Energy-Efficient Glazing Basics

Sept. 1, 2009

Choosing the right energy-efficient glazing for your building can reduce operating costs by lowering heating and cooling bills

Windows connect building occupants to the outdoors via daylight, views, and natural ventilation while protecting them from the elements. Or, at least that’s the ideal. Even so, in many older buildings (and even in some newer ones), windows fail to weatherize occupied space and are prone to water leakage, condensation, or noise transmission. The consequences can include high energy bills, occupant discomfort, and water damage. For this reason, window improvements are in high demand; while upfront costs can be a barrier, overall cost savings can be achieved if improvements are made with energy efficiency in mind.

High-performance glass is central to window energy efficiency. Given the long lifespan of glazing and the high cost of retrofits, it’s crucial to understand what’s available in today’s commercial glazing market and the attributes that each product offers.

Three properties are integral to evaluating glazing energy performance: insulating performance (U-factor), solar heat gain coefficient (SHGC), and visible light transmittance (VLT or VT). The ideal properties depend on the local climate, building type, and design. For instance, a low U-factor (less heat loss) is most important in a cold climate; a low SHGC (less solar heat gain) is a priority where overheating is a concern. Visible transmittance is important when daylight is incorporated into the project design.

Low-E coatings can combine the advantages of a reduced U-factor and SHGC while maintaining high levels of visible light transmittance. These microscopically thin layers of metal or metallic oxide can be designed to filter radiation so as to reflect UV and infrared radiation while transmitting visible wavelengths. The latest generation of low-E coatings, referred to as triple silver low-E for their three layers of silver oxide, is the most powerful solution to date for reflecting solar heat without sacrificing daylight. Where too much light transmission may lead to glare, tinted glass used in combination with low-E coatings can provide some glare control while reducing solar heat gain even further.

In climates where cooling load reduction is the priority, reflective coatings offer a tool for maximum solar control. Unlike low-E coatings, reflective coatings reject a high level of infrared solar heat and visible light, allowing building occupants access to views while reducing solar heat gain and glare.

When windows or window glass are replaced, opting for high-performance glazing over standard options comes at a relatively low incremental cost; however, since replacement of the existing glazing isn’t always practicable or affordable, retrofit window films are available with low-E, reflective, or tinted properties. These can be applied to existing glazing. Although they provide only a minor improvement to the U-factor, applied films can be very effective for reducing SHGC.

It’s important to note that glazing energy performance not only depends on the type of coating and/or tint used, but also on the design of the whole window system, including spacers, frame, multiple panes, and possible gas fills. To express the interaction of these components, the National Fenestration Rating Council has developed standards for rating the U-factor, SHGC, and VT ratings of whole windows, including all of their components. These standards allow an apples-to-apples comparison among products and are referenced in building energy codes.

Those who seek a basic assessment of various glazing options can use the Façade Design Tool at www.commercialwindows.org, or download the beta version of COMFEN, a free simulation tool under development by Lawrence Berkeley National Laboratory.

These tools, as well as more comprehensive energy modeling software, take into account the impacts of climate and orientation, and provide valuable information on how high-performance glazing can lower heating and cooling costs, improve occupant comfort, and lower peak demand on HVAC systems.

Paul Bostrom is an associate at the Efficient Windows Collaborative at the Alliance to Save Energy in Washington, D.C.

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