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This keeps it from being frozen in winter and from being overheated in summer, just as a dead-air space in the walls of a building protects from extremes of heat and cold.

The storm window reduces this to a minor percentage because the dead-air space between the two thicknesses of glass acts as an efficient means of insulation.

Or, better still, for the few earliest frames, have double-glass sash, the dead-air space making further protection unnecessary.