The U-value insulation rating

U-values – also called U-factors – are a rating of energy efficiency for windows, doors, skylights and certain other exterior building components, including walls. They're a standard used in building codes for specifying the minimum energy efficiency values, and are a good basis for comparing assembled building components.

The U-value measures how well a window or door product prevents heat from escaping a building. It's calculated in terms of Btu per hour, per square foot/meter of area, per degree of temperature (C/F) difference across the material. The lower the U-value number, the greater the heat transfer resistance – or insulating – characteristics of the material or assembled component.

U-value ratings typically fall between 0.2 and 1.2, with 0.2 being the best insulator.

R-value
The reciprocal of the U-value is the thermal resistance rating, or R-value. While a U-value rates how much heat a building component allows to pass through it, an R-value rates how much heat loss a material resists from passing through it. The higher the R-value number, the better the energy efficiency.

The U-value is used to rate assembled components like windows and doors, while the R-value is most often used to rate individual materials, like insulation batts