Window U-Factor Calculator

How It Works

The U-factor measures how well a window conducts heat — lower is better (opposite of R-value). A single-pane window has a U-factor around 1.10, meaning it conducts heat rapidly. Double-pane windows improve to about 0.49 by trapping an insulating layer of air or gas. Low-E (low emissivity) coatings reflect infrared heat back inside during winter and block solar heat in summer, bringing double-pane windows down to about 0.30. Triple-pane Low-E windows reach 0.18 or better. Windows are typically the weakest thermal link in a home — even triple-pane windows have an R-value of only about 5.5, compared to R-13 or more for insulated walls. However, upgrading windows can cut window-related energy loss by 70 to 80 percent.