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

R-value measures thermal resistance, or how well insulation resists heat flow. Higher R-value means better insulation. U-value measures how well a window, door, or assembly conducts heat. Lower U-value means better insulation. R-value applies mainly to insulation materials, while U-value covers entire assemblies like windows with frames and glass. In cold climates, aim for high R-value in insulation and low U-value in windows. Both ratings help you compare the energy performance of building materials.