Window U-Factor Calculator

This calculator blends frame and glass performance into a single Overall U-Factor, then derives R-Value, an estimated Annual Heat Loss, and an Energy Cost Impact per square foot. Overall U-Factor is an area-weighted average -- Frame U-Factor gets weighted by Frame Percentage, Glass U-Factor gets the remaining share, and Spacer Type tacks on a small fixed penalty for aluminum spacers that a warm-edge spacer avoids. This mirrors how the National Fenestration Rating Council actually labels real windows: NFRC 100 governs the whole-window U-factor calculation, and NFRC moved away from reporting center-of-glass numbers alone because, as NFRC itself notes, most of a window's heat loss occurs at the frame and the edge of the glass, not the center -- so the NFRC label on a real window already reflects this same frame-plus-glass blend rather than glass performance in isolation. Because Frame Percentage typically keeps the frame to a modest slice of a real window, an equal percentage change in Glass U-Factor moves Overall U-Factor further than the same percentage change in Frame U-Factor does.

R-Value is the mathematical reciprocal of Overall U-Factor, so the two always move in opposite directions -- a lower U-factor is always a higher R-value. Annual Heat Loss and Energy Cost Impact both scale directly off Overall U-Factor at a fixed assumed heating-degree-day figure, so whatever changes the U-factor changes those two by the same proportion. Air/Gas Gap Width has a spot on the input form for describing the space between panes, a detail that genuinely matters for real insulating-glass performance, but this calculator's math never touches it -- Overall U-Factor, R-Value, Annual Heat Loss, and Energy Cost Impact stay exactly the same no matter what gap width is entered.