What is the difference between R-value and U-value?

R-value measures how well insulation resists heat flow. Higher R-values mean better insulation. R-value applies to insulation materials in walls, attics, and floors. U-value measures how well a window, door, or skylight conducts heat. Lower U-values mean better insulation. U-value includes the entire assembly including glass, frame, and spacers. Both ratings matter, but they apply to different parts of your home.