Modern windows offer thermal efficiencies that would have been unimaginable not so long ago. Some of that improvement can be attributed to better-insulated frames, but most of it has to do with better insulated glazing units, or IGUs. A top-of-the-line triple-glazed unit filled with krypton or xenon gas may have a center-of-glass R-value of 10 or more. (The R-value of the completed window is partly determined by the frame and sash, and will usually be somewhat lower).

That kind of efficiency, however, comes at a cost. High-end IGUs are expensive in themselves, and because a triple-glazed IGU can be up to 1-1/2 inches thick, building a moveable window around one calls for a large, heavy frame and beefed-up hardware. That adds another layer of cost and helps explain why such glazing is rare on residential jobs.

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