A material’s U-factor and R-value are both measurements of its insulation value, and put a number to how easily heat energy will pass through it by conduction. For windows, the accepted measurement is the U-factor (often — though incorrectly — called the U-value). Because a window’s U-factor is a direct measure of conduction, the lower the number, the lower the rate of heat loss, and vice-versa. A single-glazed window with no storm typically has a U-factor of 1.1 or so, while a good-quality double-glazed low-E window will have a U-factor of 0.35 or less. The most efficient triple-glazed windows with insulated frames can have U-factors as low as 0.15.

R-value — the familiar measure of the insulating value of walls, floors, and ceilings — is essentially just a different method of expressing the U-factor of a material or assembly. The conversion is simple: Take the reciprocal of a U-factor, and you have its R-value. A U-factor of 0.33, for example, can be expressed fractionally as 33/100; the...