Windows have traditionally provided daylight and fresh air for building occupants and given character to the building’s façade. One of the most important elements in an architect’s palette, windows work to establish the style and rhythm of a structure, and their design and placement are a key aesthetic concern.
Yet windows can surpass these traditional aesthetic and practical considerations and become even more significant partners in creating sustainable buildings. As manufacturers continually improve their products to meet the needs of their customers, architects should be aware of new and emerging technologies in order to specify the most appropriate windows on their projects and realize the potential of windows to enhance sustainability goals.
During the past two decades, advances in glass technology have produced a new generation of materials that offer improved energy efficiency, easier maintenance, lower operating costs and higher performance. This article will define the parameters used in measuring the energy performance of fenestration, and detail how technological advances in the fenestration industry are achieving savings in energy and life cycle costs.
Measuring Energy Performance in Windows
Fenestration—including windows, doors and skylights—has the ability to reduce the heating, cooling and lighting requirements of a building, and has become critical to achieving good energy performance in a structure. It is critical to note, however, that no one window is suitable for every application. A first step is to understand how energy performance is calculated. The main energy characteristics that form the basis for quantifying a window’s energy performance are described as follows:
U Values
Manufacturers usually represent the energy efficiency of windows in terms of their U-values (measure of heat loss). When there is a difference between the ambient air temperatures inside and outside a structure, heat is either lost or gained through a window. The U-value (or U-factor) is a measure of heat escaping the interior of the home. U-value ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a window’s resistance to heat flow, the better its insulating value, and the lower the heating costs. In other words, the lower the U-value the better the energy efficiency of the window. Low U-values are most important in northern regions where outside temperatures are cold and heating costs are traditionally high.
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In product literature, some manufacturers list the U-factor only for the glass itself, rather than for the entire window unit. If it is only for the glass, the U-value may be much better than a rating of the whole-product. Low-E (low-emissivity) and gas fills can provide cost-effective energy efficiency. The lower the U-factor, the greater the energy savings; U-factors between .3 and .4 are optimal.
