Insulated Glass

One of the shortcomings of glass is its relatively poor insulating qualities. Multiple panes of glass with air spaces in between improve the insulating value considerably. Figure 3-5 illustrates the performance of single glazing with clear glass. Relative to all other glazing options, clear single glazing allows the highest transfer of solar energy while permitting the highest daylight transmission.

The inner and outer panes of glass are both clear, and they are separated by an air gap. Double glazing reduces heat loss (as reflected by the U-factor) by more than 50 percent in comparison to single glazing. Although U-factor is reduced significantly, the VT and SHGC for a double-glazed unit with clear glass remain relatively high.

Edge Spacers
The panes of glass in an insulating unit must be held apart at the appropriate distance by spacers. In addition to keeping the glass units separated, the spacer system must serve a number of functions:

accommodating stress induced by thermal expansion and pressure differences;
providing a moisture barrier that prevents passage of water or water vapor that would fog the unit;
providing a gas-tight seal that prevents the loss of any special low-conductance gas in the air space;
creating an insulating barrier that reduces the formation of interior condensation at the edge.
Warm edge spacers have become increasingly important as manufacturers switch from conventional double glazing to higher-performance glazing. For purposes of determining the overall window U-factor, the edge spacer has a thermal effect that extends beyond the physical size of the spacer to a band about 2-1/2 inches wide. The contribution of this 2-1/2-inch-wide "edge of glass" to the total window U-factor depends on the size of the window. For instance, edge of glass effects are more important for smaller windows, which have a proportionately larger glass edge area.

A more significant benefit may be the rise in interior surface temperature at the bottom edge of the window, which is most subject to condensation. With an outside temperature of 0 degrees Fahrenheit, a thermally improved spacer could result in temperature increases of 6-8 degrees Fahrenheit at the window sightline or 4-6 degrees Fahrenheit at a point one inch in from the sightline, which is an important improvement. As new highly insulating multiple layer windows are developed, the improved edge spacer becomes an even more important element.

Gas Fills and Gap Width in Multiple-Glazed Units
Another improvement to the thermal performance of insulating glazing units involves reducing the conductance of the air space between the layers. Originally, the space was filled with air or flushed with dry nitrogen just prior to sealing. In a sealed-glass insulating unit, air currents between the two panes of glazing carry heat to the top of the unit along the inner pane and settle down the outer pane into cold pools at the bottom. Filling the space with a less conductive, more viscous, or slow-moving gas minimizes the convection currents within the space, reducing conduction through the gas and the overall heat transfer between the interior and exterior.

The use of low-conductance gas fills is far less common in commercial glazing than it is in residential windows. This results from the fact that solar control technologies are more important in typical commercial buildings than techniques for reducing heat transfer by conduction. However, as higher performance facades are developed, gas fills may become more common in commercial building windows as well.

Multiple Panes and Suspended Plastic Films
By adding a second pane, the insulating value of the window glass alone is doubled (the U-factor is reduced by half). As expected, adding a third or fourth pane of glass further increases the insulating value of the window, but with diminishing effect.

Triple- and quadruple-glazed windows became commercially available in the 1980s as a response to the desire for more energy-efficient windows . As each additional pane of glass adds to the insulating value of the assembly, it also reduces the visible light transmission and the solar heat gain coefficient. Additional panes of glass increase the weight and thickness of the unit, which makes mounting and handling more difficult and transportation more expensive. Prototype windows using very thin layers of glass (0.5-1.0 mm) have been fabricated but are not in commercial production.

It is apparent that there are physical and economic limits to the number of glass panes that can be added to a window assembly. However, multiple-pane units are not limited to glass assemblies. One innovation is based on substituting an inner plastic film for the middle layer of glass. The light weight of plastic film is advantageous, and because it is very thin, does not increase the unit thickness. As with triple- or quadruple-glazed windows, windows using plastic films decrease the U-factor of the unit assembly by dividing the inner air space into multiple chambers.

. There are three glazing layers and one low-E coating with 1/4-inch krypton gas fill in the cavities, and low-conductance edge spacers. In this case, the middle glazing layer is a suspended plastic film. The low-E coatings can be applied to the glass or plastic. Figure 3-12 illustrates a window with four glazing layers (two glass panes and two suspended plastic films). The combination of multiple glass panes and plastic films with low-E coatings and gas fills achieves very low center-of-glass U-factors. In this example, both low-E coatings have low-solar-gain properties in order to reduce cooling loads. The combination of multiple glass and plastic film layers with low-E coatings and gas fills has been used to achieve center-of-glass U-factors as low as 0.08. The properties of low-E coatings and tints are discussed in the following sections.