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A Guide to Energy-Efficient Windows and Doors

Figure 10 3.2 Glazing Parts

Glazing is the generic term for the transparent—or sometimes translucent—material in a window or door. A window may be divided into one or more sashes, some of which may move and others which may be fixed. For example, a double-hung window generally has two moveable sashes, while a single-hung window has just one moveable sash (Fig. 10).

A sash may be divided into two or more lights (panes of glass) held in place by mullions and muntins. (Fig.10)

3.3 Glazing Types

Figure 11When we speak of windows, we tend to use the terms single-, double- or triple-glazed. These terms simply refer to the number of panes of glass incorporated into the window unit: single glazed—one pane; double-glazed—two panes; triple-glazed—three panes (Fig. 11).

All windows in the Carolinas should be at least double-glazed. To determine the number of glazings in a particular window, hold a light next to the glass and count the reflections (Fig. 12). In a double-glazed window, for example, you'll see two main reflections, corresponding to the number of glazings. (if you look carefully at each reflection, there are actually two reflections very close together, bouncing off both surfaces of each pane.)

Figure 12Most window manufacturers offer several types of glazing which affect the insulation value of the window and the likelihood of condensation forming on the glass. Sometimes, transparent polyester films are used to increase energy efficiency.

A variety of coatings on the glazing surfaces—in addition to polyester films or inert gases between the glazings—can increase the insulating value of a double-glazed window to more than equal that of a standard triple-glazed window. Coatings are often used with gas fill. (See Section 7 for a more detailed discussion of special coatings and gas fills.)

Most windows now incorporate sealed, insulated glazing (IG) units in which two or more glazing layers are sealed around the outside edge to prevent air or moisture from entering the air space, eliminating dirt and condensation between glazings. If moist air finds its way into the sealed air space, condensation may form between the glazings. This is usually caused by a faulty seal and cannot be corrected except by replacing the IG unit. Windows with dual seals are recommended for better longevity and lower seal failure rates.

3.4

If you look between the window panes in a conventional double-glazed window where the glass meets the frame, you will probably see a strip of material, known as a spacer. The purpose of spacers is to maintain a uniform separation between the panes of glass (Fig. 13).

Spacers have traditionally been made of hollow aluminum, containing a drying agent or desiccant designed to absorb the initial moisture present at the time of manufacture in the space between the glazings. Metal spacers conduct energy easily and are a significant source of heat loss and poor window performance. The best are insulating spacers often made from non-metallic materials. There are also hybrid spacers made out of metal and non-metallic materials. These materials do not conduct nearly as much heat (see Section 7.4).

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