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.
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
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.