R-value: resistance of the window to heat transfer. R-value is the inverse of U-factor. The higher the R-value of a window, the better its insulating properties.

Radiation: in window terminology, radiation usually refers to the invisible heat-producing spectrum of the sun’s rays (example: infrared radiation).

Rail: the top or bottom horizontal frame member of a sash.

Reflective glass: glass which is coated to reflect the suns rays, reducing heat transfer and glare.

RESFEN: a computer program that can calculate energy usage for a particular window.

Rough opening: the wall opening into which a window or door is mounted.

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