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The Value Of High Performance Windows Is Very High

Glazed, But Not Confused

by Gregory Franta, FAIA


Superwindows may reduce or eliminate the need for costly traditional heating & cooling systems


In terms of a building's energy performance, windows can be either heroes or villains. When buildings are “designed by climate,” windows can be the major contributor to human needs for heating, cooling, lighting, ventilating, and a connection to the outdoors (through views).

Windows are the main components of fenestration, which also includes skylights, clerestories, and dormers. Glazing refers to the fenestration’s light-transmitting materials, and it is typically glass, fiberglass, or plastic. Glass was first used for windows by the Romans, probably in Pompeii and prior to the birth of Christ. In Venice in the thirteenth century, flat glass technology was developed and windows became popular throughout Europe. Not surprisingly, the Industrial Revolution made windows more economical through mass production.

Even higher-performing windows with double and triple glazing are not new. Thomas Stetson first patented multi-paned windows in the United States in 1865. However, it wasn’t until more than one hundred years later that they became popular. Today, with our modern window technology and design tools, windows can be designed so they optimize building energy performance, as well as just about everything else in a modern home. Yet, this optimization is far from mainstream in the building industry.

The most fundamental climate-responsive design consideration in terms of windows is orientation. In the continental United States, east- and west-facing windows have low sun angles, meaning big solar gains and glare that’s difficult to control. Controlling solar gain and glare through overhangs or lightshelves on south- and north-facing windows is much easier. South-facing windows can provide winter solar heat gain if desirable, and north-facing windows can be great in hot climates.

When the sun’s rays strike glazing, they are reflected, absorbed, or transmitted through the glazing. The portion that is transmitted may be ultraviolet, visible, or infrared light energy. All types will produce heat when they are transmitted through windows, but only about half of the energy is visible. This distinction makes for interesting possibilities, and modern technology allows us to filter out invisible heat or visible light, depending upon our objectives. For example, a glazing that reduces solar heat gains in response to cooling loads may have a low solar heat gain coefficient (SHGC), typically less than 0.26. A glazing with the same SHGC could have a relatively low (less than 20 percent) or high daylight transmittance (greater than 50 percent) to help light the building. This can make a big difference in the amount of glass used on a project and the overall building performance, not to mention blocking 99.5 percent of potentially harmful ultraviolet light. Glass color and spectrally selective low-emmissivity (“low-e”) coatings make this possible.

Another modern opportunity is to drastically reduce heat transfer via a reduction in heat conduction through the window. With interior glass surface temperatures being closer to the interior air temperatures rather than outdoor temperatures, as with conventional glazing, thermal comfort is increased and heating and cooling loads are reduced.

Single glazing (still common in mild climates) has a rate of heat transfer of about U-1.1,1 double glazing is about U-0.5, and triple glazing is about U-0.32, all depending upon the air space size, glass thickness, and other characteristics.

Double glass with a low-e coating on one surface achieves less than U-0.30. Suspended coated (low-e) film (SCF) between panes can drop a window’s rating to below U-0.20. Combining low-e coatings on the inner surfaces of glass, using two SCFs and filling the gas-filled spaces with optimized mixes of krypton and argon gas results in less than U-0.10. Of course, these values are impacted by the spacer and frame performance, so they need to be high-performance as well. An example of a very high-performance window is on my own residence in Boulder, Colorado, where I used clear Alpenglass with fiberglass windows (made by Alpen, Inc., of Boulder) that achieves seven times the performance of thermal pane glazing (U-0.07).

Yes, high-performance windows can seem pricey up front. But to fully assess a window’s worth, one needs to understand how it impacts the entire system’s performance, the construction cost, and the life-cycle cost. Superwindows may reduce or eliminate the need for costly traditional heating and cooling systems and have other important impacts. Integrated design is the key.

1“The rate of heat loss, in British thermal units per hour, through a square foot of a surface (wall, roof, door, windows, or other building surface) when the difference between the air temperature on either side is 1 deg. Fahrenheit. The U-value is the reciprocal of the R-Value."



Copyright 2012 Ronald Sauve All Rights Reserved

This page was last modified on April 06, 2012

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