What U-factor means
U-factor is the rate heat moves through a whole assembly, such as a window, including the glass, frame and air films. Lower is better. It is roughly the inverse of R-value.
Also called U-value.
Where it matters on a insulation job
Windows are rated by U-factor, and energy codes and rebates set maximums by climate. Quote replacement windows with the U-factor on the label so the customer can compare bids on the same terms.
What the model code says
- The energy code defines U-factor as the coefficient of heat transmission through a component or assembly, air to air. (IRC 2021, Chapter 11)
Check which code edition your building department enforces for U-factor before you quote.
What it costs
The window replacement cost guide has the typical job at $750 and the usual range at $350 to $1,100. The guide shows its sources and what moves the price. The window replacement cost calculator prices a job line by line.
Work it out
- Free calculator: Window replacement cost calculator
- Cost guide: Window replacement cost
- Tools, templates and guides: Free insulation resources
Sources: 2021 International Residential Code, Chapter 11, Energy Efficiency (ICC). Checked October 9, 2026.