de-icing salt exposure.

Jamaica Station
Jamaica Station in the Queens section of New York City was significantly expanded in 2006 to link a new light rail system from JFK International Airport to the Long Island Rail Road, New York City Transit’s subway lines, and ground transportation. The addition is a seven-floor, 2323-m2 (250,000-sf) structure.

As an important gateway to the city, the design had to be attractive and functional; however, durability is always critical for transit buildings. The building features a glass and aluminum curtain wall, terrazzo floors, marble, granite, and stainless steel for demanding interior applications and the roof panels.

The Port Authority of New York and New Jersey served as the project’s architects. The terminal’s large stainless steel batten seam roof has joints carefully designed to allow controlled expansion and contraction in all directions. It also has high wind upload resistance. An internal drainage system was necessary because of the roof's size and shape.

Numerous design considerations guided roof material selection, including:

  • Proximity to the airport;
  • Coastal location;
  • Moderate pollution levels; and
  • Need for longevity and low-maintenance.

The roof has a textured, low-reflectivity surface of durable Type 316 stainless steel so pilots are not blinded. Photo courtesy Contrarian Metal Resources.
Any large roof near an airport must have a low-reflectivity surface finish so pilots are not blinded. Type 316 stainless steel was selected because it provides the desired corrosion performance and durability. Long-term maintenance was minimized by avoiding the need for a protective paint coating, which would deteriorate over time due to jet fuel residue, pollution, and coastal salt exposure.
The Jamaica Station’s addition shows its large, low-maintenance Type 316 stainless steel roof. Photo courtesy Contrarian Metal Resources.

A proprietary, textured, non-directional, low-reflectivity finish was specified for the project because of its performance on similar projects.

Conclusions
Products manufactured from stainless steel are an excellent choice for protecting the environment and creating comfortable, attractive, energy efficient structures. It can be a cost-effective, environmentally responsible option for applications where low-maintenance and a long service life are desired.

Stainless steel is also suitable for fragile ecosystems where environmental contamination must be avoided or where runoff water will be recaptured and reused. However, as with all building components, performance depends on selecting an appropriate material, finish, and design.

Notes
1 This author would like to thank the International Molybdenum Association (IMOA), the Nickel Institute, A. Zahner Co, Skidmore Owings & Merrill, Contrarian Metal Resources, and Zimmer Gunsul Frasca for their assistance in the preparation of this article. 2 See the Nickel Development Institute's publication 11 024, Stainless Steels in Architecture, Building and Construction: Guidelines for Corrosion Prevention, a 2001 document written by this author. 3 See Recycling Stainless Steel, available from International Stainless Steel Forum (ISSF). Visit www.worldstainless.org. 4 For more information, see Specialty Steel Industry of North America (SSINA)'s "LEED Fact Sheet: Stainless Sheet & Plate for the Building & Construction Market." Visit www.ssina.com. 5 See C. Leygraf and I.O. Wallinder's "Environmental Effects of Metals Induced by Atmospheric Corrosion," in ASTM International Standard Technical Publication (STP) 1421, Outdoor Atmospheric Corrosion. 6 See Watershed Protection Technologies' Technical Note #25, Is Roof Runoff Really Clean?, 1(2): 84—85. 7 Visit www.epa.gov/safewater/contaminants/index.html. 8 For further background, see various papers and articles written by this author. They include "Which Stainless Steel Should be Specified for Exterior Applications?" (International Molybdenum Association [IMOA]), "Stainless Steels in Architecture, Building and Construction: Guidelines for Corrosion Prevention" (Nickel Institute), "Stainless Steel Selection for Exterior Applications" (the January 2003 issue of The Construction Specifier) and "Architectural Metal Corrosion: The De-icing Salt Threat" (the December 2006 issue of The Construction Specifier). Also, visit www.stainlessarchitecture.org and www.imoa.info.

Additional Information
Author
Catherine Houska, CSI, is senior development manager at TMR Consulting. She is a metallurgical engineering consultant specializing in architectural metal selection, specification, and failure analysis and the author of over 100 publications. Houska can be reached via email at chouska@tmr-inc.com.

MasterFormat No.
05 13 00—Structural Stainless-Steel Framing
07 41 00—Roof Panels
07 61 00—Sheet Metal Roofing
08 44 19—Glazed Stainless: Steel Curtain Walls

UniFormat No.
B1020—Roof Structural Frame
B1020—Roof Decks, Slabs, and Sheathing
B2020—Glazed Curtain Wall

Key Words
Divisions 05, 07, 08 Curtain wall Roofing Stainless steel Sustainability

Editor's Note
This article originally appeared in the August 2008 issue of The Construction Specifier, the official publication of the Construction Specifications Institute. Visit www.csinet.org.


Editor: Rosalind P. Raymond rraymond@smacna.org  |  Asst. Editor/Writer: Cynthia Young cyoung@smacna.org

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