Insulated glass consists of two or more panes spaced 1/4" to 3/4" apart. The spacer separating the panes may be wood, metal, or a synthetic material. Sometimes a drying agent is placed between the panes to minimize fogging. In some windows, a plastic film is suspended between two panes of glass to provide extra airspaces. Multiple air spaces mean greater energy efficiency, and the number of airspaces is more important than the thickness of the airspace. For example, two 1/2" air spaces are more energy efficient than a single 1" air space.
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Increasingly, the air between the panes of insulating glass is replaced with an inert gas—usually argon, but sometimes krypton. These gases have more resistance to heat flow than air, so they improve the energy-efficiency of the window, often by as much as 15%. If the seal at the edge of a gas-filled window becomes damaged, the inert gas can slowly leak out, but there are no reports of health problems related to such leakage.
Some manufacturers combine a double-pane insulating-glass panel with an additional single-pane storm window. Two panes of glass are quickly becoming the industry standard in many climates, while three panes (and occasionally four panes) are popular in very cold climates. If a multiple-pane window unit is not properly sealed, or is poorly constructed, moisture can leak between the panes of glass and condense.
Coated glass and plastic films are offered by a number of manufacturers to make glass even more energy efficient. Plastic films suspended between two panes of glass are generally not a problem for sensitive people because the units are well sealed, but some of the glass coatings have the potential to outgas depending on their composition. These should be tested by sensitive people prior to use.
Energy-efficient glass with a low-E coating is becoming very popular. There are actually different types of low-E glass, and they have slightly different characteristics. But they all work in a similar manner—they act like a mirror and reflect radiant energy back into a room, thus preventing heat loss to the outdoors in the winter. In hot, sunny climates—or where excessive sunlight can cause overheating of the living space—windows are built with the low-E coating on a different surface of the glass to reflect radiant energy from the sun back toward the outdoors. Thus, windows can be specifically tailored to different climates—and even to different sides of the same house. Low-E coatings block a great deal of the incoming ultraviolet light. This is advantageous for your curtains, because ultraviolet light is responsible for material degradation and fabric fading. There are reports of a few hypersensitive people being bothered by low-E coatings but, fortunately, this is not a common problem.
In some window and door applications, shatter-resistant glass (or plastic) is required to minimize occupant injury. Acrylic and polycarbonate plastic sheets meet this requirement as does tempered safety glass, but the plastic materials are often avoided by sensitive people because of minor outgassing potential.
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