A Greener Curtain Wall
Steel, concrete, aluminum and glass have been the predominate materials used in large commercial buildings for more than a century. The first steel-framed skyscraper was the Home Insurance Building built in Chicago in 1885. The use of these materials has allowed buildings of today to be tall, lightweight, fire-resistant, and incredibly efficient.
Until very recently, there has not been much of a reason to challenge the use of these materials in buildings. Our ever-growing understanding of the negative impacts our species has had on the environment is forcing a rethinking of everything we do, including the materials we use to build with. Steel, concrete, aluminum, and glass all require a huge amount of energy to produce. And they are used in great quantity in buildings as structure and skin. According to the paper “Life-Cycle Energy Use in Office Buildings,” by Raymond J. Cole and Paul C. Kernan, structure and envelope represent the single largest component of a building’s total embodied energy. As a result, these four materials represent the largest contribution to green house gas emissions in buildings. And this is a big deal. So much so that the current version of CALGreen—the State of California’s Green Building Code—includes provisions for calculating a building’s contribution to climate change (GHG), using either whole building analysis or materials analysis. For the materials analysis path, the code states:
|“A5.409.3 Materials and system assemblies. If whole building analysis of the project is not elected, select a minimum of 50 percent of materials or assemblies based on life cycle assessment of at least three for the impacts listed in Section A5.409.2.3, one of which shall be climate change.”|
Swapping out steel, concrete, glass, and aluminum for most materials and systems is not easy to do. For large commercial buildings, there are few available, or even feasible, alternatives. There is one, however, that shows promise.
A NEW KIND OF CURTAIN WALL
Nearly every large commercial building in the US is clad, in part or in whole, with glazed aluminum curtain wall. A curtain wall system is a lightweight exterior cladding made of framing members (typically aluminum and/or steel) that support glass and metal panels. Curtain wall is hung from the building and supported by the edge of each floor slab. It does not support floors or roof. Aluminum framed curtain wall has been used in the U.S. since the 1930s.
As technological advances in curtain wall have progressed, manufacturers have been offering systems with a wide range of materials and configurations, including the framing, without compromising performance or cost. Framing members are typically made of rectangular extruded aluminum sections, with steel reinforcement added where required. Steel, fiberglass, and PVC curtain wall framing is an option today, with timber and bamboo recently added to the growing list. You read that correctly—bamboo!
Since its formation in 1992, the German company Raico has been redefining how curtain wall works and is incorporated into buildings. It has developed several systems that can be attached to any type of framing without changing performance characteristics. Framing can be of aluminum, steel, wood or—yes, even bamboo. In every case, water penetration, air infiltration, and thermal performance remain the same. This has opened the door of opportunity for innovative companies looking to capitalize on this exciting new alternative.
United States companies CT Windows, IC2 Technologies, and Lamboo now offer Raico curtain wall systems coupled to laminated timber and bamboo framing systems. Replacing aluminum framing members with timber or bamboo can help reduce a building’s total GHG emissions. In the paper “A Comparative Study on Environmental Life Cycle Impacts of Curtain Walls,” by Rahman Azari and Yong-Woo Kim, the authors conduct a Life Cycle Analysis that shows aluminum curtain wall systems have more than seven times more global warming potential than a timber framed curtain wall. The same is true for acidification and eutrophication.
WHERE CAN I GET ONE?
A sample of a timber framed curtain wall was recently dropped off at my firm, and was displayed in the “new and interesting materials” display shelf. The sample generated quite a bit of interest among architects. The manufacturers I spoke with on the telephone are actively and aggressively working to get these systems to market. At the moment, few, if any, systems have been used in the U.S. There are a small handful of examples in Europe (where else?) but none of note in the United States. Why? The reasons are many but the main one has to do with U.S. building codes.
The international Building Code allows the use of wood in all types of buildings, even in non-combustible assemblies. But the code is very prescriptive, and very strict in what it allows—and what it does not. Wood framed curtain wall, while not expressly prohibited in the code, is not expressly allowed either. Getting wood (or bamboo) framed curtain wall on a commercial building in the U.S. is possible, but owners and designers must be prepared to jump through several hoops to achieve the goal. The best contemporary example of a team that was successful in this quest is represented in the exquisitely designed Arena Stage at the Mead Center for American Theater in Washington D.C., designed by Bing Thom Architects. The building uses laminated wood columns 45 to 63 feet tall which support a laminated wood framed glazed curtain wall. To convince skeptical building code officials, the design team used computer modeling to we show that effects of a fire on the structure would be minimal, and there would be plenty of time for safe building evacuation. A char analysis was also done, showing how char that develops on the exterior of the exposed wood members actually protects the interior of the wood during the fire event. Calculations were provided showing that the integrity of the wood members would not be compromised.
Wood use in buildings is being given a second life, thanks to building code revisions and the desire to minimize buildings’ environmental footprint. New technologies such as timber and bamboo-framed curtain wall allow designers to use these materials in place of traditional materials such as aluminum and steel, without any adverse impact on the system’s ability to perform structurally, thermally, and environmentally (water penetration and air infiltration). If manufacturers of these systems can pave the way for designers to convince owners and code officials that these systems are completely viable alternatives, they might start showing up on a building near you!