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Sustainability 104: doing glass walls right

Glazed curtain wall
Glazed curtain wall
Photo by Rick Zimmerman

Since many of today’s newest urban structures are designed to be swathed in broad expanses of glass, it’s reasonable to seek sustainability and energy savings in glazing and fenestration (window) systems. And, since commercial buildings across the nation typically consume nearly half of all the energy of our built environment, there exists a sizable ‘base’ of energy use from which we can glean savings as payback for initial investment. Further, much of the energy consumption of commercial buildings — whether for heating, cooling, lighting or air tempering — can be affected by buildings’ exterior envelopes. Glass walls done right can be the economic gift that keeps on giving, year after year.

In recent years, the most widely used benchmark for sustainability has become the LEED (Leadership in Energy and Environmental Design) standards of the U.S. Green Building Council (USGBC). To attain LEED certification, various ‘points’ are scored for each different sustainability performance. That performance often hinges on the type and quality of a particular building component, whether it be green roof or sun shading or water reclamation system, and so on. Glazed assemblies for exterior walls are just such components.

There are a variety of ways in which glazed assemblies for exterior walls gain sustainability ‘points’. These include their ability to: provide daylighting and natural views to occupants; provide increased or improved natural ventilation; provide effective natural lighting while optimizing heat gain/heat loss; be sourced from local suppliers and material sources; and be recycled, repurposed or otherwise reused at the end of their initial life cycle.

Being ubiquitous, glazed wall assemblies are influenced or controlled by many different codes, regulations, industry standards, agencies and advisory groups, ranging from the U.S. Department of Energy, through local building codes, to component manufacturers, trade groups and utility companies. Currently, all but a dozen states in the union apply state energy codes that meet or exceed the most widely accepted industry standard of energy efficiency: ASHRAE Standard 90.1-2004, developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. By 2010, that group had tightened that standard by almost 30% — meaning performance efficiencies were required to be 30% more efficient — with an expected further tightening to 50% within the next few years. To best fine-tune energy efficiency to local climate conditions, the Department of Energy has subdivided the nation into eight different climate zones, ranging from Miami to the Yukon. Meanwhile, progress in energy efficiency of glazing systems in Europe and the UK are several decades more advanced than the American marketplace.

The first and most direct place to create a more sustainable glazed wall is at a thermal break in its framing: a gap that prevents thermal transfer from interior framing components to exposed exterior framing components, and vice versa. Conduction of heat through solid materials of a framing system can account for up to half of all heat transfer, so there’s much to be gained by thermal breaks. Almost all window and glazing manufacturers have available thermal break systems and components. Thermal breaks may be achieved by air gaps or by thermal barrier materials (such as polyurethane or polyamide nylon) having low conductivity. Since polyamide nylon has a coefficient of thermal expansion and contraction similar to that of the aluminum of which much framing is made, the two materials make an ideal marriage.

Glazing systems can also minimize the other two methods of heat transfer — convection from air circulation in framing cavities, and radiation from warm surfaces to cold — by employing cavity breaks and warm edge spacers.

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