The following article was first published in Nickel, the magazine devoted to nickel and its applications.
Sustainable Benefits of Austenitic Stainless Steel Roof Includes Energy Savings
The roof over your head does more than keep the elements out. Properly insulated, it also keeps heat in during the winter months and out in summer. Nickel-containing S30400 stainless steel is a poor conductor of heat and therefore can help to insulate a roof and make a building more energy-efficient. Architects who choose it as a roofing material may soon be able to take advantage of this and other properties of stainless steel in the Leadership in Energy and Environmental Design (LEED) certification system.
When the David L. Lawrence Convention Center in Pittsburgh, Pennsylvania, opened in 2003, it was the largest building in the United States to earn a Gold Certification under the LEED system. The Gold LEED status recognizes the centre’s brownfield redevelopment, accommodation of alternative transportation, reduced water use, efficient energy performance, use of materials that emit no or low amounts of toxins, and innovative design. Had the sustainable attributes of nickel-containing stainless steel been fully accounted for under LEED, the certification could very well have been platinum, says Catherine Houska, senior market development manager with Pittsburgh-based TMR Architectural Metals Consulting and a consultant to the Nickel Institute.
“The shape of the roof structure is part of the sustainable design initiative that promotes natural ventilation in the interior of the exhibition hall,” says David Roland, project director with Rafael Viñoly Architects PC in New York, N.Y. “Stainless steel was chosen for its aesthetic qualities, durability and low maintenance, and to pay homage to Pittsburgh’s steel heritage.”
Measuring 280 by 96 metres, the roof is sheathed with 23,000 square metres of 0.6-millimetre (24-gauge) S30400 stainless steel (weighing about 136 tonnes). The solar reflectance and thermal emittance values (see yellow box below) of the stainless steel roof are less than those required by LEED in 2003, so the roof did not qualify for a reduced heat island point. Also, the roof was not recognized for the contribution it made to the building’s reduced energy consumption.
The emergence of “green” design reflects a growing awareness that buildings need to reduce their consumption of energy and to use materials that do not pose a risk to the health of their occupants and the surrounding environment. When the U.S. Green Building Council (USGBC) developed the LEED rating system, it was meant to guide and recognize sustainable achievements for “all building types and (to emphasize) state-of-the-art strategies for sustainable site development, water savings, energy efficiency, materials and resources selection, and indoor environmental quality.”
Introduced in 2000, LEED is used today to certify buildings around the world. The rating system addresses several areas of construction, including new construction, existing buildings, commercial interiors, schools, and homes. It also evaluates site planning, water management, energy, material use, and indoor environmental quality. Depending on how many LEED points are earned, a building can qualify for bronze, silver, gold or platinum certification. LEED has evolved since its inception to incorporate maturing sustainable building concepts and technologies, and continues to do so. In May, 2008, USGBC opened the door for public comment on the latest draft version of LEED 2009. This provided the stainless steel industry with a 30-day opportunity to suggest significant amendments.
LEED provides no explicit guidance for the use of stainless steel, though there are three areas where stainless steel can help architects earn points: the re-use of building materials from existing structures, diversion of construction waste from disposal, and the recycled content of “new” stainless steel.
The high recycled content of austenitic stainless steel (ranging from 60 to 85% depending on geographic location) contributes significantly to the aggregate recycled content of materials used in new buildings. The Speciality Steel Industry of North America reports that the average post-consumer recycled content of the 300 series stainless steel grades is about 75-85%. The stainless steel industry could be at least as proactive as other industries in making designers and architects aware of the recycled content of its products, according to Houska.
Another area where the stainless steel industry could play a “LEED role” is in the reduction of “heat islands,” which refer to urban areas that are significantly warmer than their undeveloped, rural surroundings. The current LEED rating system gives points for roofs that meet required Solar Reflectance Index (SRI) values for low- and steep-sloped roofs (see below).
It is commonly thought that only specially painted roofs could meet those requirements, but recent tests funded by Contrarian Metal Resources in Pennsylvania, U.S.A., prove that this is not so: Contrarian’s InvariMatte® finish sheet stainless steel met the SRI requirement for steep-sloped roofs.
The non-directional matte surface finish used on the Pittsburgh Convention Center roof was produced by J&L Specialty Steel Inc., also of Pittsburgh. This non-reflective finish, achieved by abrasive blasting, is called Architex®. Today, a similar product by Contrarian Metal Resources is sold under the brand name InvariMatte®.
The roof sections are supported by cables or cantilevered at the end walls, and form curves like those of the cables on the nearby bridge that spans the Allegheny River.
Skylights at the interior cables allow light to enter the exhibition hall below while revealing the roof structure above.
The roof panels were supplied in 7-metre lengths. They were folded into a continuous extrusion which was fastened to a composite assembly consisting of rosin paper, waterproofing, oriented strand board sheathing, and rigid insulation over a steel deck. The sheets are held in place by a stainless steel batten cap fastened into the extrusion, which acts as a rail to guide expansion and contraction. Any water that migrates into the roof system weeps out of the extrusions at the roof eave or gutter transition, according to James Mersich, manager of roof fabricator Overly Manufacturing Company in Greensburg, Pennsylvania.
At the base of the roof, a large shallow gutter collects all the water runoff. It was fabricated from 1.2 mm (18-gauge) S30403 stainless steel with a 2B (semi-reflective) finish. Large sections were welded together in the factory, then welded together on the job site into 9.1-metre sections between expansion joints.
The snow and ice guard system, which consists of angles, attachment brackets, bolts, washers and nuts made from S30400, keeps snow from sliding over the edge and damaging the gutter or other building components or harming pedestrians.
What is SRI?
When the David L. Lawrence Convention Center in Pittsburgh was constructed in 2003, roofs had to meet minimum solar reflectance (the percentage of energy a surface reflects) and thermal emittance (the percentage of energy a material radiates after it is absorbed) values in order to earn LEED points for reducing heat islands. In 2006, LEED replaced these values with a single Solar Reflectance Index (SRI). In accordance with ASTM E 1980 standards, SRI is calculated from eight different parameters including wind speed, thermal emissivity, solar flux and air temperature.
To earn LEED points, a low-sloped roof (equal or less than 2:12) has to have an SRI of at least 78. Steep-sloped roofs require an SRI of at least 29.
“The real world is more complex than the SRI system suggests,” says James Halliday, president of Contrarian Metal Resources, based in Allison Park, Pennsylvania. “There is a lot more going on than heat bouncing off a roof. A poor thermal conductor like stainless steel insulates and contributes to energy conservation.”
The SRI of stainless steel will also be more sustainable over the life of a building, compared with other roofing materials, because it is so durable and will be completely recyclable at the end of its life.