96 Fenestration

The terms "Fenestration," "window," and "glazing" are often used interchangeably. To describe the important aspects of performance in this area requires that terms be defined carefully. "Fenestration" refers to the design and position of windows, doors and other structural openings in a building. When we speak of windows, we are actually describing a system of several components. Glazing is the transparent component of glass or plastic windows, doors, clerestories, or skylights. The sash is a frame in which the glass panes of a window are set. The Frame is the complete structural enclosure of the glazing and sash system. Window is the term we give to an entire assembly comprised of the sash, glazing, and frame.

Because a window is a thermally nonhomogeneous system of components with varying conductive properties, the thermal performance cannot be accurately approximated by the one-dimensional techniques used to evaluate common opaque building envelope components. The thermal performance of a window system will vary significantly, depending on the following characteristics:

• The dimension of the space between panes

• The type of gas between the panes

• The emissivity of the glass

• The frame in which the glass is installed

• The type of spacers that separate the panes of glass

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