Cost lever
U-factor
the winter number
How well a whole window keeps heat in — the rate it loses heat. Lower is better; most quality NC windows land around 0.25–0.30.
What it means for you
The winter-comfort number. Coming off single-pane, this is where a good window earns its keep. Compare it across quotes before you compare price.
Where you'll see it
Top-left box on the NFRC sticker. It's whole-window, not just the glass, so it already bakes in the frame.
Related terms
Defined by Marc Hanley
Verified against NFRC label guide, ENERGY STAR
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