U-Factor
U-factor measures how well a window, door, or assembly transmits heat — the inverse of R-value. Lower U-factors mean better insulation. U-factor is the standard metric for windows because window R-value is dominated by the frame and edge effects, not the glass thickness. NFRC-rated whole-window U-factor includes frame, edge, and center-of-glass performance combined. U = 1/R, so a U-0.30 window is roughly equivalent to R-3.3.