When looking at different models of windows you want to be cognizant of the rating the manufacturer applies. That rating will either be a “U Value” – which gives an indication of the window’s tendency to transfer heat either in or out, or by an “R Value” – which describes the window’s resistance to transferring heat in or out.
U-Values are a measure of heat conductance
R-Values are a measure of heat flow resistance
If a window’s R-value is high, it will lose less heat than one with a lower R-value.
Conversely, if a window’s U-value is low, it will lose less heat than one with a higher U-value.
In other words, U-values are the reciprocals of R-values (U-values = 1/R-value). However please note: Most window manufacturers use R-values in rating their windows.
Usually, window R-values range from 0.9 to 3.0 (U-values range from 1.1 to 0.3), but some highly energy-efficient exceptions also exist.
When comparing different windows, you should ensure that all U- or R-values listed by manufacturers:
- Are based on current standards set by the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE)
- Are calculated for the entire window, including the frame, and not just for the center of the glass
- Represent the same size and style of window
- The type of glazing material (e.g., glass, plastic, treated glass)
- The number of layers of glass
- The size of the air space between the layers of glass
- The thermal resistance of conductance of the frame and spacer materials
- The “tightness” of the installation