961Multiple Glass Panes

Because of the low resistance provided by the glazing itself, the major contribution to thermal resistance in single pane glazing is from the indoor and outdoor air films. Assuming 0.17 outdoor and 0.68 indoor air film resistances, a single paned glazing unit might be expected to have an overall resistance of better than 0.85 (hr-ft2-°F)/Btu, or a U-factor of 1.18 Btu/(hr-ft2-°F). The addition of a second pane of glass creates an additional space in the assembly, increasing the glazing R-value to 1.85, which results in a U-factor of approximately 0.54 Btu/(hr-ft2-°F). Similarly, the addition of a third pane of glass might increase the overall R-value to 2.85 (hr-ft2°F)/Btu, yielding a U-factor of 0.35 Btu/(hr-ft2-°F). This one-dimensional estimate does not hold true for the entire window unit, but only in the region described by ASHRAE as the "cen-ter-of-glass" (see Figure 9.21). Highly conductive framing or spacers will create thermal bridging in much the same fashion as metal studs in the insulated wall evaluated in Section 9.31.

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